Vitamin D

Vitamin D

Vitamin D is essential for vision health, immune system integrity and the health of our brains. It is absorbed through the skin from exposure to sunlight, or taken in through some foods or supplements. In the body it is stored in fat tissue. When needed the liver and kidneys convert it to an active form called calcitriol. Fat-soluble vitamin D is actually a group of vitamins that perform a number of essential functions, including proper absorption of magnesium and calcium.1

Vitamin D synthesis comes primarily from a process involving the sun. Our skin contains a biochemical precursor to vitamin D, a form of cholesterol called 7-dehydrocholesterol. When the skin is exposed to UV light rays, cholecalciferol is created, and a process begins that passes through the liver, then to the kidneys where it is converted into calcitriol, the active form of vitamin D.2

Your body can create vitamin D from enough sun exposure. However, people in northern climates or those who do not spend enough time outdoors, may have a deficiency. Milk is vitamin D-fortified to help calcium absorption, but this may not be enough. Make sure your milk is fortified with vitamin D3, not D2 (which is not as beneficial and can be toxic in large doses).

Furthermore, a deficiency in vitamin D reduces vitamin B production. Merely supplementing with D3 does not solve the problem, extra vitamin B must also be taken.3

Vitamin D for Seniors

Seniors require four times the amount of sunshine to get the same amount of vitamin D as a 20-year-old.4 The accepted normal range for vitamin D levels runs from 20 to 50 nanograms per milliliter. Vitamin D's role is especially important for seniors because one of its functions is to support bone tissue and protect against bone loss by aiding the absorption of calcium. It is not surprising that there is a strong correlation between osteoarthritis and AMD, both which may aggrevated by low levels of vitamin D.4a

In the Eye

Vitamin D is a membrane antioxidant; it protects membranes against oxidative damage. In the eye its ability to reduce retinal cell damage, inhibit inflammation, and reduce the development of extra blood vessels (angiogenesis) provide essential functions. 4b

  • Diabetic retinopathy. Low levels of vitamin D are seen in patients with diabetic retinopathy.5 Vitamin D levels are linked to artery intima-media thickness in diabetics.6
  • Dry eyes. Vitamin D deficiency is linked to dry eye.7 The cornea contains receptors that recognize vitamin D.8 In dry eye patients, cornea nerve cells were less dense, shorter, thinner, and covered less total area of the cornea.9
  • Macular degeneration. Low levels of vitamin D are connected to an increase in macular degeneration, and advancement from early AMD to the more advanced form, wet macular degeneration.9a and supplementing with vitamin D may lower risk,10 especially in women younger than age 75. It supports the health of the retinal microvascular system.11
  • Related disorders. Low levels of vitamin D contribute to weak bones and may play a role in cancer and respiratory infections. It is known for its function in supporting the integrity of the immune system.12

In the Brain

Vitamin D is linked to neurological development and is being investigated as a therapeutic tool for cognitive impairment and dementia. While it offers some protection and support, clinical interventional studies don't link increased D with improved cognition13 unless the patient had been deficient in vitamin D. This deficiency may play a role in Alzheimer's.

Food Sources

Sunlight is a great source of D. Foods include fatty fish, vitamin D fortified foods, and smaller amounts of beef liver, cheese, and egg yolks.


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4. Kruse, J. (2013). Epi-paleo Rx: The prescription for disease reversal and optimal health. New Orleans, LA: Optimized Life, PLC.
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5. Aksoy H, Akçay F, Kurtul N, Baykal O, Avci B. (2000). Serum 1,25 dihydroxy vitamin D (1,25(OH)2D3), 25 hydroxy vitamin D (25(OH)D) and parathormone levels in diabetic retinopathy. Clin Biochem. 33:47–51.
6. Targher, G., Bertolini, L., Padovan, R., Zenari, L., Scala, L., et al. (2006). Serum 25-hydroxyvitamin D3 concentrations and carotid artery intima-media thickness among type 2 diabetic patients. Clin Endocrinol (Oxf) 65:593–597.
7. Yoon, S.Y., Bae, S.H., Shin, Y.J., Park, S.G., Hwang, S.H. et al. (2016). Low Serum 25-Hydroxyvitamin D Levels Are Associated with Dry Eye Syndrome. PLoS One. Jan 25;11(1):e0147847.
8. Yildirim, P., Garip, Y., Karci, A.A., Guler, T. (2015). Dry eye in vitamin D deficiency: more than an incidental association. Int J Rheum Dis. Jan;19(1):49-54.
9. Shetty, R., Sethu, S., Deshmukh, R., Deshpande, K., Ghosh, A., et al. (2016). Corneal Dendritic Cell Density Is Associated with Sub basal Nerve Plexus Features, Ocular Surface Disease Index, and Serum Vitamin D in Evaporative Dry Eye Disease. BioMed Res Int. 2016:4369750.
9a. Kan E, Kan EK, Yucel OE. (2020). The Possible Link Between Vitamin D Levels and Exudative Age-related Macular Degeneration.Oman Med J. Jan 5;35(1):e83.
10. Vitamin D linked to lower macular degeneration risk. (2016). Naturaleyecare. Retrieved Nov 2 2017 from
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12. Hewison, M. (2012). Vitamin D and immune function: an overview. Proc Nutr Soc. Feb;71(1):50-61.
13. Landel V, Annweiler C, Millet P, Morelio M, Feron F. (2016). Vitamin D, Cognition and Alzheimer's Disease: The Therapeutic Benefit is in the D-tails. J Alzheimers Dis. May 11;53(2):419-44.