Vitamin D
- kim32739
- Oct 1
- 3 min read
Updated: Oct 14
Sunlight and Vitamin D are hot topics in the cold season. As summer transitions to fall here's some compiled information about Vitamin D (see list of sources at the bottom for more info).
Disclosure: I am neither a nutritionist nor a doctor, so please do your own research and/or consult an expert before making any decisions based on this post.
What do we know about Vitamin D?
Vitamin D is known to support bone density and bone health, helping people to avoid osteoporosis, rickets, and other bone-related conditions. It promotes Calcium absorption in our gut. Vitamin D is associated with reduction in risk of certain cancers, such as colon and breast cancer. It also has roles related to immunity, cell growth, and reduction of inflammation.
You can get Vitamin D from exposure to UV light (such as from sunlight), food sources, and supplements. Major food sources include some fish sources (cod liver oil is particularly rich), egg yolks, mushrooms (particularly if exposed to UV light), and fortified foods such as milk and orange juice.
Where is our knowledge sporadic and evolving?
How much is enough? A study published in 2010 By the National Academy of Medicine (at the time it was known as the Institute of Medicine) concluded that anything above 20 nanograms per milliliter is generally considered healthy. A study published in 2011 in the Journal of Clinical Endocrinology and Metabolism concluded a higher level is necessary for health - 30 nanograms per mililiter. The latter number was adopted by the medical community, though the scientific evidence supporting one cut-off versus the other is inconclusive.
How much exposure to UV light is sufficient? There are many factors that affect the amount of UV light that get to our skin (such as angle of the sun, cloud-cover, and pollution levels) and several factors that affect our absorption of UV light (such as skin pigmentation and the use of sunscreen). Research has been inconclusive. Some Vitamin D researchers have suggested that 5-30 minutes of sun exposure between 10:00am and 3:00pm twice per week to the face, arms, legs, or back should be sufficient.
How much should I get from food sources? This depends of course on how much UV light you are exposed to. The Food and Nutrition Board at the Institute of Medicine of the National Academies has put out Recommended Daily Allowance (RDA) guidelines which assume minimal sun exposure: for individuals between one and seventy years of age the RDA is 600 IUs (15 micrograms). For comparison, one tablespoon of cod liver oil contains 1,360 IUs, 100 grams of portabella mushrooms may contain between 100 and 800 IUs (exposed to UV light) or 10 IUs (not exposed to UV light), one cup of Vitamin D fortified orange juice contains 137 IUs, and one large egg yolk contains 41 IUs.
How informative are Vitamin D tests? Tests for Vitamin D generally measure the amount of calcidiol (or 25-hydroxyvitamin D). This chemical is an intermediary between the Vitamin D we absorb through the skin or digestion, which is inert, and the active form that our body uses for cellular functions. Interestingly, before taking its active form, Vitamin D is chemically altered in the liver and then the kidney. While the tests for calcidiol are fairly reliable, the extent to which calcidiol levels actually represent the levels of active Vitamin D in our body is unclear.
What are the trade-offs and how steep are they? UV light is a source of Vitamin D, but it is also tied to skin cancers. Ironically, Vitamin D has been shown to play a role in reducing the risk of other types of cancers (such as breast and colon cancer). Some fish are a great source of Vitamin D, but fish often contains pollutants such as heavy metals, so it pays to do your research about what fish to avoid from a pollution standpoint. There is also such a thing as too much Vitamin D.
Photo Credit: Ray Bilcliff
Sources of information:
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