But there’s more to this powerhouse than building bones. Studies also show that vitamin D plays a notable role in ocular conditions, such as age-related macular degeneration (AMD) and diabetic retinopathy.
Synthesis in the SunVitamin D is the only vitamin formed with the help of sunlight. Activated vitamin D (known as calciferol) is hormone-like and fat-soluble. The kidneys produce it to help regulate calcium, and thus prevent bone diseases.1 We now know that vitamin D also regulates cells, systems and organs.1,2
Vitamin D synthesis begins when 7-dehydrocholestrol in the skin is converted to pre-vitamin cholcalciferol by UVB radiation (290nm to 320nm). This precursor molecule is then converted to the non-active storage form called 25-hydroxycholcalciferol—also called 25 (OH) D, or 25-hydroxyvitamin D—via hydroxylation in the liver.1 Various body tissues activate calcitriol for local use.
Ways To Get EnoughThe three ways to obtain vitamin D are through food, sunlight and supplementation. Vitamin D3 (cholcalciferol) is abundant in fatty fish. Along with long-chain essential fatty acids, vitamin D3 is found in cold-water fish, such as sockeye salmon and sardines.1 The less potent vitamin D2 is used to fortify milk (see “Sources of Vitamin D”).
It takes only about 12 minutes of mid-day summer sun exposure for Caucasians to produce 3,000 IU of natural vitamin D3. Of course, achieving this level takes much longer for a person of color living in a northern climate.The recommended daily allowance (RDA) for vitamin D is based on age, but makes no allowance for race, gender, season or location. For people aged one to 70 years, the RDA is 600 IU. For people over 70, 800 IU.3 Populations vulnerable to deficiency include those living in northern regions, people of color, indoor workers, infants, the housebound elderly and those advised by their doctors to avoid sunlight.
Implications of DeficiencyVitamin D deficiency has been linked to several types of cancer, cardiovascular disease, diabetes, multiple sclerosis, schizophrenia and influenza. Rickets in African Americans has returned to hospitals in Northern cities.1 Researchers have also studied the potential role of vitamin D as it relates to autism. Research has found that the degree of autism varies with distance from the equator.4
The 25 (OH) D blood test, a measure of vitamin D reserves, is inexpensive and widely available. The typical normative laboratory reference range is 30ng/ml to 100ng/ml. Any 25 (OH) D liver reserve value below 20ng/ml is considered deficient.
Ocular Wellness and Vitamin DSeveral epidemiological studies suggest an association between vitamin D deficiency and AMD. One such study in 2007 suggested that vitamin D may protect against age-related retinal changes. Researchers hypothesize that the beneficial effects are due to its anti-inflammatory activity.5
In 2011, researchers evaluated monozygotic twin pairs with discordant AMD phenotypes to assess differences in behavioral and nutritional factors and found that the twin with the earlier stage of AMD, smaller drusen size and area, and less pigmentary disturbances had higher dietary vitamin D intake.6 In a cross-sectional study of 517 patients, vitamin D deficiency was associated with an increased prevalence of retinopathy in young people with type 1 diabetes.7 The inflammatory and angiogenic effects of vitamin D deficiency in both types 1 and 2 diabetes may contribute to early retinal vascular damage; however, further investigations are needed.8 Whether vitamin D supplementation in diabetic patients can prevent or improve the prognosis for retinopathy remains to be investigated. Researchers also found that myopes had lower levels of blood vitamin D by an average of 3.4 ng/ml compared with non-myopes when adjusted for age and dietary intake. Adjusted for dietary variables, myopes appear to have lower average blood levels of vitamin D than non-myopes.9
Many other vitamin D-associated conditions, such as cardiovascular disease, multiple sclerosis, inflammatory and neoplastic disease, have secondary ocular manifestations and the potential for sight-threatening complications.
Our RoleEye care providers, particularly those in northern latitudes, should alert those vulnerable patients to the possibility of vitamin D deficiency. Dermatologist and researcher Michael Holick, MD, of Boston University advocates “sensible sunlight exposure”—five minutes, two to three times per week for Caucasians; five times that for people of color—and raising the RDA of vitamin D for all age groups. A great source of information is www.vitaminDcouncil.org.We should urge our patients to have their 25 (OH) vitamin D liver reserve status checked, and increase consumption of cold-water fish and vitamin D.
Thanks to Stuart Richer, OD, PhD, for contributing to this article.
1. Richer SP, Pizzimenti JJ. The importance of vitamin D in systemic and ocular wellness. J Optom. 2013;6:124-33.2. Pike JW, Glorieux FH, Feldman D. Vitamin D. 2nd ed. Waltham, MA: Elsevier; 2004.3. Institute of Medicine, Food & Nutrition Board, and Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.4. Grant WB, Soles CM. Epidemiologic evidence supporting therole of maternal vitamin D deficiency as a risk factor forthe development of infantile autism. Dermatoendocrinol. 2009 Jul;1(4):223-8.5. 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):661-9.6. Seddon JM, Reynolds R, Shah HR, Rosner B. Smoking, dietarybetaine, methionine, and vitamin D in monozygotic twins with discordant macular degeneration: epigenetic implications. Ophthalmology. 2011 Jul;118(7):1386-94.7. Kaur H, Donaghue KC, Chan AK, et al. Vitamin D deficiency is associated with retinopathy in children and adolescents with type 1 diabetes. Diabetes Care. 2011 Jun;34(6):1400-2. 8. Payne JF, Ray R, Watson DG, et al. Vitamin D insufficiency indiabetic retinopathy. Endocr Pract. 2012 Mar-Apr;18(2):185-93. 9. Mutti DO, Marks AR. Blood levels of vitamin D in teens and young adults with myopia. Optom Vis Sci. 2011 Mar;88(3):377-82. 10. Dietary Supplement Fact Sheet: Vitamin D. National Institutes of Health Web site. Available at: www.nih.gov/factsheets/vitaminD.com. Accessed Oct. 16, 2013.