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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).
| 25 (OH) D Level | ng/ml | nMol/L |
| 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 |
The definitions of normal and optimal are still debated. Heaney suggests there are two ways to define the optimal level: that level at which calcium absorption does not change further on giving extra vitamin D, and that level which will avoid increases in parathyroid hormone. Both approaches revealed serum 25(OH)D levels of 32 ng/ml.
Low vitamin D levels are associated with lower bone density and higher risk of fractures. But high levels should also be avoided, as shown in these two studies using data from the NHANES survey:
This is drawing of a graph by Bischoff-Ferrari. The publisher would not allow permission to use the actual graph without a fee, so I drew this rough sketch which I hope will satisfy copyright laws but also give an idea of the relationship between bone density and vitamin D levels
Data from Looker AC, using only postmenopausal Caucasian women.
Similar findings were seen in the Women's Health Initiative:
Data presented at the 2007 ASBMR meetings by Cauley, from the Women's Health Initiative, comparing 400 women who had experienced a hip fracture to matched controls.
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.
This table shows overall results of studies of vitamin D on bone density or fracture rates. Most of the data suggest no signficant benefit of vitamin D, with a few studies suggesting more fractures in the vitamin D groups.
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 with initial low levels of vitamin D, 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 2/27/08
More about vitamin D.