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There are three sources of vitamin D: natural sunlight, fortification of dietary foods, particularly dairy products and some cereals and oily fish. The radiation that converts vitamin D in the skin is the same wavelength that causes sunburn, so careful application of sunscreen can inhibit vitamin D production. At northern latitudes, there is not enough radiation to convert vitamin D, especially during the winter. After the age of 70 the skin does not convert vitamin D effectively.
| Vitamin D supplementation | |
| People with poor sunlight exposure | 400-1000 units/day |
|---|---|
| Adults older than 70 | 800-1000 units/day |
| Patients with cystic fibrosis | 800-1000 units/day |
| Patients with malabsorption | Up to 50,000 units/day, check levels |
| Patients with liver disease | May need active metabolites |
| Patients with kidney disease | Need active metabolites |
| Patients with sarcoidosis Patients with kidney stones | Be careful not to give excess Check levels, don't exceed 20 ng/mL |
Many vitamin D supplements also contain high contents of vitamin A - and recent studies show that vitamin A can increase bone resorption. The labels do not have to list the vitamin A, but it is often there, especially if it comes from cod liver oil.
Cholecalciferol is preferred because it sustains blood levels for a longer time (Armas). A recent ASBMR abstract in 2008 confirmed this (Glendenning).
| 25 (OH) D Level | ng/ml (used in USA) | nMol/L (international) |
| Deficient | less than 8 | less than 20 |
|---|---|---|
| Insufficient | 8-20 | 20-50 |
| Optimal | 20-60 | 50-150 |
| High | 60-90 | 150-225 |
| Toxic | greater than 90 | greater than 225 |
Binkley N, Low Vitamin D status despite abundant sun exposure.
J Clin Endocrinol Metab 92:2130, 2007. copyright 2007, The Endocrine Society
The above graph shows results from 93 healthy 24-year-old Hawaiian surfers. The authors of this paper concluded that: "the goal of vitamin D replacement therapy should be no greater than the maximum that appears attainable, a serum 25(OH)D concentration of approximately 60 ng/ml." However, many laboratories currently have listed their normal range as 32-150 ng/ml, even though the only way to acheive a value above 60 is to take a drug!
There is disagreement about whether the lower limit of the optimal vitamin D levels should be 20 or 30 ng/ml, but none of the experts in the field still think that levels lower than 20 ng/ml are desirable. This is really different from ten years ago, when levels above 15 ng/dl were considered sufficient. A new paper by Melamed ML followed 13,331 adults in the US for 8.7 years to see if the vitamin D level was related to mortality. Notice that the mortality increases with either high or low levels of vitamin D (just like some of the other steroid hormones).
copyright American Medical Association, used with permission
Another recent study (Dobnig H) from Germany found lower mortality in people in the top 25% of vitamin D levels, compared to those with the lower 25%. The overall levels, however, were quite low; almost everybody was lower than 33 ng/ml. A meta-analysis of clinical trials of vitamin D (average dose was about 500 units/day) found an overall reduction in mortality of 7% (Autier P).
Vitamin D probably protects against colon cancer (Wei MY, Giovannucci E). Early studies suggested the men with low vitamin D levels were more likely to get prostate cancer, but newer, larger studies do not find this (Freedman DM, Ahn J, Freedman DM).
1,25(OH)2 vitamin D is more difficult and expensive to measure than 25(OH)D; moreover, it is not a good measure of vitamin D status. When patients are vitamin D deficient, the parathyroid hormone increases and drives the renal 1-alpha-hydroxylase, so that 1,25(OH)2 vitamin D levels increase. Only in severe deficiency, when substrate is depleted, does the 1,25(OH)2 vitamin D become low. Partially treated vitamin D deficiency also results in marked elevations of 1,25(OH)2 vitamin D levels.
Some doctors, thinking they are sophisticated because they know that 1,25(OH)2 vitamin D is more active, order the wrong measurement. Do not fall into this trap and waste money on this expensive but often misleading test! There are only a few situations where you would actually want to know the 1,25(OH)2 vitamin D levels: unexplained hypercalcemia (looking for granulomatous disease or lymphoma), suspected genetic childhood rickets, suspected tumor-induced osteomalacia, some cases of nephrolithiasis or hypercalciuria. Patients with stages 4-5 chronic kidney disease have decreased 1,25(OH)2 vitamin D levels but even in those patients the 25(OH) vitamin D is a better test of the stores of vitamin D, and the PTH is a better indicator of mineral abnormalities.
Finally, new studies suggest that some cells generate intracellular 1,25(OH)2 vitamin D to fight tuberculosis, suppress cancer growth, or modify immune response. They need adequate substrate, as measured by 25(OH) vitamin D.
Vitamin D levels are associated with fractures, as shown in the following two graphs:
Data from Looker AC, using only postmenopausal Caucasian women.
Data from the Women's Health Initiative, comparing 400 women who had experienced a hip fracture to matched controls. (Cauley JA)
This table shows overall results of studies of vitamin D and fracture rates. Most of the studies did not find a signficant benefit of vitamin D.
Click on the table to see an Excel file with more details.
Recently there have been several meta-analyses of this topic, which show different points of view.
After reading these studies and analyses, I conclude that vitamin D therapy can reduce fractures in frail elderly patients, especially when they have vitamin D insuffiency, and that adequate calcium intake is also needed. Otherwise, the difference in fracture rates is really quite modest in the vitamin D clinical trials.
Updated 3/26/09
More about vitamin D.