Vitamin D deficiency is one of the most commonly undiagnosed medical conditions in the world.1 In fact, more than 60% of the global population is estimated to have insufficient vitamin D levels. The primary consequences of vitamin D deficiency include increased risk of hypertension, diabetes and diabetic retinopathy, age-related macular degeneration and various forms of cancer.2
The ABCs of Vitamin D
Vitamin D is a fat-soluble steroid hormone that facilitates calcium absorption in the gut. It also helps the body maintain adequate serum calcium and phosphate concentrations, which enables normal bone mineralization and prevents hypocalcemic tetany. Finally, vitamin D is needed for bone growth and bone remodeling.3 In fact, results from several clinical trials suggest that vitamin D supplementation can slow bone density loss or decrease the risk of osteoporotic fracture in men and women.4-7 Vitamin D has also been shown to protect against certain autoimmune diseases, such as multiple sclerosis and type I diabetes, as well as both colorectal and breast cancer.8,9 Because it reduces inflammation and blood pressure, vitamin D supplementation also has a protective effect against cardiovascular disease.10,11 However, current recommendations for vitamin D intake might be inadequate to prevent cardiovascular disease.
Vitamin D Sources
Skin exposure to ultraviolet-B (UV-B) radiation allows the body to synthesize vitamin D3. UV-B radiation converts a biochemical in the skin to vitamin D, which is then metabolized to calcitriol, the active form of vitamin D. Formation of vitamin D by UV-B can be six times more efficient in light-skinned individuals than in dark-skinned individuals, which likely accounts for widespread vitamin D deficiency among African Americans who live in northern latitudes.12
In addition to sun exposure, vitamin D can also be obtained from supplements and certain foods, such as fish oil and fortified milk. In fact, a recent quantitative meta-analysis concluded that a mean daily vitamin D supplement of 528 IU can reduce an individual’s risk of mortality by 7% to 8%.13
Keep in mind, however, that adequate dietary intake of vitamin D alone is significantly more challenging than simply going outside on a sunny day.
In a recent AARP Magazine article, Michael F. Holick, Ph.D., M.D., of the Vitamin D, Skin and Bone Research Laboratory of Boston University Medical Center said, “To get the vitamin D value of 10 minutes’ exposure to sunlight, you’d have to eat 6.5 pounds of shiitake mushrooms, or 150 egg yolks, or 3.75 pounds of fresh farmed salmon, or 30 servings of fortified cereal, or 2.17 pounds of sardines, or 30 cups of fortified orange juice in one day.”14 Do the calorie count on that diet regimen—you will quickly realize that relying solely on food sources of vitamin D may actually further worsen your health.
Vitamin D Deficiency
Vitamin D deficiency can be caused by a host of systemic complications, such as gastrointestinal disorders associated with fat malabsorption, obesity, a history of bariatric surgery, ill-advised dieting and poor dietary habits, as well as decreased cutaneous production of vitamin D with advanced age. Also, several socioeconomic concerns, including increased urbanization, decreased outdoor activity, the use of sunscreen, air pollution and global dimming, and wearing clothing that covers the majority of the skin can lead to vitamin D deficiency.
One prospective cohort study showed that patients with low levels of vitamin D are at an increased risk for cardiovascular mortality compared to patients with higher vitamin D levels.15-17 Another recent study found that 40.7% of patients who were deficient in 25-hydroxyvitamin D [serum 25(OH)D], a biomarker for an individual’s overall vitamin D stores, suffered from chronic migraine.18 The study also showed that the longer individuals had chronic migraine, the more likely they were to be vitamin D deficient.
Vitamin D and Ocular Disease
Data from several recent studies suggest that increased intake of vitamin D3 is beneficial to ocular health.19,20 One study indicated that overall levels of serum vitamin D were inversely associated with early AMD.19 Accordingly, this study also showed that increased milk intake was inversely associated with early AMD. Strangely, however, patients who regularly consumed milk and took a daily vitamin D supplement had no reduced risk for early AMD. However, this study did not show any association between increased vitamin D intake and a lower risk for neovascular AMD.19 Interestingly, results from a different study suggested that higher levels of bone mineral density may be associated with a lower risk for AMD.20 But, the underlying causal mechanism is still unknown.
Recommendations
Most physicians consider a serum 25(OH)D level greater than 32ng/mL (80nmol/L) to be ideal. Serum 25(OH)D can be expected to rise by about 1ng/mL (2.5nmol/L) for every 100 IU of additional vitamin D3 ingested each day.2
But, for optimum preservation of bone health, patients should consume approximately 1,000mg to 1,200mg of dietary calcium per day. Because vitamin D3 is known to be at least three times more potent than vitamin D2, adults should take at least 400 IU of vitamin D3 per day.21,22 Additionally, for optimum vitamin D utilization, most adults should receive 10 to 15 minutes of sun exposure at least three times a week, as close to noon as possible. If regular sunlight exposure is unattainable, adults should take 800 IU of vitamin D3.
Be aware, however, that toxicity-hypercalcemia (too much calcium) can cause bone loss, kidney stones, and calcification of the heart and kidneys. Because the consequences of hypervitaminosis D and hypercalcemia are severe, the Institute of Medicine’s Food and Nutrition Board established a very conservative upper limit of 2,000 IU (50mcg) of vitamin D3 per day for children and adults.23 Nonetheless, other reports suggest that as much as 10,000 IU of vitamin D3 per day is tolerable.24,25
It is an unfortunate reality that many Americans are deficient in vitamin D. However, an effective diet, the use of supplements and routine exposure to sunlight can help your patients maintain healthy levels of vitamin D.
Drs. Shechtman and Karpecki thank Larry J. Alexander, O.D., of McKinney, Texas, for contributing this article. Dr. Alexander is the senior director of educational development for Optovue, Inc., the ophthalmic digital imaging company. He is not a consultant to any pharmaceutical or nutraceutical companies.
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8. Vieth R, Bischoff-Ferrari H, Boucher BJ, et al. The urgent need to recommend an intake of vitamin D that is effective. Am J Clin Nutr. 2007 Mar;85(3):649-50.
9. Bodnar LM, Simhan HN, Powers RW, et al. High prevalence of vitamin D insufficiency in black and white pregnant women residing in the northern United States and their neonates. J Nutr. 2007 Feb;137(2):447-52.
10. Li YC, Kong J, Wei M, et al. 1,25-Dihydroxyvitamin D(3) is a negative endocrine regulator of the renin-angiotensin system. J Clin Invest. 2002 Jul;110(2):229-38.
11. Giovannucci E, Liu Y, Hollis BW, Rimm EB. 25-hydroxyvitamin D and risk of myocardial infarction in men: a prospective study. Arch Intern Med. 2008 Jun 9;168(11):1174-80.
12. Harris SS. Vitamin D and African Americans. J Nutr. 2006 Apr;136(4):1126-9.
13. Giovannucci E. Can vitamin D reduce total mortality? Arch Intern Med. 2007 Sep 10;167(16):1709-10.
14. Pierre C. The sunshine vitamin. AARP The Magazine: Online Edition. Available at: www.aarpmagazine.org/health/sunshine_vitamin.html (Accessed March 22, 2010).
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16. Martins D. Prevalence of cardiovascular risk factors and the serum levels of 25-hydroxyvitamin D in the United States: data from the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2007 Jun 11;167(11):1159-65.
17. Holick MF. Vitamin D deficiency. N Engl J Med. 2007 Jul 19;357(3):266-81.
18. Wheeler S. American Headache Society 50th Annual Scientific Meeting: Abstract S33. Presented June 28, 2008.
19. Parekh N, Chappell RJ, Millen AE, et al. Association between vitamin D and age-related macular degeneration in the Third National Health and Nutrition Examination Survey, 1988 through 1994. Arch Ophthalmol. 2007 May;125(5):661-9.
20. Seitzman RL, Mangione CM, Cauley JA, et al. Bone mineral density and age-related maculopathy in older women. J Am Geriatr Soc. 2007 May;55(5):740-6.
21. Armas LA, Hollis BW, Heaney RP. Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab. 2004 Nov;89(11):5387-91.
22. Houghton LA, Vieth R. The case against ergocalciferol (vitamin D2) as a vitamin supplement. Am J Clin Nutr. 2006;84(4):694-697.
23. Food and Nutrition Board, Institute of Medicine. Vitamin D. Dietary Reference Intakes: Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride. Washington D.C.: National Academies Press;1999:250-87.
24. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr. 1999 May;69(5):842-56.
25. Bischoff-Ferrari HA, Giovannucci E, Willett WC, et al. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr. 2006 Jul;84(1):18-28.