Vitamin D in Winter
By Reed Mangels, PhD, RD
Vitamin D is known as the sunshine vitamin because humans are able to produce vitamin D after sunlight exposure. This essential vitamin is needed for healthy bones, plays an important role in the immune system, and helps our nerves function properly.
Certainly, in winter, many of us aren’t out in the sun (or at least not with large amounts of skin exposed) to the same extent that we are in summer. For that reason alone, other sources of vitamin D may be needed in the winter. In fact, anything that prevents skin from being directly exposed to the sun affects the amount of vitamin D that our bodies can produce. Clothing, sunscreen, air pollution, clouds, window glass, even darkly pigmented skin all limit vitamin D production. That’s not to say, however, that a lot of sun exposure is needed for vitamin D production. Around 10 to 15 minutes a day of sun on hands and face appears adequate for many people. It gets more complicated in the winter, however. In addition to being bundled up, having fewer daylight hours, and staying inside more, in winter, there’s not enough of the type of sunlight needed to stimulate vitamin D production in some areas. In locations above 42 degrees north latitude (Chicago, Boston, and Portland, Oregon, for instance), vitamin D production does not occur from late October through early March. Even as far south as Atlanta (about 35 degrees north latitude), vitamin D production doesn’t occur from November to February.1, 2
Vitamin D is stored in our liver and fat tissue. Although it could be possible to rely on stores in the winter months, studies consistently show that blood vitamin D levels drop significantly in the winter, suggesting that additional vitamin D is needed.3, 4 There also are other factors that can reduce vitamin D production including being an older person and having darker skin. And there are concerns about sun exposure and increased risk of skin cancer – although the short sun exposure needed for vitamin D production seems to be pretty low risk.
The RDA for vitamin D is 600 IU per day for children and adults up to 71 years old, and 800 IU per day for those age 71 years and older.5 Some plant milks are fortified with vitamin D and can be an important source of this vitamin for vegans. Still, with 40 to 240 IU in a cup of fortified plant milk, one would need to drink a lot of plant milk to meet the recommended amount of vitamin D. The same is true for nonvegans relying on fortified cow’s milk for vitamin D – multiple cups would be needed. A supplement of vitamin D can be helpful in meeting vitamin D needs especially in winter or other situations where sunlight exposure is limited. A recent study found that a 600 IU/day vitamin D supplement in the winter months helped promote an adequate vitamin D status, regardless of location.6
To read more about vitamin D, see:
References
1. Holick MF. Vitamin D deficiency. N Engl J Med 2007; 357:266-81.
2. Wang SQ, Halpern AC. Estimated equivalency of vitamin D production from natural sun exposure versus oral vitamin D supplementation across seasons at two US latitudes. J Am Acad Dermatol. 2010;62(6):929.e1-929.e9299.
3. Outila TA, Kärkkäinen MU, Seppänen RH, Lamberg-Allardt CJ. Dietary intake of vitamin D in premenopausal, healthy vegans was insufficient to maintain concentrations of serum 25-hydroxyvitamin D and intact parathyroid hormone within normal ranges during the winter in Finland. J Am Diet Assoc. 2000;100(4):434-441.
4. Kroll MH, Bi C, Garber CC, et al. Temporal relationship between vitamin D status and parathyroid hormone in the United States. PLoS One. 2015;10(3):e0118108.
5. Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.
6. Mendes MM, Hart KH, Williams EL, Mendis J, Lanham-New SA, Botelho PB. Vitamin D supplementation and sunlight exposure on serum vitamin d concentrations in 2 parallel, double-blind, randomized, placebo-controlled trials. J Nutr. 2021;151(10):3137-3150.
