Did you know that there are 2 types of vitamin D? Vitamin D-2 is a synthetic form of the vitamin, and it is made from irradiated yeast. It is used to fortify foods and in many vitamin preparations. Vitamin D-3 is made by human skin, which can easily produce all the vitamin D that the body requires. During the late spring and summer, in temperate regions, human skin can produce 10,000 IU of vitamin D-3 per day if the whole body is exposed to the sun for much of the day (1).
The current “adequate intake” for vitamin D is 400 to 800 IU daily, depending on age. This is far less than the 10,000 IU per day that human skin can synthesize. Indeed, 10,000 IU of vitamin D is well in excess of the Food and Nutrition Board’s guidelines, which specify that healthy adult should not consume more than 2000 IU daily or they may be risking vitamin D toxicity (2). There is no evidence that people ever develop vitamin D toxicity and/or hypercalcemia as a result of exposure to large amounts of tropical sunlight for prolonged periods of time. Therefore, it seems likely that amounts of vitamin D-3 well in excess of what is currently considered the upper limit of safety are perfectly natural for the human body. Indeed, there is growing evidence that the recommended “adequate intake” for vitamin D is likely inadequate for many people.
Harm from lack of sun
More and more Americans are avoiding even tiny amounts of sunshine on their skin for fear of getting wrinkles, age spots, and skin cancer. Sunscreens are spread over exposed areas of the body to block the dreaded UV rays. However, it is these same UV rays that enable skin cells to synthesize D-3 from cholesterol. In addition to this sun phobia, many older Americans increasingly stay indoors out of fear for their safety on the streets and/or increasing disability. Without regular sun exposure, the human body becomes dependent on dietary sources of vitamin D. Vitamin D is found naturally in significant amounts in few foods. Fatty fish are the best source and modest amounts are also present in liver and eggs. Cow’s milk is fortified with vitamin D-2, and, for most Americans, this represents the major dietary source of vitamin D. However, D-2 appears to be of less value in making the biologically active form of vitamin D. Indeed, it may be that synthetic vitamin D-2 and/or contaminants associated with it may be more toxic to the body than the naturally produced vitamin D-3 (3).
A study of 290 consecutive hospitalized patients found that 57% were deficient in vitamin D. Not surprisingly, two thirds of those with intakes below recommended levels were deficient. However, 37% of those patients with intakes above the recommended amount were also shown to be deficient because their blood levels of a metabolite vitamin D were low (4). In the absence of sunlight, it appears that at least 1000 IU of vitamin D is needed to maintain adequate levels in the blood (5).
Cancer and the sun
Many more Americans die of breast, colon, prostate, and ovarian cancers than all skin cancers... combined. Because these cancers are more common in people who live farther away from the equator (and presumably get less sunshine vitamin D) there is a growing suspicion that a lack of vitamin D may significantly increase the risk of developing all these cancers (6, 7, 8). Cells in these organs convert vitamin D to a hormone that is needed to regulate cell proliferation (9). Cancer is unregulated cell proliferation.
A lack of sunshine may lead cells in the skin to proliferate more rapidly. Psoriasis is a hyperproliferative skin disease and there is some evidence that it may be due in part to a lack of UV light (10).
A lack of vitamin D leads to pathological increases in parathyroid hormone, increased bone loss, osteomalacia (weak bones) and osteoporosis, which increase the risk of hip fractures (11, 12). Supplements of vitamin D in the 700-800 IU/day range have been shown to result in fewer fractures than a placebo (13, 14).
Inadequate levels of vitamin D can cause hormonal and other changes in the body, which can lead to muscle weakness, pain, and fatigue (15). In some cases, people with inadequate levels of vitamin D may be incorrectly diagnosed with fibromyalgia, chronic fatigue syndrome or depression (16).
The bottom line:
Natural vitamin D-3 produced by human skin exposed to natural sunlight is the best source of vitamin D. D-2, derived from irradiated yeast, appears to be more toxic and less useful to the body than natural D-3. Foods fortified with vitamin D usually contain vitamin D-2. Consuming more fatty fish and spending more time outdoors in the sun should help reduce the risk of at least several common cancers, as well as osteomalacia and osteoporosis. It may also help relieve psoriasis, muscle pain and weakness. For those unable to get sufficient sunshine, a supplement of 1000 IU daily of vitamin D-3 should supply a safe, adequate amount for nearly all adults.
1. Vieth R. Am J Clin Nutr 1999;69:842-56
2. Vieth R, et all. Am J Clin Nutr 2001;73:288-94
3. DeLuca HF. Federation Proc 1974;332211-9
4. Thomas MK, Lloyd-Jones DM, Thadhani RI, et al. N Engl J Med 1998;338:777-83
5. Glerup H, et al. Calcif Tissue Int 2000;66419-24
6. Garland CF, et all. Am J Clin Nutr 1991;54(suppl):193S-201S
7. Hanchette CI, Schwartz GG. Cancer 1992;70:2861-9
8. Ahonen MH, Tenkanen L, Teppo L, et al. Cancer Causes Control 2000; 11:847-52
9. Holick MF. Lancet 2001;357:4-6
10. Holick MF. Am J Clin Nutr1994; 60:619-30
11. Eastell R, Riggs BL. Vitamin D. Academic Press. San Diego, CA1997:695-711
12. Chapuy M-C, Meunier PJ. Vitamin D. Academic Press. San Diego, CA 1997:679-93
13. Dawson-Hughes B, et all. N Engl J Med 1997;337:670-6
14. Chapuy MC, et al. N Engl J Med 1992;327:1637-42
15. Gloth FM III, et al. JAMA 1995;274:1683-6
16. Prabhala A, Garg R, Arch Intern Med 2000;160:1199-1203
By Dr. James J. Kenney, PhD, RD, FACN.
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