The skinny on Vitamin D
Vitamin D is a fat-soluble vitamin that has recently garnered much media attention. The difficulty with Vitamin D is that it is naturally present in very few foods. It is made in our bodies when our skin is exposed to ultraviolet rays from the sun. This Vitamin D is initially biologically inactive. It then gets hydroxylated by the liver to form 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. 25(OH)D gets hydroxylated in the kidney to form the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.
Vitamin D has a multitude of functions in the human body. It promotes calcium absorption in the gut, help with bone growth and remodeling and reduces inflammation. Current research shows that Vitamin D is involved in the regulation of over 200 genes responsible for cellular growth, differentiation, cell death and blood vessel generation. The cell death (apoptosis) genes are especially important when it comes to considering the anti-cancer effects of Vitamin D.
The best marker to check for Vitamin D status is 25(OH)D, which has a half-life of 15 days. This is done via a blood test by your doctor. The optimal level according to the National Institutes of Health Office of Dietary Supplements is ? 30ng/ml. Unlike people with Kidney disease, we don’t check 1,25(OH)2D in the normal population. 1,25(OH)2D has a short half-life of about 15 hours and is closely regulated by the parathyroid hormone, blood calcium and phosphate levels.
Inadequate intake of Vitamin D is a leading contributor to osteoporosis, which affects 10-12 million Americans. The people at most risk for Vitamin D deficiency include: elderly people; people with dark skin tones, who may require as much as six times the UV light to produce Vitamin D; obese patients, whose fat tissue sequesters the Vitamin D; and breastfed babies who receive small amounts of vitamin D from breast milk.
The current U.S recommended daily allowance for vitamin D is as follows:
- Birth-age 50: 200 IU/day (update: new guidelines coming out soon recommend 400 IU/day)
- Age 51-70: 400 IU/day
- Age 71 and up: 600 IU/day
It is interesting to note that the Canadian Cancer Society and many endocrinologists recommend a daily intake of at least 1000 IU/day. This is difficult to obtain through diet alone. Here is a nice little table from the NIH Health Office of Dietary Supplements website listing food sources of Vitamin D
| Food | IUs per serving* | Percent DV** |
|---|---|---|
| Cod liver oil, 1 tablespoon | 1,360 | 340 |
| Salmon, cooked, 3.5 ounces | 360 | 90 |
| Mackerel, cooked, 3.5 ounces | 345 | 90 |
| Tuna fish, canned in oil, 3 ounces | 200 | 50 |
| Sardines, canned in oil, drained, 1.75 ounces | 250 | 70 |
| Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup | 98 | 25 |
| Margarine, fortified, 1 tablespoon | 60 | 15 |
| Ready-to-eat cereal, fortified with 10% of the DV for vitamin D, 0.75-1 cup (more heavily fortified cereals might provide more of the DV) | 40 | 10 |
| Egg, 1 whole (vitamin D is found in yolk) | 20 | 6 |
| Liver, beef, cooked, 3.5 ounces | 15 | 4 |
| Cheese, Swiss, 1 ounce | 12 | 4 |
Finally, a question frequently asked is whether supplementing with Vitamin D3 is better than Vitamin D2. This is an extremely controversial question and as of yet we don’t have a definitive answer. Some studies show Vitamin D3 to be more than three times as effective as vitamin D2 in raising serum 25(OH)D concentrations and maintaining those levels for a longer time. However, new data shows Vitamin D2 to be equivalent to Vitamin D3 in maintaining circulating levels. The bottom line is you should aim to incorporate more Vitamin D rich foods in your diet, allow for moderate sun exposure as recommended by the American Academy of Dermatology, and discuss with your doctor if you should supplement with Vitamin D.
References:
- DeLuca HF, Zierold C. Mechanisms and functions of vitamin D. Nutr Rev 1998;56:S4-10.
- Cranney C, Horsely T, O’Donnell S, Weiler H, Ooi D, Atkinson S, et al. Effectiveness and safety of vitamin D. Evidence Report/Technology Assessment No. 158 prepared by the University of Ottawa Evidence-based Practice Center under Contract No. 290-02.0021. AHRQ Publication No. 07-E013. Rockville, MD: Agency for Healthcare Research and Quality, 2007.
- Armas LAG, Hollis BW, Heaney RP. Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab 2004;89:5387-91.
- Houghton LA, Vieth R. The case against ergocalciferol (vitamin D2) as a vitamin supplement. Am J Clin Nutr 2006;84:694-7.
- Holick MF. Evolution and function of vitamin D. Recent results. Cancer Res 2003;164:3-28.
- Armas LAG, Hollis BW, Heaney RP. Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab 2004;89:5387-91. • Holick et al. Vitamin D2 Is as Effective as Vitamin D3 in Maintaining Circulating Concentrations. J Clin Endocrinol Metab.2008; 93: 677-681.
