Importance of Vitamin D

By Joel Fuhrman, M.D.

This “Sunshine” Vitamin Is Vital For Maintaining Strong Bones!

Vitamin D is a fat-soluble vitamin that your body makes after exposure to ultraviolet (UV) rays from the sun. Vitamin D functions as a hormone because it sends a message to the intestines to increase the absorption of calcium and phosphorus. By promoting calcium absorption, vitamin D helps to form and maintain strong bones.

Vitamin D also works in concert with a number of other vitamins, minerals, and hormones to promote bone mineralization. Research also suggests that vitamin D is important to maintain a healthy immune system, regulate cell growth, and prevent cancer. Vitamin D has been shown to protect against the development of autoimmune disease such as inflammatory bowel disease, multiple sclerosis, and rheumatoid arthritis. It also has been shown to be helpful in decreasing disease severity for those suffering with autoimmune disease.1

Sun exposure is perhaps the most important source of vitamin D because exposure to sunlight provides most humans with their vitamin D requirement. The further you live from the equator, the longer you need to be exposed to the sun in order to generate vitamin D. Season, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis. For example, sunlight exposure from November through February in Boston is insufficient to produce significant vitamin D synthesis in the skin. Recent data have demonstrated that getting sunshine during the summer months is simply not enough; besides, most adults work indoors for the majority of the day, avoiding the sun. Sunscreens block UV rays that produce vitamin D, so they could contribute to our epidemic of vitamin D deficiency.

It is extremely important for individuals with limited sun exposure to ingest supplemental vitamin D. I still recommend that you routinely use sunscreen to help prevent skin cancer, wrinkling, and aging of the skin, especially because the ozone layer has been depleted. The risk of skin damage and skin cancer is real.

Americans age 50 and older are at increased risk of developing vitamin D deficiency. As people age, skin cannot synthesize vitamin D as efficiently, and the kidney is less able to convert vitamin D to its active hormone form. It is estimated that as many as 30-40 percent of older adults with hip fractures are vitamin D insufficient.2 Therefore, older adults especially benefit from supplemental vitamin D.

Melanin is the pigment that gives skin its color. The high melanin content in darker skin reduces the skin’s ability to produce vitamin D from sunlight. It is very important for African-Americans and other populations with dark-pigmented skin to consume recommended amounts of vitamin D. Some studies

suggest that older adults in these groups, especially women, are at extremely high risk of vitamin D deficiency. It is thought that the main reason prostate cancer is so prevalent in black men is because of increased need for vitamin D.

Vitamin D And Osteoporosis

It is estimated that over 25 million adults in the United States have, or are at risk of developing, osteoporosis. Adequate storage levels of vitamin D help keep bones strong and help prevent osteoporosis in older adults. Vitamin D deficiency results in diminished calcium absorption, and has been linked to a higher incidence of osteoporosis- related bone fractures seen in post-menopausal women and older Americans.

In a review of women with osteoporosis hospitalized for hip fractures, 50 percent were found to have vitamin D deficiency.3 Daily supplementation with 20g (800 IU) of vitamin D has been shown to reduce the risk of osteoporotic fractures in elderly populations with low blood levels of vitamin D.4 The Decalyos II study examined the effect of combined calcium and vitamin D supplementation in a group of elderly women who were able to walk indoors with a cane or walker. The women were studied for two years, and results showed that supplementation significantly reduced the risk of hip fractures, and that vitamin D was more effective than calcium.5 Clearly, any woman being counseled or treated for osteoporosis should have her vitamin D status checked with a blood test and appropriately supplemented to assure a normal level.

Vitamin D And Cancer

Laboratory, animal, and epidemiologic evidence suggests that vitamin D may be protective against cancer. Epidemiologic studies suggest that a higher dietary intake of calcium and vitamin D, and/or sunlight-induced vitamin D synthesis, correlates with lower incidence of cancer, including lymphoma, breast, prostate, and colon cancer.6 In fact, for over 60 years, researchers have observed an inverse association between sun exposure and cancer mortality,7 and those with more sun exposure had fewer cancers. The inverse relationship between higher vitamin D levels in blood and lower cancer risk in humans shows a significantly lower risk among those with the highest vitamin D intake.

In addition to its significant cancer- protective effects, recent studies demonstrate that vitamin D also can inhibit the growth of existing breast and prostate cancer cells. Likewise, it helps inhibit the progression and metastasis of a wide spectrum of cancers, suggesting therapeutic value in the treatment of those who already have cancer.8

Interestingly, one dermatologist, Dr. Michael Holick, even wrote a book, The UV Advantage, advocating a moderate dose of sunlight. He was promptly kicked off the faculty of the Boston University School of Dermatology. Dr. Boni E. Elewski, president of the American Academy of Dermatology, maintained that even a few minutes of sun can be dangerous and promote skin cancer. Dr. Holick’s critics pointed out that the Indoor Tanning Association contributed $150,000 to his research.

In Dr. Holick’s defense, we must consider the keynote address that was presented at the meeting of the American Association for Cancer Research, where Dr. Edward Giovannucci, professor of medicine and nutrition at Harvard, said that his research suggests that deaths from cancer in cases where vitamin D would have been of benefit outnumber skin cancer deaths 30 to one.“I would challenge anyone to find a nutrient or any factor that has such considerable anti-cancer benefits as vitamin D,” he said.

Avoiding sun damage or not should not be the point here, because even a little bit of sun, as Dr. Holick suggests, is not really necessarily going to guarantee you an ideal blood level of vitamin D. Most of us simply need to supplement with an appropriate dose of vitamin D. The RDA of 400 IU may be an appropriate dose to obtain a normal blood level in many people, but there is still a significant portion of people who need more. To be safe, you should take more vitamin D or measure the vitamin D level in the blood to assure you are taking enough. While common sense might lead you to think that the vitamin D your body produces from sunshine is superior to the vitamin D from supplements, the documented beneficial effects of both sources are the same, and with supplements you don’t risk damaging your skin.

Measuring Vitamin D Levels In Your Blood

Assessment of vitamin D status is usually made by measuring 25-hydroxy- vitamin D; however, the optimal serum concentration is somewhat controversial. The data sheets from most blood laboratories list 20 ng/ml as the cutoff for vitamin D deficiency. However, recent studies have demonstrated that parathyroid hormone (PTH) levels begin to rise as 25-hydroxy-vitamin D falls below 30, and recently there has been a growing consensus that 30 ng/ml should be used as a cutoff for the diagnosis of vitamin D inadequacy. As pointed out by T. Colin Campbell, Ph.D., in The China Study, it may be true that vegetarians and those eating little animal protein have a more efficient conversion of the 25-hydroxy-vitamin D into its biologically active form, 1,25-dihydroxy, and perhaps a blood level below 30 is not so bad in a vegan or vegetarian. I do not think there is enough evidence to take a chance with running a D level below 20, unless your level of 1,25-dihydroxyvitamin D comes out on the blood test above 40. Otherwise, I think a blood level of at least 25 ng/ml is still advisable in those who consume little or no animal products.

Recently, a large study assessed the vitamin D status of postmenopausal women receiving therapy to treat or prevent osteoporosis. Amazingly, they found that 52 percent of the 1,536 women had inadequate vitamin D levels—and these were women being treated with drugs for osteoporosis. We know that vitamin D levels are inadequate in the vast majority of American women, and that this deficiency is a major cause of disability and death.

What Are The Health risks of too much vitamin D?

Vitamin D toxicity can cause nausea, poor appetite, constipation, weakness, confusion, and weight loss. Sun exposure does not result in vitamin D toxicity. Vitamin D toxicity is only a possibility from high intakes of vitamin D from supplements. The Food and Nutrition Board of the Institute of Medicine has set the recommended upper intake level to 50g (2,000 IU) for children, adults, and pregnant and lactating women. Vitamin D is one of those vitamins where the right amount is essential—not too much and not too little.


1. Cantorna MT, Zhu Y, Froicu M, Wittke A. Vitamin D status, 1,25-dihydroxy-vitamin D3, and the immune system. Am J Clin Nutr 2004 Dec;80(6 Suppl):1717S-20S. Hein G, Oelzner P. Vitamin D metabolites in rheumatoid arthritis: findings— hypotheses-consequences. Z Rheumatol 2000;59 Suppl 1:28-32.

2. Holick MF. McCollum Award Lecture, 1994: Vitamin D: new horizons for the 21st century. Am J Clin Nutr 1994;60:619-30.

3. LeBoff MS, Kohlmeier L, Hurwitz S, Franklin J, Wright J, Glowacki J. Occult vitamin D deficiency in postmenopausal US women with acute hip fracture. J Am Med Assoc 1999;251:1505-11.

4. Reid IR. Therapy of osteoporosis: Calcium, vitamin D, and exercise. Am J Med Sci 1996;312:278-86.

5. Chapuy MC, Pamphile R, Paris E, Kempf C, Schlichting M, Arnaud S, Garnere P, Meunier PJ. Combined calcium and vitamin D3 supplementation in elderly women: confirmation of reversal of secondary hyperparathyroidism and hip fracture risk: the Decalyos II study. Osteoporosis Int 2002;13:257-64.

6. Martinez ME, Willett WC. Calcium, vitamin D, and colorectal cancer: a review of the epidemiologic evidence. Cancer Epidemiol Biomark Prev 1998;7:163-68. Lieberman DA, Prindiville S, Weiss DG, Willett W. Risk factors for advanced colonic neoplasia and hyperplastic polyps in asymptomatic individuals. J Am Med Assoc 2003; 290:2959-67.

7. Heaney RP. Long-latency deficiency disease: insights from calcium and vitamin D. Am J Clin Nutr 2003;78:912-9.

8. O’Kelly J, Koeffer HP. Vitamin D analogs and breast cancer. Recent Results Cancer Res 2003;164:333-48. Giovannucci E. The epidemiology of vitamin D and cancer incidence and mortality: a review. Cancer Causes Control 2005; 16(2):83-95.

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