Recently there have been several reports that have re-analyzed previous studies and come to somewhat contradictory recommendations about calcium and vitamin D. One is from the the U.S. Preventive Services Task Force and another from the American College of Physicians Journal Club, both based on an analysis by Chung. The study found only a modest benefit on fractures with combinations of calcium and vitamin D. The USPSTF draft recommendation is mostly about vitamin D, and says that low doses of vitamin D with or with our supplements does not prevent cancer or fractures. However, they still recommend it to prevent falls. Meanwhile the Institute of Medicine recommends that women should take 1200mg of calcium a day (total amount including food) for best bone health.
I have read these various reports and think that for now the best advice is to try to get at least 500mg of calcium from the diet in the form of dairy products such as yogurt, cheese, and milk. If possible, 3 servings of dairy and some green calcium-containing vegetables will provide all the calcium that is needed. If that is too difficult, then 500mg of supplement should be safe and is probably better than an inadequate intake.
It is important to remember that these new reports are largely still based on the Women's Health Initiative, which did not show a benefit for fracture prevention. However, the women were already taking an average of 1100mg of calcium a day, so an extra 1,000 mg did not help. The bones need an adequate amount of calcium, but once that is achieved, extra will not go into the bones. A new report by Prentice examined only the women who were not taking extra calcium, and after 5 years they did show fewer hip fractures than those with placebo.
For adult men and women a total elemental calcium intake of 1,000 to 1,200 mg/day is the best recommendation with the current evidence. This includes both dietary calcium and calcium supplements. The total daily intake should not exceed 2,000 mg. Calcium alone can't prevent or treat osteoporosis, but an adequate calcium intake is needed for optimal bone health.
Nutrition LabelsFood labels show the amount of calcium per serving. These are based on the RDA of 1000mg/day. Add a zero to the %RDA for calcium to calculate the mg of calcium. For example, this label from a yogurt container shows that the calcium is 25% of the RDA, which is 250mg.
The U.S. Department of Agriculture has a very detailed list of foods containing calcium, ranked in order of calcium content, that you can download. Just be aware that some sources do not have bioavailable calcium.
You can also check calcium content of any food by searching the UDSA Nutrient Data Laboratory.
Here is a simple list you can print: Calcium content or with illustrations.
This figure shows amount of ELEMENTAL calcium per tablet. Sometimes the labels are confusing; they either give the calcium per serving instead of per tablet, or they give the amount of calcium carbonate instead of elemental calcium. For example, the label on Citracal caplets says each serving contains 630 mg of calcium, but a serving is 2 tablets so each one has 315 mg of calcium. TUMS labels are also confusing:
|TUMS (per tablet)||Calcium |
|Ultra (maximum strength)||1000||400 mg|
|TUMS 500||1200||500 mg|
Calcium carbonate is the most cost-effective source of calcium. Contrary to some advertisements, the intestinal absorption of calcium citrate and calcium carbonate is the same except when there is no acid in the stomach (Heaney).
Not all calcium preparations are absorbed equally, and one of the reasons is that they have different rates of dissolution. It is less expensive to manufacture calcium carbonate in a compact form that will not readily dissolve. The dissolution of 27 brands of calcium varies from 33% to 75%. Those brands in the group with the best dissolution are: Calcium (Giant Food), Calcium Concentrate 600 (Hudson ), Natural Calcium (Giant Food), and Os-Cal (Marion). (Brennan). Chewable tablets are a safe bet.
I was skeptical about the importance of dissolution until one of my patients told me that she had forgotten to take her calcium tablets, which she purchased from a health-foods store. She put them in the pocket of her apron, and discovered them there after the apron had been through the cycles on the washer and the dryer!
|Another patient used a calcium supplement from a health food store, and, when she came in to have her bone density measured, densities were seen near the spine. These changed position after a couple of hours, and were not seen on a scan several days later (after she switched to a chewable form of calcium).|
Side effects from a reasonable dose of calcium (1,000 mg/day) are very low. Some patients find that calcium makes them constipated. In some blinded trials this complication is no more frequent than with placebo. A study by Prince found that higher doses of calcium supplements (1200mg/day in addition to 900mg/day from dietary sources) did cause constipation in 13% of subjects. To help prevent constipation, don't take more calcium than necessary, increase intake of fruit juices and water, try getting calcium from food sources instead of tablets, take calcium with magnesium, or try calcium citrate or calcium chews. Gastritis is occasionally seen, which might be caused by taking calcium carbonate between meals, thus stimulating rebound acid production.
Very high intakes of calcium could lead to alkalosis (the "milk-alkali syndrome").
Calcium supplements may increase the risk of kidney stones, if the dose is too high (over 2,000mg/day). On the other hand, a large survey in 45,619 men showed that those with the lowest dietary calcium had the most kidney stones. This is because the most common kind of kidney stone is caused by oxalate, and calcium inhibits the oxalate absorption from the intestines. A similar large observational study in women also found that there were more kidney stones in women with low calcium intakes (Sorensen). Old-fashioned advice about avoiding calcium increases the risk of both osteoporosis and another kidney stone! Decreasing salt and protein may also help decrease the incidence of calcium-oxalate kidney stones. Patients with hypercalciuria and low bone density can be treated with low doses (12.5 to 25mg/day) of thiazide, using potassium bicarbonate as necessary to keep serum potassium normal.
A study by Bolland suggested that calcium supplements could increase the risk of heart attacks. There were 1,400 women in this clinical trial, followed for 5 years. This side effect was not seen in a much larger study by Jackson and investigators for the Women's Health Initiative, which enrolled 36,282 women and studied them for 7 years. Also, a subset of patients received cardiac CT scans to look for calcifications in the coronary arteries, and there was no increase in the group who had received calcium (Manson). There are also some studies suggesting increased calcifications in the blood vessels with too much supplementation, but a recent review by Spence did not find a consensus that this was true. A meta-analysis, also by Bolland, did find a mild increased risk of myocardial infarction in those taking calcium supplements. This included 15 clinical trials of calcium (but not with vitamin D). Of note, there was no risk in the patients whose total calcium intake was lower than 1400 mg/day. I have been recommending a total intake of 1200mg/day for fracture prevention, based on my review of studies about osteoporosis, so this paper (even if it is true) does not change my recommendations.
Dietary protein and caffeine can increase urine loss of calcium. These effects can be measured and are statistically significant, but becomes clinically significant only at large intakes. In fact, a latte will result in positive calcium balance! Calcium absorption is inhibited to a modest degree by fiber in the diet. Although the calcium content in spinach is high, almost none gets absorbed.
Vitamin D must be adequate for optimal absorption of calcium. The vitamin D has a very long half-life and does not need to be taken at the same time as the calcium.
Except in severe cases of hypomagnesemia (such as seen with some medications or alcohol abuse), magnesium is not required for absorption of calcium. If magnesium is very low, the serum calcium will also be low, and the PTH can be suppressed. The optimal doses of magnesium have not been well defined.
More information about nutrition and osteoporosis is in the nutrition pages.
A meta-analysis by Tang found 29 randomized trials of calcium, with or without vitamin D. There was an overall 12% reduction in the risk of osteoporotic fractures with calcium. Addition of vitamin D did not make very much difference. The benefit was greatest in the people who had lowest calcium intake, were elderly, and had high compliance rates.
The Women's Health Initiative study (Jackson) did not find that calcium supplementation helped prevent fractures, but there was an increased risk of kidney stones. The average calcium intake at the baseline was 1148mg, which was already at the recommended level. The women took an extra supplement of 1,000 mg/day with 400 units of vitamin D. That means those in the calcium group were taking over 2,000 mg/day of calcium. Perhaps that is why the calcium did not seem to help. We need enough calcium, but extra calcium does not help the bones, it just stresses the kidneys. The women who were not already taking calcium supplements did show a reduction in the rate of hip fractures.
A study of 76,507 postmenopausal women from the NORA study (Nieves) estimated calcium intake from a questionnaire and asked about fractures 3 years later. The calcium intake was not related to the risk of fractures. Those with life-long highest calcium intakes, however, did have higher bone density. The average calcium in the diet was 600mg/day and the average total calcium intake (food plus supplement) was 900mg/day.
There have been many older randomized trials (Excel file) of calcium supplementation, but they have not been large enough to determine effect on fractures, and effects on bone density were modest. Several studies have randomly assigned menopausal women to placebo, calcium, or estrogen; those on estrogen did not lose bone, those on calcium lost some bone, and those on placebo lost the most. More contemporary studies have also given vitamin D and are listed in the next page.
The major mechanism whereby calcium effects bone is probably through inhibition of PTH secretion. Calcium could alter the physical-chemical properties of the bone mineral. With inadequate calcium, the bone is not optimally mineralized. Direct effects of calcium on the calcium receptor could also play a role.