For many, winter brings cold, dark days and an increased interest in vitamin D. Sometimes called the sunshine vitamin, vitamin D has received more interest in recent years, for good reason. In this article, we will cover what it is, why it is important, and how you can make sure you are not deficient.

What is Vitamin D?

Vitamin D is technically not a vitamin because humans can produce it themselves through exposure to sunlight. Vitamin D is biologically inert and must undergo two chemical processes in the body for activation. The first process occurs in the liver and converts vitamin D to a prehormone called calcidiol; the second process occurs primarily in the kidney and forms the physiologically active hormone calcitriol.^[1] When we do not get enough vitamin D, we do not have enough calcitriol inside our tissues, and not having enough of a hormone can be very bad news.

Human DNA contains more than 2,700 binding sites for calcitriol, including sites involved in virtually every major human disease.^[2] The widespread presence of these binding sites suggests that calcitriol—and therefore vitamin D—is very important throughout the body. We often hear about its importance for bone health because vitamin D deficiency causes rickets in children and osteomalacia or osteoporosis in adults, but vitamin D has many other critical functions in the body. Indeed, a growing body of evidence shows that vitamin D is important for preventing and treating numerous conditions, including asthma, heart failure, and autoimmune diseases such as multiple sclerosis and inflammatory bowel disease (e.g., Crohn’s disease, ulcerative colitis).^[3][4][5]

How Do We Get Vitamin D?

Vitamin D is somewhat scarce in standard human diets because there are few natural dietary sources. One common source is oily fish (e.g., salmon, sardines, etc.), but to get enough vitamin D from oily fish alone, you would need to eat two or more large servings every day! Because of the lack of natural vitamin D in most foods, milk in many countries, including in the United States, is fortified with vitamin D. However, fortified dairy and nondairy milk generally contain very low levels of vitamin D. Some other products can contain added vitamin D too, but this varies by region and food supplier. Mushrooms are one of the few natural plant-based sources of vitamin D, but only when they have been exposed to sunshine (or ultraviolet B).^[6] Because most of the mushrooms sold in grocery stores are not grown in natural conditions with very much sunshine exposure, if any, their vitamin D content is usually relatively low.

That leaves sunshine, the primary source of vitamin D for most humans on the planet. When the ultraviolet light from the sun penetrates human skin, it triggers a chemical reaction that generates vitamin D. If a person with fair skin wearing a bathing suit goes out in the sun for around 20 minutes, they can produce about 20,000 IU (international units) of vitamin D. For comparison, a large fatty piece of fish might contain 1,000 IU.^[2] Importantly, although individuals with darker skin also generate vitamin D from sun exposure, they are not quite as efficient because their darker skin contains more UV-blocking melanin. This is one reason why vitamin D deficiency can be more severe among people of color.^[7] People also lose some of their ability to synthesize vitamin D from sunshine as they age, putting the elderly at a greater risk for deficiency.^[8]

Insufficient Sun Exposure

Geographic location, specifically latitude (distance from the equator), influences vitamin D levels: the further we are from the equator, the lower the strength of the sun and the lower our potential to produce vitamin D. Season also makes a difference. In the summer, the sun is higher in the sky, which means less UV radiation is filtered by the atmosphere and the daytime is longer. Certain parts of the US are exposed to sun strong enough to produce sufficient quantities of vitamin D all year (e.g., Miami), but other places, like New York, do not get strong enough sun for about four months (November to February). Some cities even further north than New York do not get strong enough sun for about six months (October to March).

Other factors that reduce sun exposure and vitamin D production include excess pollution, always staying in the shade (including in the shadows of skyscrapers), sunscreen, glass windows, and clothing. Most modern human activity is indoors and away from the sun, increasing the risk of vitamin D deficiency. For example, office workers and people who cover their skin for religious reasons are at a higher risk for deficiency.^[9][10]

The Shadow Rule

I tell my patients to follow Kerley’s shadow rule: if your shadow is shorter than your body, you can make vitamin D (as long as you are in the sun and not covered head to toe in either clothes or sunscreen). If your shadow is longer than your body, you are probably not making vitamin D, regardless of how sunny or warm it is.

Also, the vitamin D requirement is not a justification to burn your skin. Each person’s skin is different, and you should be mindful of not overdoing it. Although there is no evidence that moderate sunshine exposure is harmful—and you can get plenty of vitamin D with moderate exposure—consistent evidence suggests sunburns are associated with skin damage.

Should You Supplement Vitamin D?

Vitamin D deficiency affects around 1 billion people, making it one of the biggest nutritional issues worldwide.^[2] Indeed, over 75 percent of Americans are vitamin D deficient.^[11]

A 2014 review of 32 studies reported that low vitamin D levels were associated with a 90 percent increased risk of early death!^[12] Additionally, two separate reviews of 78 vitamin D supplementation studies reported that taking a vitamin D supplement decreased the death rate in healthy adults.^[13][14] Another large review reported that taking a vitamin D supplement decreased the death rate in cancer patients.^[15]

A vitamin D blood level of around 30 ng/ml (75 nmol/L) is optimal (the blood test is called 25-hydroxyvitamin D or 25[OH]D). Most humans only need moderate, regular sun exposure to obtain this blood level. If the sun is strong enough (remember Kerley’s shadow rule), exposure every other day for 10–20 minutes will be sufficient. Darker skin will generally need longer. However, this may not be possible farther from the equator or during the winter. It might also be impractical if your schedule and lifestyle make getting outside difficult. In these scenarios, a vitamin D supplement is worth considering.

There are two more common types of vitamin D: vitamin D2 (generally plant-based) and vitamin D3 (generally derived from sheepskin). Vitamin D3 is thought to be more effective than vitamin D2 in raising blood levels and improving health.^[16][13][14] Not all D3 supplements, however, are derived from sheepskin: look for products labeled “plant-based” or “vegan.” For adults, 2,000 IU (50 micrograms) per day is a good starting point, but exact requirements will depend on your body size (larger bodies require more vitamin D), diet, sun exposure, and genetics.^[17] And yes, it is possible to take too much. Therefore, you should discuss the best approach with your doctor. If you choose to take a vitamin D supplement, it is also preferable to take it with food and at regular intervals (e.g., daily, as opposed to taking large doses at irregular intervals).^[18]

Vitamin D supplements are not the cure-all that some claim. But the nutrient is essential. Aim for moderate sun exposure when possible and consider vitamin D supplementation when it is not.

Five Summary Facts About Vitamin D

1. Vitamin D is not technically a vitamin; it is a precursor to the hormone calcitriol.
2. Vitamin D is important not only for bone health but also for other body functions.
3. Over 75 percent of Americans are vitamin D deficient.
4. Mushrooms are one of the few natural plant-based sources of vitamin D.
5. Sunshine is the primary source of vitamin D for most humans on the planet.

References

1. Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D and Calcium, Ross AC, Taylor CL, Yaktine AL, Del Valle HB, eds. Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011.
2. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281. doi:10.1056/NEJMra070553
3. Jolliffe DA, Greenberg L, Hooper RL, et al. Vitamin D supplementation to prevent asthma exacerbations: a systematic review and meta-analysis of individual participant data [published correction appears in Lancet Respir Med. 2018 Jun;6(6):e27]. Lancet Respir Med. 2017;5(11):881-890. doi:10.1016/S2213-2600(17)30306-5
4. Moretti HD, Colucci VJ, Berry BD. Vitamin D3repletion versus placebo as adjunctive treatment of heart failure patient quality of life and hormonal indices: a randomized, double-blind, placebo-controlled trial. BMC Cardiovasc Disord. 2017;17(1):274. Published 2017 Oct 30. doi:10.1186/s12872-017-0707-y
5. Dankers W, Colin EM, van Hamburg JP, Lubberts E. Vitamin D in Autoimmunity: Molecular Mechanisms and Therapeutic Potential. Front Immunol. 2017;7:697. Published 2017 Jan 20. doi:10.3389/fimmu.2016.00697
6. Cardwell G, Bornman JF, James AP, Black LJ. A Review of Mushrooms as a Potential Source of Dietary Vitamin D. Nutrients. 2018;10(10):1498. Published 2018 Oct 13. doi:10.3390/nu10101498
7. Ames BN, Grant WB, Willett WC. Does the High Prevalence of Vitamin D Deficiency in African Americans Contribute to Health Disparities?. Nutrients. 2021;13(2):499. Published 2021 Feb 3. doi:10.3390/nu13020499
8. Chalcraft JR, Cardinal LM, Wechsler PJ, et al. Vitamin D Synthesis Following a Single Bout of Sun Exposure in Older and Younger Men and Women. Nutrients. 2020;12(8):2237. Published 2020 Jul 27. doi:10.3390/nu12082237
9. Divakar U, Sathish T, Soljak M, et al. Prevalence of Vitamin D Deficiency and Its Associated Work-Related Factors among Indoor Workers in a Multi-Ethnic Southeast Asian Country. Int J Environ Res Public Health. 2019;17(1):164. Published 2019 Dec 25. doi:10.3390/ijerph17010164
10. Tsur A, Metzger M, Dresner-Pollak R. Effect of different dress style on vitamin D level in healthy young Orthodox and ultra-Orthodox students in Israel. Osteoporos Int. 2011;22(11):2895-2898. doi:10.1007/s00198-010-1492-y
11. Binkley N, Ramamurthy R, Krueger D. Low vitamin D status: definition, prevalence, consequences, and correction. Endocrinol Metab Clin North Am. 2010;39(2):287-contents. doi:10.1016/j.ecl.2010.02.008
12. Garland CF, Kim JJ, Mohr SB, et al. Meta-analysis of all-cause mortality according to serum 25-hydroxyvitamin D. Am J Public Health. 2014;104(8):e43-e50. doi:10.2105/AJPH.2014.302034
13. Bjelakovic G, Gluud LL, Nikolova D, et al. Vitamin D supplementation for prevention of mortality in adults. Cochrane Database Syst Rev. 2014;(1):CD007470. Published 2014 Jan 10. doi:10.1002/14651858.CD007470.pub3
14. Chowdhury R, Kunutsor S, Vitezova A, et al. Vitamin D and risk of cause specific death: systematic review and meta-analysis of observational cohort and randomised intervention studies. BMJ. 2014;348:g1903. Published 2014 Apr 1. doi:10.1136/bmj.g1903
15. Keum N, Giovannucci E. Vitamin D supplements and cancer incidence and mortality: a meta-analysis. Br J Cancer. 2014;111(5):976-980. doi:10.1038/bjc.2014.294
16. Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. doi:10.3945/ajcn.111.031070
17. Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr. 1999;69(5):842-856. doi:10.1093/ajcn/69.5.842
18. Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. doi:10.1016/j.jand.2014.09.014