The Vitamin D Questions: How Much Do You Need and How Should You Get It?

Vitamin D

Two years ago my doctor recommended I start taking vitamin D. I’m annoyed at having to take a supplement every day, but I do what the doc says.

Your body needs exposure to ultraviolet light to produce its own vitamin D, but too much UV light causes skin cancer. Russ Hobbie and I address this trade-off in Section 14.10 of Intermediate Physics for Medicine and Biology.

There has been an alarming increase in the use of tanning parlors by teenagers and young adults. These emit primarily UVA, which can cause melanoma. Exposure rates are two to three times greater than solar radiation at the equator at noon (Schmidt 2012). Many states now prohibit minors from using tanning parlors. Proponents of tanning parlors point out that UVB promotes the synthesis of vitamin D; however, the exposure to UVB in a tanning parlor is much higher than needed by the body for vitamin D production. Tanning as a source of Vitamin D is no longer recommended at any age level (Barysch et al. 2010).

To learn more about how much vitamin D people need, I read an article by Deon Wolpowitz and Barbara Gilchrest.

Wolpowitz D, Gilchrest BA (2006) “The Vitamin D Questions: How Much Do You Need and How Should You Get It?Journal of the American Academy of Dermatology, Volume 54, Pages 301–317.

Below are excerpts from their conclusion.

Given the scarcity of naturally occurring vit [vitamin] D in many otherwise adequate diets, human beings may once have depended on unprotected exposure to natural sunlight as the primary environmental source of vit D, at least during those periods of the year when sunlight can produce [previtamin] D3 in the skin… However, chronic unprotected exposure tocarcinogenic UV radiation in sunlight not only results inphotoaging, but also greatly increases the risk of skin cancer. This risk is further exacerbated by the extended lifespan of human beings in the 21st century. Fortunately, there is a noncarcinogenic alternative-intestinal absorption of vit D-fortified foods and/or dietary supplements…

All available evidence indicates that younger, lighter-skinned individuals easily maintain desirable serum 25-OH vit D levels year-round by incidental protected sun exposure and customary diet. Daily intake of two 8-oz glasses of fortified milk or orange juice or one standard vit or incidental protected exposure of the face and backs of hands to 0.25% minimum erythema [reddening of the skin] dose of UVB radiation 3 times weekly each generates adequate serum 25-OH levels by classic criteria. Dietary supplementation of vit D is efficient, efficacious, and safe. Thus, it is prudent for those at high statistical risk for vit D deficiency, such as patients who are highly protected against the sun [or old folks like me], to take daily supplemental vit D (200–1000 IU) with concurrent dietary calcium to meet current and future RDA [recommended daily allowance] levels.

So, maybe my physician was right when she put me on vitamin D. But I decided to double my dose in the winter. Is that a good idea? A recent paper out of South Korea has some interesting data.

Park SS, Lee YG, Kim M, Kim J, Koo J-H, Kim CK, Um J, Yoon J (2019) “Simulation of Threshold UV Exposure Time for Vitamin D Synthesis in South Korea,” Advances in Meteorology, Volume 2019, Article 4328151.

Sang Seo Park and his colleagues analyze UV exposure, and estimate the threshold exposure time at noon to sunlight in Seoul for vitamin D synthesis (blue) and erythema (red).

Threshold exposure time to sunlight in Seoul for vitamin D synthesis (blue) and erythema (red). From Park et al. (2019).

This is a semilog plot, so the difference between July and January is about a factor of ten. I think increasing my vitamin D dose in the winter is, if anything, conservative. I could probably do without the supplement in the summer. Rochester (42° latitude) is farther north than Seoul (38°), so the seasonal effect might be even greater where I live.

In conclusion, I think Russ and I are correct to stress the harmful effects of UV light in IPMB. If you’re worried about not getting enough vitamin D, take a supplement in the winter.

Originally published at

Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.

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