It’s that time of year again. Many of us wake up and go to work in the dark. We work all day and return home in the dark. With the days getting shorter and the weather turning cold, many start thinking about vitamin D supplementation.

Questions I typically field from patients include Do I need to be concerned about getting enough? How much do I need? and Who in particular is at risk of deficiency?

First, let us establish that vitamin D is not an essential nutrient, which is classically defined as any substance we must consume in our diet because we cannot make it adequately on our own. Vitamin D does not fit this definition because we can make it from ultraviolet B (UVB) radiation in sunlight. Very few foods even contain vitamin D naturally. Fish liver, mushrooms, certain types of fish, and a few other foods contain some vitamin D; various types of dairy and nondairy milk are also frequently fortified with vitamin D. But you see, from a natural perspective, vitamin D should not even be a nutritional issue at all. The reason it is a nutritional issue is that many people do not get enough vitamin D from sun exposure, meaning they must eat vitamin D-containing foods and absorb the nutrient through their gastrointestinal tract.

Vitamin D deficiency has been linked to cancers, multiple sclerosis, frailty, falls, and many other ailments.^[1] Yet, for all the scientific interest, there have not been consistent results that show vitamin D supplementation does anything useful for chronic diseases.^[12 Some studies have shown that vitamin D slightly reduces the risk of falls, especially in deficient people (for example, institutionalized older adults).^[3]

Getting enough vitamin D from the sun is easy in the spring, summer, and fall, no matter how far north you live. Factors that make a difference in how much vitamin D you make include skin color (the darker your skin color, the longer it takes to make a sufficient amount), time of day, length of day, and skin covering, including sunblock.^[1] If you are exposed to the midday sun (between 10 a.m. and 3 p.m.) for 5–30 minutes twice a week on the arms and legs, you will get sufficient vitamin D.^[4] For pale skin, you might need as little as five minutes; for dark skin, you might require at least 30 minutes. You can even make vitamin D on days when there is not a lot of sunshine directly on you: clouds, shade, and severe pollution reduce vitamin D synthesis by about 50–60 percent, but you still make some vitamin D.^[1] However, although windows do not block all “tanning” radiation, ultraviolet A (UVA), they do block UVB radiation, so you won’t make any vitamin D driving in a car unless the windows are down. Finally, sunblock with a sun protection factor (SPF) strong enough to prevent sunburn will stop almost all vitamin D synthesis where you apply it.^[5]

Severe vitamin D deficiency causes rickets and osteomalacia, which are problems with bone mineralization resulting from inadequate calcium and phosphorus. Vitamin D helps with calcium absorption in the intestine. By undermining that absorption, insufficient vitamin D levels trigger other changes too, including low phosphorus and hormone system changes.

Rickets and osteomalacia are uncommon but genuine risks for several populations. Populations with mild to moderate risk of vitamin D deficiency include:^[4]

Mild to moderate risk of vitamin D deficiency:

• people living at northern latitudes during winter months,
• the elderly, who do not synthesize vitamin D as efficiently,
• obese individuals, and
• those taking certain medications (anticonvulsants, steroids, AIDS therapy).

Groups with moderate to high risk of vitamin D deficiency include:

• institutionalized people (including nursing home residents),
• women who cover all of their skin for religious reasons,
• exclusively breastfed infants,
• people with certain kidney or liver diseases, and
• those with endocrine disease (thyroid and parathyroid problems) or some other granulomatous diseases (sarcoidosis, tuberculosis, lymphomas).

Exclusively breastfed infants should be given 400 IU of vitamin D daily, which can be orally administered as a droplet. Individuals older than nine who fall into the categories above should take 600 IU of vitamin D a day, and elderly institutionalized people should take 800 IU per day. Keep in mind that more is not better. It is possible to have too much vitamin D. The maximum adult dose is 4000 IU per day, and the upper limit is significantly lower for children.

You may wonder how often to get your levels checked by your doctor. While I haven’t seen any recommendations based on unequivocal scientific evidence, I can offer the following as a general guideline reflecting my approach. In my hometown of Rochester, New York, I check vitamin D levels in those individuals with calcium, bone, or parathyroid hormone problems. I also often screen people who fall into the categories listed above, plus pregnant or breastfeeding women. For those not in a high-risk category, I do not suggest routine vitamin D screening. I simply encourage everyone to get as much outdoor time as possible while strictly avoiding sunburn, and I recommend weight-bearing exercise for good bone health.

As with all information published by CNS, this is a good topic to discuss with your doctor. As winter settles in, I recommend a vitamin D supplement of 600 IU a day for adolescents and adults. This should hold us over until that next glorious spring with its promise of warmer weather, longer days, and outdoor fun. Until then, I recommend as much outdoor activity as possible: walking your dog, cross-country and downhill skiing, hiking, ice skating at an outdoor location, snowshoeing, or more traditional activities like golf, tennis, and biking in the less snowy areas of the country. The idea is to have fun and stay active and fit while keeping your vitamin D levels sufficient.

References

1. D​ietary Supplement Fact Sheet: Vitamin D. Office of Dietary Supplements, 2011. (Accessed April 15th, 2012, at http://ods.od.nih.gov/factsheets/vitamind-HealthProfessional/.)
2. IOM (Institute of Medicine). Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: The National Academies Press; 2011.
3. ​Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Guidelines for preventing and treating vitamin D deficiency and insufficiency revisited. J Clin Endocrinol Metab 2012;97:1153-8.
4. ​Holick MF. Vitamin D deficiency. The New England journal of medicine 2007;357:266-81.
5. ​Misra M, Pacaud D, Petryk A, Collett-Solberg PF, Kappy M. Vitamin D deficiency in children and its management: review of current knowledge and recommendations. Pediatrics 2008;122:398-417.