Nutrition in Infancy: Physiology, Development, and Nutritional Recommendations
The first year after birth is one of dramatic change for human infants. Growth is more rapid during infancy than at any other time in life. Infants progress from being newborns with no head control to being babies who pull themselves up to a standing position and begin to take steps. They move from securing their nourishment with a reflexive suck while they are held snuggly in the caregivers arms to joining the family at the table and feeding themselves foods with a precise pincer grasp.
GROWTH AND MATURATION
From birth to 1 year of age, normal human infants triple their weight and increase their length by 50%. Throughout this important first year of life, infant feeding and nutrition influence both physical and psychosocial growth and development. Growth and maturation can be compromised or accelerated by undernutrition or overnutrition. The first six months of life are a critical period for brain growth: Brain growth is promoted by overall physical growth. (1) The stage of maturation determines the developmental readiness to progress in the acceptance of foods, texture, and self-feeding. Through feeding and other interactions, parents and infants establish secure and trusting relationships
The rate of growth in the first 4 months of life is faster than at any other time in life. A young infant uses a substantial portion of his or her energy intake to meet needs for growth. The period of 4 to 8 months is a time of transition to a slower growth pattern, and by 8 months the growth pattern is more similar to that of a 2 year old than a newborn. Assessment of physical growth is the primary method of determining infant nutritional status.
Weight and Length
Birth weight is determined by the mother's own medical history, her nutritional status before and during pregnancy, health related events that occur during pregnancy, and fetal characteristics. Maternal prepregnancy weight and weight gain during pregnancy are especially important determinants of infant birthweight. After parturition, genetics, environment, and nutrition determine rates of gains in weight and height. Immediately after birth there is a weight loss due to a loss of fluid and some catabolism of tissue. This loss averages 6% of body weight but occasionally exceeds 10%. Birth weight is usually regained by the tenth day. Thereafter, weight gain during infancy proceeds at a rapid but decelerating rate. Average weight gains for the first six months of life are shown in Table 1. The rate of growth slows substantially in the second half of the first year of life. By 4 months of age, most infants weigh twice their birth weights, and by 12 months they usually weigh three times what they weighed at birth. Males increase in weight to twice their birth weights earlier than do females, and smaller newborns may increase in weight to twice their birth weight sooner than do heavier neonates.
Length usually increases by 50% during the first year of life. The average length gain is 25 to 30 cm (10 to 12 inches), but a period of "catch-up" or "lag-down" growth may occur. The majority of infants who are born small but are genetically destined to be longer, shift percentiles on growth grids during the first 3 to 6 months. However, larger infants at birth whose genotypes are for smaller size tend to grow at their fetal rates for several months before the lag-down in growth becomes evident. During this lag-down period length drops from a higher to a lower percentile rating on the growth chart. Often a new percentile rating is not apparent until the child is 13 months of age.(2) Racial differences have been noted in rates of growth. American Black males and females are smaller than Caucasians at birth, but they grow more rapidly during the first 2 years (3,4).
Growth is expected in a healthy infant, and physical growth is an indicator of the health and nutritional status of infants and children. Height, weight, and head circumference data are plotted on growth charts to assess how growth is proceeding. Measurements must be accurately obtained and accurately recorded so acceleration or deceleration can be monitored.
The most commonly used growth grids in North America are those prepared by an expert committee of the National Center for Health Statistics (NCHS). These charts have recently been revised, and are soon scheduled to be available on the NCHS web site at http://www.cdc.gov/nchswww.
Previous growth grids were based on data from were based on data collected from 1929 through 1975 on 867 white, mostly bottle fed infants. The DARLING study of healthy, full term breastfed and formula fed babies found that growth of breastfed infants differed from both the NCHS standards and the growth of formula fed infants. (5) Mean weight of these breastfed infants fell below the median on the former NCHS charts from 6 to 18 months. Length gain was similar between the two groups so the breastfed infants were leaner than the formula fed infants between 4 and 18 months. There were no adverse consequences of this pattern of growth in breastfed infants.
The new charts are based on a representative sample of U.S. infants that includes both breastfed and bottle fed infants. Infants are assessed using charts that go from birth to 36 months. Growth grids are provided for weight for age, length for age, head circumference for age, and weight for length. The grids are prepared so that age values lie along the axis and height or weight values are plotted along the abscissa. Measurements at one age rank the baby's height or weight in relation to 100 other infants of the same age. Weight-height percentiles rank the baby's weight in relation to 100 other babies of the same length. Sequential measurements plotted on the growth grid indicate if the baby is maintaining, reducing, or increasing the percentile rating as growth proceeds.
Assessment of incremental growth or growth velocity is an additional method of determining if growth is appropriate. This method is especially useful in assessing short term growth or growth of infants who are at high risk of growth problems. Table 1 presents incremental growth standards for infancy.
Table - 1 Weight Gain in Grams per Day in One Month Increments
|Up to 1 month
|Up to 1 month
Adapted from Guo et al. (33)
Changes in Body Composition
Changes during growth occur not only in height and weight but also in the components of the tissue. Increases in height and weight and skeletal maturation are accompanied by changes in water, lean body mass, and fat.
Total body water as a percentage of body weight decreases throughout infancy from approximately 70% at birth to 60% at 1 year of age. Reduction of body water is almost entirely extracellular. Extracellular water decreases from about 42% of body weight at birth to 32% at 1 year of age. At the same time, intracellular water increases with the rapid growth of lean body mass toward the end of the first year.
The fat content of the body develops slowly during fetal life. Fat accounts for 0.5% of body weight at the fifth month of fetal growth and 16% at term. After birth, fat accumulates rapidly until approximately 9 months of age. Between 2 and 6 months of age the increase in adipose tissue is more than twice as great as the increase in the volume of muscle. Sex-related differences appear in infancy. Females deposit a greater percentage of weight as fat than the male.
Changes in Body Proportions
Increases in height and weight are accompanied by dramatic changes in body proportions. The head proportion decreases as the torso and leg proportion increases. At birth, the head accounts for approximately one fourth of the total body weight. When growth has ceased, the head accounts for one eighth of the total body length. Between birth and adulthood, leg length increases from approximately three eighths of the newborn's birth length to one half of the adult's total body height.
Feeding is the fundamental interaction from which the relationship between parents and infants evolves and the infant's psychosocial development proceeds. A parent's responsiveness to the infant's cues of hunger and satiation, and the close physical contact during a feeding facilitate a healthy development. For optimal development in early infancy babies need to be fed as soon as they express hunger, so that they learn that their needs will be met. As they grow older and learn to trust that their needs will be met they can wait longer for the initiation of the meal. All babies need to be held and cuddled while they are fed. Propping the bottle is unsafe and developmentally unsound in the first months of life.
Parents' ability to interpret cues and negotiate the feeding experience with their babies fosters healthy parent-child interactions and a sense of parental competency. Infants are born with characteristics that contribute to their overall temperament. Some infants are more irritable and less easily soothed. Some infants adapt easily to a regular schedule, others continue to be unpredictable and irregular for several months. Parents may benefit from the opportunity to learn about temperamental characteristics to better understand their infants and to prevent the development of problematic feeding interactions.
Ideally, the infant should be in a quiet, wakeful state when feeding is initiated. An infant who is distressed and crying may find it difficult to feed appropriately. The quiet wakeful state facilitates caregiver infant interactions during feeding. Identification of the infant's cues of hunger and satiation are basic to developing a strong caregiver-infant feeding relationship. Cues change rapidly as development proceeds during the early years. Please see Table in Chapter for a listing of infant hunger and satiety cues. The observant parent will recognize the changes and respond appropriately as the baby grows and develops.
Digestion and Absorption
The physiological development of the gastrointestinal (GI) tract is influenced by several factors. In utero the fetal GI tract is exposed to amniotic fluid that contains physiologically active factors such a growth factors, hormones, enzymes, and immnoglobulins. As discussed in Chapter 6, human milk provides epithelial growth factors and hormones as well as digestive enzymes to enhance the newborns ability to digest and absorb feedings.
These play a role in mucosal differentiation and GI development as well as the development of swallowing and intestinal motility. Digestion and absorption in the newborn requires:
The full-term newborn is prepared to digest and absorb an adequate supply of nutrients for normal growth and development from breastmilk or formula. The digestive capacity of the infant matures and increases during the first year of life. Feeding stimulates release of several hormones that are related to GI motility, intestinal development, and pancreatic cell function. The developing stomach and intestine provide an increasing ability to handle various nutrients and textures provided by food.
Esophageal motility is decreased in the newborn compared to older infants and children. In addition the lower esophageal sphincter (LES) is primarily above the diaphragm and LES pressure is less for the first months of life. Gastric emptying may be delayed in early infancy and intestinal motility is more disorganized. Due to these physiological realities, infants commonly experience regurgitation or "spitting up." Stomach capacity at birth, 10 to 12 ml, increases to 200 ml by 12 months. Newborns require small, frequent feeds.
Transit time through the small intestine is slower for infants than for adults. This may help to assure adequate digestion and absorption of nutrients. Passage through the large intestine is more rapid. Infants are at increased risk of dehydration if water and electrolyte resorption in the large intestine are further compromised.
Enzymatic secretions provide the capacity for infants to digest and absorb the milk and food consumed. Quantitative information about exact amounts of these enzymes at stages of infancy is lacking, but some relative findings have been established. (6)
Proteins: Protein digestion and absorption is limited by several factors in infancy. Gastric acidity may limit digestion, although not as much as previously thought. Concentrations of chymotrypsin and carboxypeptidase in the duodenum are only 10% to 60% of the adult levels. Babies can digest adequate protein even though the quantity of enzymes is limited. Newborns can completely digest about 1.95 g/kg/day of protein, 4-month-old babies about 3.75 g/kg/day. In other words, a 3.5 kg newborn would be expected to digest 6.75 g of protein. An intake of 12.5 oz (369.6 ml) of human milk will meet the suggested energy intake and provide approximately 4.0 g protein. A formula-fed infant who consumes the same quantity will receive 5.54 g protein; either protein intake is adequately digested by the infant.
Fats: Pancreatic lipase activity is low in-the newborn, but other lipases from breastmilk, the tongue (lingual lipase), and the stomach (gastric lipase) offer compensatory mechanisms for fat digestion. Human milk and colostrum have bile-salt-stimulated lipase (BSSL), and the fat in human milk is more easily absorbed than that of infant formula. (7) The newborn bile acid pool, although present, is about half that of an adult on the basis of body surface area
Carbohydrates: Sugars are well utilized. Maltase, isomaltase, and sucrase activity reach adult levels by 28 to 32 weeks gestation. Lactase, present in low levels at 28 weeks gestation, increases near term. True lactase deficiency in infancy is very rare even in populations that have high rates of lactase insufficiency in adulthood. Pancreatic amylases are low or absent up to 4 months of age. Salivary amylase, present at birth, rises to adult concentrations between 6 months and 1 year of age. Even though a large percentage of the salivary amylase is suspected of being inactivated by hydrochloric acid in the stomach, young infants do digest some starch. This is thought to be due to the presence of glycosidase and glucoamylase present in the brush border of the small intestines. These enzymes hydrolyze starch to glucose.
Table 2 Summary of Digestive Factors in Early Infancy
|In Early Infancy Compared to Adult levels
|Lower production: rapid fall in pH after a meal
|Intestinal Mucosal peptidases
|Very low levels
|Lingual, gastric and breastmilk bile salt stimulated lipase
|Stays active in stomach
|Very low levels
|Fermentation and absorption in large intestine
The newborn has an immature kidney, and can maintain water and electrolyte balance only within a fairly narrow range of intakes and losses. The functional development of the nephron is not complete until 1 month of age. The tubules are short and narrow and do not reach mature proportions until approximately 5 months. In addition, the pituitary gland produces only limited quantities of the antidiuretic hormone (ADH) vasopressin, which normally inhibits diuresis. These factors limit the newborn's ability to concentrate urine and to cope with fluid and electrolyte stress, i.e., electrolyte-dense formula, limited fluid intake, and diarrhea.
Renal solute load. The major percentage of solutes presented to the kidney for excretion are the nitrogenous end products of protein metabolism, sodium, potassium, phosphorus, and chloride. If none of these elements were utilized in new body mass or lost by nonrenal routes, such as perspiration, they would need to be excreted in the urine. They are therefore referred to as the potential renal solute load. The potential renal solute load can be calculated by assuming all nitrogen is excreted, dividing dietary nitrogen by 28 and adding sodium, potassium, chloride, and phosphorus in the feed expressed as milliosmoles, abbreviated mOsm. (8) Most healthy adults are able to achieve urine concentrations of 1300 to 1400mOsm/l. A healthy newborn may be able to concentrate urine to 900 or even 1100 mOsm/l, but an isotonic urine of 280 to 310 mOsm/l is the goal.
Table 3 Potential Renal Solute Load of Representative Milks and Formulas
Potential Renal Solute Load, mOsm/l
|Cows milk based formula
|Soy based formula
|Whole cows milk
Difficulties with the renal solute load are unlikely in normal infants fed human milk or a correctly prepared formula. Problems may occur when elevated environmental temperature or fever increase evaporative loss, when diarrhea occurs, and/or when infants reduce the volume of fluids they consume.
NUTRIENT NEEDS OF INFANTS
Estimates of energy and nutrient needs in infancy have been made from intakes of infants growing normally and from the nutrient content of human milk. These are only guidelines, and each individual infant will have individual requirements at different stages of infancy. The Recommended Dietary Allowances (RDAs) are planned to provide a margin of safety to allow for maximum protection, and the newer Dietary Reference Intakes for infancy are based on observed levels that appear to be adequate. Because of the declining growth rates during the latter part of the first year, recommended intakes have been set for two 6-month periods, from birth to 6 months and from 6 months to 1 year. (9)
Current RDA for energy intake in infancy is 108 kcal/kg/day from birth through 6 months of age, and 98 kcal/kg/day for the second half of the first year. (9) These values were estimated from WHO data on intakes from healthy infants with an additional 5% allowance for underestimation of intake. Energy requirement is higher in the first weeks of life, but it was determined that data was insufficient to establish more precise breakdowns. A recent review of studies of well nourished infants found that current recommendations are significantly higher than actual energy requirements and could lead to overfeeding if they were closely adhered to. (10) Breastfed infants have lower requirements for total energy expenditure and energy cost of growth than formula fed infants.
The energy requirement in infancy is determined primarily by body size, physical activity, and rates of growth. Since large variations in these variables are seen among infants at any age and in any one infant from month to month, ranges of energy needs are large. Total energy needs (kcal/day) rise during the first year, but energy needs per unit of body size decline in response to changes in rates of growth. Energy expended for growth declines from approximately 32.8% of intake during the first 4 months to 7.4% of intake from 4 to 12 months. (11) The contribution of physical activity to total energy expenditure is quite variable but can be expected to increase with age as motor skills develop. Some infants are quiet and cuddly while others spend a considerable amount of time crying, kicking, or just exploring with motor skills they have acquired. The most appropriate way to judge the adequacy of babies' energy intake is to monitor the adequacy of their linear growth and weight gain.
Protein and Amino Acids
Infants require protein for synthesis of new body tissue during growth, as well as synthesis of enzymes, hormones, and other physiologically important compounds. Increases in body protein are estimated to average about 3.5 g/day for the first 4 months, and 3.1 g/day for the next 8 months.(11) The body content of protein increases from about 11.0% to 15.0% over the first year. The recommended intake is 2.2 g/kg for the first 6 months and 1.6 g/kg from 6 to 12 months.(9) The American Academy of Pediatrics has set minimum protein standards for infant formula of 1.8 g/100 kcal with a protein efficiency ratio equal to that of casein. (12;13) Nine amino acids are dietary essentials in infancy. See Table -5. The subcommittee responsible for determining the amino acid requirements for infancy for the Recommended Dietary Allowances concluded that the composition of human milk should be used as a reference pattern for amino acid requirements in infancy. (9)
TABLE 4 Recommended Energy and Protein Intake for Infants
|Age in Months
|Reference Weight (kg)
|Energy Recommendations (kcal/kg/day)
|Protein Recommendations (g/kg/day)
|6 - 12
From National Academy of Sciences: Recommended dietary allowances, Ed 10, Washington, DC, 1989, National Academy Press.
TABLE -5 Estimated Amino Acid Requirements of Infants 3 - 4 Months of Age
|Methionine plus cystine
|Phenylalanine plus tyrosine
From Energy and Protein Requirements, Report of a Joint FAD/WHO Ad Hoc Committee, World Health Organization technical report series No 522, FAO Meet Rep No 52, Geneva, 1973, World Health Organization.
Fat: Fat, the most calorically concentrated energy nutrient, supplies between 40% and 50% of the energy consumed in infancy. The energy provided by fat spares protein for tissue synthesis. Its caloric concentration is an asset during periods of rapid growth when energy demands are great. Fat provides about 50% of the energy when human milk or commercial infant formulas are the only source of nutrients. Fat provides 47 to 49% of the energy content of commercial formula and about 52% of the energy content of human milk.
In the second half of infancy, as foods become increasingly important in the infant diet, nutrient and energy content of those foods must be considered. Excessive consumption of traditional high carbohydrate weaning foods such as cereal, vegetables, and fruits may compromise both energy and nutrient intake for some infants. It is common for the percent of energy from fat to drop precipitously between 6 and 9 months, and then to rise again as infants begins to eat more high fat family foods. (14)This pattern of food and energy consumption may pose a problem for children with health or developmental concerns that interfere with the ability to eat or digest foods or for children from vegan or health conscious families who exclude higher fat, nutrient dense foods from the diet. Growth failure has been reported in some infants and toddlers when fat intake is inadequate. (14)
Long Term Effects of Lipid Intake in Infancy: Lipid and cholesterol intakes in infancy may establish metabolic programming that influences health through out the lifespan. Research in this area is preliminary, but there are indications that feeding choices in infancy influence factors such as bile acid metabolism, cholesterol metabolism, and obesity.
Fatty Acids: Major considerations of fatty acids in the infant diet include adequacy of essential fatty acids to prevent essential fatty acid deficiency and assurance of adequate long chain fatty acids to promote growth and neurological development.
Linoleic anda-linolenic fatty acids are the essential fatty acids for humans. (12). Arachidonic is sometimes considered essential as well because it can prevent fatty acid deficiency when linoleic acid is inadequate. Linoleic is the 18 carbon fatty acid from the omega-6 family. a-Linolenic is the 18 carbon fatty acid from the omega-3 family. Deficiency of linoleic or arachidonic (20 carbon, omega-6 fatty acid) acids results in the traditional fatty acid symptoms of scaly skin, hair loss, diarrhea, and impaired wound healing. Human milk contains 3 to 7% of kilocalories as linoleic acid. The American Academy of Pediatrics and the Food and Drug Administration specify that infant formula should contain at least 300 mg of linoleate per 100 kilocalories or 2.7% of total kilocalories as linoleate.
These essential fatty acids serve as precursors for longer chain fatty acids that are required for normal cell function. Humans have the capacity to make these longer chain fatty acids if the essential fatty acids are present. However, this process is limited, especially in preterm infants. Considerable recent interest has focused on the importance of dietary sources of preformed long chain fatty acids of the omega 6 and 3 families.
Human milk provides both parent and longer chain essential n-3 and n-6 fatty acids. The brain and other neural tissue preferentially accumulate the omega-3 long chain fatty acid, Docosahexanoic acid (DHA) in utero and in the first months after birth. Human infants have limited capacity to synthesize DHA from the parent omega-3, linolenic acid, and the availability of preformed DHA from the diet has been proposed as one possible explanation for higher intelligence quotients for breastfed children. (15;16). Improved visual function has been reported in many, but not all, studies of infants who received formula with DHA compared to controls fed non-supplemented formula. However some studies of DHA supplementation have found that the addition of DHA to formulas for preterm infants lowered growth rates. A balance of both omega 3 and omega 6 fatty acids appears to be the key factor (17).
The dietary ratio of n-6 to n-3 fatty acids is critical as these fatty acids compete for enzymatic pathways and each can be metabolized to potent eicosanoids. In the US there are currently no commercially available infant formulas with DHA, and the addition of DHA and arachidonic acid (AA) to infant formulas is under discussion. (18) The case for adding DHA and AA. to preterm infant formula is thought to be stronger than that for including long chain polyunsaturated fatty acids (LC-PUFA) in all formulas. Preterm infants have very low LC-PUFA stores at birth. The appropriate commercial ingredient source for these fatty acids for infant formulas is not clear at this point.
Infants require more water per unit of body size than do adults. A larger percentage of water is located in the extracellular spaces. As noted before, young infants have an immature kidney. These two factors make the infant vulnerable to water imbalance. The water requirement is determined by water loss, water required for growth, and solutes derived from the diet.
TABLE -6 Range of Average Water Requirements of Infants and Children Under Ordinary Circumstances
|Amount of Water (ml/kg/day)
Modified from Behrman RE, Khegman RM, editors: Nekon Textbook of Pediatrics, Ed 14, Philadelphia, 1992, WB Saunders.
Water is lost by evaporation through the skin and respiratory tract (insensible water loss), through perspiration when the environmental temperature is elevated, and by elimination in urine and feces. During growth additional water is necessary since water is needed as a constituent of tissue and for increases in the volume of body fluids. The amount of water required for growth, however, is very small. The body requirement for water is the sum of the above demands.
Water lost by evaporation in infancy and early childhood accounts for more than 60% of that needed to maintain homeostasis, as compared to 40% to 50% in the adult. At all ages approximately 24% of the basal heat loss is by evaporation of water through the skin and respiratory tract. This amounts to 45 ml of insensible water loss per 100 kcal expended. Fomon estimates evaporative water loss at 1 month of age to average 210 ml/day and at age 1 year, 500 ml/day. (11) Evaporative losses increase with fever and increased environmental temperature. Increases in humidity decrease respiratory loss. Loss of water in the feces averages 10 ml/kg/day in infancy.
The range of water requirements is shown in Table -6. The National Research Council recommends an intake of 1.5 ml/kcal of energy expenditure for infants. (9). Water intoxication resulting in hyponatremia, irritability, and coma can result if infants are fed too much water. This has been reported to occur when families do not have the resources to obtain adequate infant formula. (19) Under normal conditions, infants fed breast milk or infant formulas do not need additional water.
Minerals and Vitamins
Need for minerals and vitamins are influenced by growth rates, mineralization of bone, increases in bone length and blood volume, and by energy, protein, and fat intakes. Recommended intakes have been established for nutrients for which there is adequate information. Starting in 1997 the Food and Nutrition Board of the National Research Council began to publish the results of a new approach to nutrient recommendations. New Dietary Reference Intakes (DRIs) have been established for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline (20) as well as calcium, phosphorus, magnesium, Vitamin D, and fluoride (21). New DRIs for antioxidant vitamins and minerals are under consideration. In general, with the exception of vitamin K and possibly vitamin D, healthy infants who receive human milk or commercial infant formula do not need vitamin supplements. (12).
TABLE -7 1997 Infant Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride (20)
AI: Adequate Intake
Table -8 1989 Infant Recommended Dietary Intakes for, Fat-Soluble Vitamins, Vitamin C, and Minerals (9)
TE: Tocopherol equivalents
RE: Retinol equivalents
Table -9 1998 Infant Dietary Reference Intakes For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline (21)
AI: Adequate Intake
Calcium and Phosphorus: The recommended intakes of calcium and phosphorus are based on the adequate intake that is observed in infants fed primarily with human milk. Calcium is well absorbed from human milk. About 61% of the calcium in human milk is absorbed compared to 38% of the calcium in formula. (11;22) Commercial infant formulas have higher calcium content than human milk to compensate for decreased rates of absorption. In the past a great deal of attention has been given to the ratio of calcium to phosphorus in the diet, Healthy full term infants can adjust to a range of phosphorus intakes, and phosphorus levels that are found in commercial infant formulas and from food in the infants diet are unlikely to be a cause of concern. (23)
Iron: Rates of iron deficiency in U.S. infants have declined with increased breastfeeding prevalence and consumption of iron fortified formulas. Iron deficiency anemia remains a concern for infants who do not receive breastmilk, iron fortified formulas, or foods with adequate iron in the second half of the first year of life. Iron deficiency in infancy may have long term developmental consequences.
Infant iron needs are supplied from two different sources, prenatal reserves and food sources. Before birth, the fetus accumulates iron in the last trimester of pregnancy. Premature infants have limited reserves that are quickly depleted. Even with the advantage of full-term iron stores, the rapidly growing infant is at risk for iron deficiency because of the increase in blood volume as the baby grows larger. The concentration of hemoglobin at birth averages 17 to 19 g/100 ml of blood. During the first 6 to 8 weeks of life it decreases to approximately 10 to 11 g/100 ml because of a shortened life span of the fetal red blood cell and decreased formation of red blood cells. After this age there is a gradual increase in hemoglobin concentration to 13 g/100 ml at 2 years of age.
Iron absorption is highly variable. See Table -10. Two levels of ferrous sulfate fortification are available in commercial infant formulas, very low iron formulas and iron fortified formulas. Iron-fortified commercial infant cereals are fortified with electrolytically reduced iron. Absorption of iron from infant cereal averages 5%. Many have recommended mixing this cereal with a fruit juice containing vitamin C to enhance the iron absorption. Healthy full-term breastfed infants can maintain satisfactory hemoglobin levels without supplemental iron in early infancy. However, if they continue to be fed only breast milk after 6-7 months they are at risk of negative iron balance and may deplete reserves in the later half of infancy. (24;25)
Table -10 Iron Absorption In Infancy
|Percent Reported Absorbed
|Hallberg et al. (34)
|Human Milk in 5 to 7 month olds who are also eating solid foods.
|Abrams et al.(22)
|Iron Fortified Cows milk based Formula
|Hurrel et al.(35)
|4 to 5%
|Fomon et al.(36)
Zinc: Zinc deficiency is unlikely in healthy, full term infants fed breastmilk or a commercial infant formula in developed countries, but it is frequently reported in developing countries and in infants with failure to thrive. Compared to iron, relatively little is known about true zinc requirements and zinc absorption in infancy. It appears that infants adapt to varied zinc intakes by increased or decreased absorption rates. Rates of absorption from formula have been reported to range from 41% when total zinc intake was low to 17% when total intake was high. (26)
Fluoride: Adequate fluoride intake is an essential component of preventive dental health care. On the other hand, excessive fluoride intake can cause fluorosis. Fluoride adequacy should be assessed when infants are 6 months old. Dietary fluoride supplements are recommended for those infants who have low fluoride intakes. Breast milk has a very low fluoride content. (27) Fluoride content of commercial formulas has been reduced to about 0.2 to 0.3 mg per liter to reflect concern about fluorosis. (28) Formulas mixed with water will reflect the fluoride content of the water supply. The Food and Drug administration does not require manufacturers of beverages to determine or disclose the fluoride content of their products. Fluorosis is likely to develop with intakes of 0.1 mg/kg or more. (28)
Table -11 Fluoride Supplementation Schedule
|Fluoride Concentration in Local Water Supply, ppm
|6 mo. to 3 y
|6 y to at least 16 y
American Dental Association, American Academy of Pediatrics, American Academy of Pediatric Dentistry, 1994. (12)
Vitamin A: In the United States, concern about excess vitamin A is more of a health risk than inadequate vitamin A in infancy. (12;29) Human milk, commercial infant formula, and cows milk are good sources of vitamin A. In developing countries low Vitamin A intakes are associated with infectious diseases. These relationships are under study in more developed countries. At this time, the American Academy of Pediatrics recommends vitamin A supplementation under two specific circumstances in infants and toddlers who have complications of measles. Supplements are recommended for infants between 6 and 24 months of age who are hospitalized with select serious complications and for older than 6 months who have risk factors such as opthamaologic evidence of vitamin A deficiency, immunodeficiency, impaired absorption, moderate to severe malnutrition, and recent immigration from areas with high mortality from measles. (12)
Vitamin D: Vitamin D acts in concert with other nutrients, principally calcium, phosphorus, and protein to promote bone mineralization. Vitamin D requirements are dependent on the amount of exposure to sunlight. Rickets has been reported in some high risk U.S. infants with dark skin. (30), and the American Academy of Pediatrics recommends supplements of 10mg (400 IU) per day for breastfed infants. On the other hand the Pediatric Nutrition Handbook (12) states that for white infants adequate exposure to sunlight to produce vitamin D is 30 minutes per week clothed only in a diaper, or 2 hours per week fully clothed with no hat. These exposures are mediated by time of year as well as latitude.
Vitamin E: Defining appropriate intakes for vitamin E is complicated by large variations in the susceptibility to peroxidation of fatty acids in the diet and tissues. The recommended intake during infancy reflects the tocopherol concentration of human milk in which 6% of the kilocalories are provided by polyunsaturated fatty acid.
Vitamin K: Vitamin K is necessary for blood coagulation and other physiological processes. Infants have low vitamin K stores at birth and are at risk for hemorrhagic disease of the newborn, which usually occurs between 2 and 10 days after birth. Breastfed infants are at higher risk of this condition. This condition develops in one in 200-400 infants who do not receive prophylactic vitamin K soon after birth. Late onset vitamin K-responsive bleeding is less common, but has been reported in breastfed infants who do not receive vitamin K therapy early in life. Since 1961 the Committee on Nutrition of the American Academy of Pediatrics has recommended a prophylactic intramuscular dose of vitamin K at birth. This recommendation was recently revisited by a Canadian committee, and a 1 mg of vitamin K shortly after birth was recommended . (31) Oral doses may also be given, but these are not as effective in maintaining adequate vitamin K status for the first months of life.
Water-soluble Vitamins: In 1998 Dietary Reference Intakes were established for nine water soluble vitamins. (21) See Table-9. In general, the Committee on the Scientific Evaluation of Dietary Reference intakes, found insufficient evidence to establish an RDA for these vitamins during infancy. Instead, a level of Adequate Intake (AI)was established. For young infants, the AI was based on the daily mean nutrient intake supplied by human milk. New Vitamin C recommendations are currently under consideration. Major considerations for each water soluble vitamin are listed below:
Vitamin and mineral supplements: After initial supplements of vitamin K, full-term infants who receive milk from a well-nourished lactating mother will receive all the vitamins they need, with the possible exception of vitamin D and fluoride. The American Academy of Pediatrics recommends a supplement of 400 IU of vitamin D per day for breastfed infants, especially African American infants and others with darker skin. Fluoride supplement decisions are based on fluoride content of the water supply as described above. Infants who receive a commercially available formula prepared with fluoridated water, will not need a vitamin supplement. The need for supplemental iron depends on the composition of the diet consumed. Solely breastfed infants should receive a source of iron by 6 months of age. Those who receive iron-fortified cereals will probably not need additional iron supplements. Infants receiving iron-fortified formula will need no supplemental iron.
Development of Oral Structures and Functions
In 1937 Gesell and Ilg published their now classic observations made during extensive studies of the feeding behavior of infants. (32) Their observations are as valid today as they were then. Cineradiographic techniques developed since then have permitted more detailed descriptions of the actions involved in sucking, suckling, and swallowing.
It is important to recognize that even though the normal neonate is well prepared to suck and swallow at birth, the physical and motor maturation during the first year alter both the form of the oral structures and the methods by which the infant extracts milk from a nipple. Each of these changes influences the infant's eating skills. At birth the tongue is disproportionately large in comparison with the lower jaw and essentially fills the oral cavity. The lower jaw is moved back relative to the upper jaw, which protrudes over the lower by approximately 2 mm. When the mouth is closed, the jaws do not rest on top of each other; the tip of the tongue lies between the upper and lower jaws. There is a "fat pad" in each of the cheeks. It is thought that these pads serve as a prop for the muscles in the cheek, maintaining rigidity of the cheeks during suckling. The lips of the neonate are also instrumental in suckling and have characteristics appropriate for their function at this age. A mucosal fold disappears by the third or fourth month, when the lips have developed muscular control to seal the oral cavity. The newborn infant sucks reflexively, the young infant beginning at 2 or 3 weeks of age suckles, and as infants grow older they learn mature sucking. Some description of these two processes therefore seems important.
Suckling:The processes of breast and bottle suckling are similar, but there are subtle differences that make it difficult for some infants to easily go from bottle feeding to breastfeeding once bottle feedings have been established.The nipple of the breast becomes elongated during breastfeeding so that it resembles a rubber nipple in shape, and both assume a similar position in the infant's mouth. The infant grasps the nipple in the mouth. The oral cavity is sealed off by pressure from the middle portions of the lips assisted by the mucosal folds of the jaws. The nipple is held in the infant's mouth with the tip located close to the junction of the hard and soft palate. During the first stage of suckling, the lower jaw and tongue are lowered while the mouth is closed, thus creating a negative pressure. The tip of the tongue moves forward. The lower jaw and tongue are next raised, compressing the beginning base of the nipple. The compression is moved from the beginning base to the tip of the nipple as the tip of the tongue withdraws, thus stroking or milking the liquid from the nipple. The moved back position of the lower jaw maximizes the efficiency of the stroking action. As the tongue moves back, it comes in contact with the tensed soft palate, thus causing liquid to squirt into the side food channels. The location of the larynx is further elevated by the muscular contractions during swallowing. As the liquid is squirted back in the mouth, the epiglottis is positioned so that it parts the stream of liquid, passing it to the sides of the larynx instead of over it. Thus liquid does not pass over the laryngeal entrance during early infancy because of the relatively higher position of the larynx and the parting of the stream of liquid by the epiglottis.
Sucking: Mature sucking is an acquired feature of the orofacial muscles. It is not a continuous process. Upon accumulation of sufficient fluid in the mouth, a swallowing movement interrupts sucking and breathing. The closure of the nasopharyngeal and laryngeal sphincters in response to the presence of food in the pharynx is responsible for the interruption of the nasal breathing.
Swallowing: During swallowing the food lies in the swallow preparation position on the groove of the tongue. The farther back portion of the soft palate is raised toward the adenoidal pad in the roof of the epipharynx. The tongue presses upward against the nipple so that the swallow of milk follows gravity down the sloping tongue and reaches the pharynx. As the milk in the mouth moves downward, the rear wall of the pharynx comes forward to displace the soft palate toward the back surface of the tongue and the larynx is elevated and arched backward. The accumulated milk is expressed from the pharynx by peristaltic movements of the pharyngeal wall toward the back of the tongue and the larynx. The milk spills over the joining folds of the pharynx and epiglottis folds into the side food channels and then into the esophagus. The tonsils and Iymphoid tissue play an important role as infants swallow. They help keep the airway open and the food away from the rear pharyngeal wall as the infant is held in a reclining position, thus delaying nasopharyngeal closure until food has reached the pharynx.
Mature Feeding: As the infant grows older the oral cavity enlarges so that the tongue no longer fills the mouth. The tongue grows differentially at the tip and attains motility in the larger oral cavity. The elongated tongue can be protruded to receive and pass solids between the gum pads and erupting teeth for mastication. Mature feeding is characterized by separate movements of the lip, tongue, and gum pads or teeth.
Sequence of Development of Feeding Behavior
Newborns: The "rooting reflex" caused by stroking of the perioral skin including the cheeks and lips causes an infant to turn toward the stimulus, so that the mouth comes in contact with it. Stimulus placed on the lip causes involuntary movements toward it, closure, and pouting in preparation for sucking. These reflexes thus enable the infant to suck and receive nourishment. Both rooting and suckling can be elicited when the infant is hungry but are absent when the infant is satiated. During feeding the neonate assumes a tonic position, the head rotated to one side and the arm on that side fisted. The infant seeks the nipple by touch and obtains milk from the nipple with a rhythmic suckle. (32) Semisolid foods, introduced by spoon at an early age into the diets of many infants, are secured in the same manner as milk, by stroking movements of the tongue with the tongue projecting as the spoon is withdrawn. Frequently, the food is expelled from the mouth. Age 16 to 24 weeks. By 16 weeks of age the more mature suckling pattern becomes evident, with the tongue moving back and forth as opposed to the earlier up-and-down motions. Spoon feeding is easier because the infant can draw in the lower lip as the spoon is removed. The tonic neck position has faded, and the infant assumes a more symmetric position with the head at midline. The hands close over the bottle. By 20 weeks of age the infant can grasp on tactile contact with a palmer squeeze. By 24 weeks of age the infant can reach for and grasp an object on sight. In almost every instance the object goes into the mouth.
Age 24 to 28 weeks: Between 24 and 28 weeks of age chewing movement, an up-and-down movement of the jaws, begins. This movement, coupled with the ability to grasp and the hand-to-mouth route of grasped objects, as well as sitting posture, indicates a readiness of the infant to finger feed. Infants at this age grasp with a palmer grasp. Therefore, the shape of the food presented to the child to finger feed is important. Cookies, melba toast, crackers, and teething biscuits are frequently introduced at this stage.
Age 28 to 32 weeks: Between 28 and 32 weeks of age the infant gains control of the trunk and can sit alone without support. The sitting infant has greater mobility of the shoulders and arms and is better able to reach and grasp. The grasp is more digital than the earlier palmer grasp. The infant is able to transfer items from one hand to the other and learns to release and resecure objects voluntarily. The beginning of chewing patterns, up-and-down movements of the jaws, is demonstrated. The tongue shows more maturity in regard to spoon feeding than in drinking. Food is received from the spoon by pressing the lips against the spoon, drawing the head away, and drawing in the lower lip. The infant is aware of a cup and can suck from it. Milk leaks frequently from the corners of the mouth as the tongue is projected before swallowing. By 28 weeks of age infants are able to help themselves to their bottle in sitting postures, although they will not be able to tip the bottle adaptively as it empties until about 32 weeks of age. By the end of the first year they can completely manage bottle-feeding alone. By 32 weeks of age infants bring their heads forward to receive the spoon as it is presented to them. The tongue shows increased motility and allows for considerable increased manipulation of food in the mouth before swallowing. At the end of the first year, infants are able to manipulate food in the mouth with definite chewing movements.
During the fourth quarter of the first year, the child develops an increasingly precise pincer grasp. The bottle can be managed alone and can be rescued if it is lost. The child can drink from a cup if help is provided. Infants at this age are increasingly conscious of what others do and often imitate the models set for them. (32) By 1 year of age the patterns of eating have changed from sucking to beginning rotary chewing movements. Children understand the concept of the container and the contained, have voluntary hand-to-mouth movements and a precise pincer grasp, and can voluntarily release and rescue objects. They are thus prepared to learn to feed themselves, a behavior they learn and refine in the second year.
TABLE -12 Sequence of Development of Feeding Behavior
|Oral, Fine, Gross Motor Development
|Rooting and suck and swallow reflexes are
present at birth
Tonic neck reflex present
|Head control is poor
Secures milk with suckling pattern, the tongue projecting during a swallow
By the end of the third month, head control is developed
|Rooting reflex fades
Bite reflex fades
Tonic neck reflex fades by 16 weeks
|Changes from a suckling pattern to a
mature suck with liquids
Sucking strength increases
Munching pattern begins
Grasps with a palmer grasp
Grasps, brings objects to mouth and bites them
|Gag reflex is less strong as chewing of
solids begins and normal gag is developing
Choking reflex can be inhibited
|Munching movements begin when solid foods
Rotary chewing begins
Has power of voluntary release and resecural
Holds bottle alone
Develops an inferior pincer grasp
|Bites nipples, spoons, and crunchy foods
Grasps bottle and foods and brings them to the mouth
Can drink from a cup that is held
Tongue is used to lick food morsels off the lower lip
Finger feeds with a refined pincer grasp
Modified from Gessell A, Ilg FL: Feeding behavior of infants, Philadelphia, 1937, JB Lippincott.
Margin Notes and Boxes
Growth velocity: Rapidity of motion or movement; rate of childhood growth over normal periods of development, as compared with a population standard. May also be referred to as incremental growth.
Growth acceleration: Period of increased speed of growth at different points of childhood development.
Growth deceleration: Period of decreased speed of growth at different points of childhood development.
Intracellular water: Water found in fluids outside of cells.
Intracellular water: Water found in fluids inside of cells; in body composition this reflects lean body mass.
Adipose tissue: Loose connective tissue in which fat cells (adipocytes) accumulate and are stored.
Lean body mass: Collective fat-free mass of body composition; most metabolically active portion of body tissues.
Temperament: inherited pattern of physiologic and behavioral reactions to situations. Components commonly include: activity level, rhythmicity, approach, adaptability, intensity, mood, persistence, distractibility, and threshold.
State: The infants condition. State is important in understanding an infants response to his/her environment. May be thought of a continuum that includes, quiet sleep, active sleep, drowsy, quiet alert, active alert, and crying (37)
Gastric pH: Chemical symbol relating to H+ concentration or activity in a solution; expressed numerically as the negative logarithm of H + concentration: pH 7.0 is neutral-above it alkalinity increases, and below it acidity increases. The hydrochloric acid (MCI) gastric secretions make gastric pH about 2.0.
Trypsin: Protein-splitting enzyme formed in the intestine by action of enterokinase on inactive precursor trypsinogen.
Chymotrypsin: One of the protein-splitting and milk-curdling pancreatic enzymes, activated in the intestine from precursor chymotrypsinogen; breaks peptide linkages of the amino acids phenylalanine and tyrosine.
Renal solute load: Collective number and concentration of solute particles in solution, carried by the blood to the kidney nephrons for excretion in the urine, usually nitrogenous products from protein metabolism, and the electrolytes Na+, K+, Cl-, and HPO4.
Mililosmoles (mOsm/L): Standard unit of osmotic pressure; equal to the gram molecular weight of solute divided by the number of particles (ions) into which a substance dissociates in solution. The term osmolality refers to this concentration of solutes per unit of solvent
Energy intake: Energy value of carbohydrate, fat, and protein in food, measured in kilocalories per kilogram.
Naming system for fatty acids: First number refers to carbon chain length. Number after the colon refers to number of double bonds. The omega number (also presented as "n" or "w" refers to the position of the first double bond from the methyl end of the carbon chain.
Essential Fatty Acids: those that must be provided in the diet because humans do not have the enzymes capable of forming the omega-3 or omega-6 double bonds. These include:
Linoleic acid(18:2, omega-6)
Arachidonic acid(20:2, omega-6)
a-Linolenic acid(18:3, omega-3)
Important fatty acids formed from essential fatty acids:
Docosatetraenoic acid (22:4 omega-6)
Eicosapentaenoic acid (EPA)(20:5 omega-3)
Docosahexaenoic acid (DHA)(22:6 omega-3)
Eicosanoids: Potent physiological mediators such as prostaglandins, leukotrienes, and thromboxanes made from long chain fatty acids.
Insensible water loss Daily water loss through the skin and respiration, so-named because a person is not aware of it. An additional smaller amount is lost in normal perspiration, the amount varying with the surrounding temperature.
Hyponatremia Abnormally low levels of sodium (Na+) in the blood; can be easily caused by excess water intake to point of water intoxication, with resulting dilution of the major electrolyte (Na+) in extracellular circulating fluids.
Adequate Intake (AI): the observed or experimentally set intake by a defined population or subgroup that appears to sustain a defined nutritional status, such as growth rate, normal circulating nutrient values, or other functional indicators of health. AI is used if sufficient scientific evidence is not available to derive an Estimated Average Requirement. The AI is not equivalent to an RDA.
Estimated Average Requirement (EAR): Daily intake value that is estimated to meet the requirement, as defined by the specified indicator of adequacy in 50 percent of individuals.
Recommended Dietary Allowance (RDA): The intake that meets the nutrient needs of almost all (97-98 Percent) individuals in a group.
Ferrous sulfate Iron fortification compound in infant formulas. Has been shown to be effective in prevention of iron deficiency anemia.
Iron absorption Degree of iron absorption, relatively small at best, depends upon the form of the iron (heme or non-heme) and its acid reduction either by accompanying food such as orange juice or by the gastric HCI secretions, from the ferric form (Fe+ + +) in foods to the ferrous form (Fe++) required for absorption.
American Academy of Pediatrics Iron Recommendations for Full Term Infants (12)
Fluorosis: Effects of excess fluoride on dental enamel. Fluorosis can be classified on a continuum from "Questionable" slight aberrations from the normal glossy translucency of enamel to "severe"- all surfaces affected with pitting and widespread brown stains.
Retinol: Chemical name for vitamin A derived from its function relating to the retina of the eye and light-dark adaptation. Daily RDA standards are stated in retinol equivalents (RE) to account for sources of the preformed vitamin and its precursor beta-carotene.
Tocopherol: Chemical name for vitamin E, so-named by early investigators because their initial work with rats indicated a reproductive function, which did not turn out later to be the case with humans, in whom it functions as a strong anti-oxidant to preserve structural membranes such as cell walls.
Cineradiographs: Fluoroscopic motion film records of internal structures and their functions.
Palate: The partition separating the nasal and oral cavities, with a hard bony front section and a soft fleshy back section.
Larynx: Structure of muscle and cartilage lined with mucous membrane, connected to top part of the trachea and to the pharynx; essential sphincter muscle guarding the entrance to the trachea and functioning secondarily as the organ of the voice.
Orofacial muscles: Adjoining muscles of the mouth and face.
Pharynx: The muscular membranous passage between the mouth and the posterior nasal passages and the larynx and esophagus.
Adenoidal pad: Normal Lymphoid tissue in the nasopharynx of children.
Epipharynx: Nasopharynx; the part of the pharynx that lies above the level of the soft palate.
Lymphoid tissue: Tissues related to the body system of Iymphatic fluids.
Pincer grasp: Later digital grasp of the older infant, usually picking up smaller objects with a precise grip between thumb and fore-finger.
Palmar grasp: Early grasp of the young infant, clasping an object in the palm and wrapping whole hand around it.
1. Dobbing J. Nutrition and the developing brain. Lancet 1973;1:48-48.
2. Smith DW, Truog W, Rogers JE, et al. Shifting linear growth during infancy: illustration of genetic factors in growth from fetal life through infancy. J.Pediatr. 1976;89:225-230.
3. Goldenberg RL, Cliver SP, Cutter GR, et al. Black-white differences in newborn anthropometric measurements. Obstet.Gynecol. 1991;78:782-788.
4. Robson JR, Larkin FA, Bursick JH, Perri KP. Growth standards for infants and children: a cross-sectional study. Pediatrics 1975;56:1014-1020.
5. Dewey KG, Heinig MJ, Nommsen LA, Peerson JM, Lonnerdal B. Growth of breast-fed and formula-fed infants from 0 to 18 months: the DARLING Study. Pediatrics 1992;89:1035-1041.
6. Hamosh M. Digestion in the newborn. Clin Perinatol 1996 Jun;23(2): 1996;23:191-209.
7. Hernell O BL. Human milk bile salt-stimulated lipase: functional and molecular aspects. J Pediatr 1994;125:S56-S61
8. Ziegler EE, Fomon SJ. Potential renal solute load of infant formulas. J Nutr. 1989;119:1785-1788.
9. Food and Nutrition Board, Subcommittee on the Tenth Edition of the RDAs. Recommended Dietary Allowances. 10th ed. Washington, DC: National Academy Press, 1989.
10. Butte NF. Energy requirements of infants. Eur.J.Clin.Nutr. 1996;50Suppl1:S24-36.
11. Fomon SJ. Nutrition of Normal Infants. St. Louis: Mosby-Year Book, Inc., 1993.
12. American Academy of Pediatrics. Pediatric Nutrition Handbook. Fourth ed. 1998.
13. Committee on Nutrition AAoP. Commentary on breastfeeding and infant formulas, including proposed standards for formulas. Pediatrics 1976;57:278-285.
14. Michaelsen KF, Jorgensen MH. Dietary fat content and energy density during infancy and childhood; the effect on energy intake and growth. Eur.J.Clin.Nutr. 1995;49:467-483.
15. Lucas A, Morley R, Cole TJ, Lister G, Leeson Payne C. Breast milk and subsequent intelligence quotient in children born preterm. Lancet 1992;339:261-264.
16. Lanting CI, Fidler V, Huisman M, Touwen BC, Boersma ER. Neurological differences between 9-year-old children fed breast- milk or formula-milk as babies 1994;344:1319-1322.
17. Fergusson DM, Horwood LJ. Early solid food diet and eczema in childhood: a 10-year longitudinal study. Pediatr.Allergy Immunol. 1994;5:44-47.
18. AnonymousLong chain polyunsaturated fatty acids in neonatal nutrition. J.Am.Coll.Nutr. 1994;13:546-548.
19. Keating JP, Schears GJ, Dodge PR. Oral water intoxication in infants. An American epidemic. Am.J.Dis.Child 1991;145:985-990.
20. Food and Nutrition Board IoM. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington. D.C.: National Academy Press, 1998.
21. Food and Nutrition Board IoM. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, D.C.: National Academy Press, 1998.
22. Abrams SA, Wen J, Stuff JE. Absorption of calcium, zinc, and iron from breast milk by five- to seven-month-old infants [published erratum appears in Pediatr Res 1997 Jun;41(6):814]. Pediatr Res. 1997;41:384-390.
23. Wheeler RE, Hall RT. Feeding of premature infant formula after hospital discharge of infants weighing less than 1800 grams at birth. J.Perinatol. 1996;16:111-116.
24. Calvo EB, Galindo AC, Aspres NB. Iron status in exclusively breast-fed infants. Pediatrics 1992;90:375-379.
25. Pisacane A, De Vizia B, Valiante A, et al. Iron status in breast-fed infants. J Pediatr 1995;127:429-431.
26. Davidsson L. Minerals and trace elements in infant nutrition. Acta Paediatr.Suppl. 1994;83:38-42.
27. Levy SM, Kiritsy MC, Warren JJ. Sources of fluoride intake in children. J.Public Health Dent. 1995;55:39-52.
28. Schuman AJ. How much fluoride is too much Contemporary Pediatrics 1995;12:65-74.
29. Hathcock JN, Hattan DG, Jenkins MY, McDonald JT, Sundaresan PR, Wilkening VL. Evaluation of vitamin A toxicity. Am.J Clin.Nutr. 1990;52:183-202.
30. Sills IN, Skuza KA, Horlick MN, Schwartz MS, Rapaport R. Vitamin D deficiency rickets. Reports of its demise are exaggerated. Clin.Pediatr (Phila.) 1994;33:491-493.
31. McMillan DD. Administration of Vitamin K to newborns: implications and recommendations. CMAJ. 1996;154:347-349.
32. Gesell A, Ilg FL. Feeding Behaviors of Infants. Philadelphia: J.B. Lippincott, 1937.
33. Guo SM, Roche AF, Fomon SJ, et al. Reference data on gains in weight and length during the first two years of life [see comments]. J.Pediatr. 1991;119:355-362.
34. Hallberg L, Rossander-Hulten L, Brune M, Gleerup A. Bioavailability in man of iron in human milk and cow's milk in relation to their calcium contents. Pediatr Res. 1992;31:524-527.
35. Hurrell RF, Davidsson L, Reddy M, Kastenmayer P, Cook JD. A comparison of iron absorption in adults and infants consuming identical infant formulas. Br.J Nutr. 1998;79:31-36.
36. Fomon SJ, Ziegler EE, Rogers RR, et al. Iron absorption from infant foods. Pediatr Res. 1989;26:250-254.
37. Barnard, K. Caregiver/Parent-Child Interaction Feeding Manual. 1994. University of Washington School of Nursing, Seattle,WA, NCAST Publications.