Sources of Vitamin D
Many people understand that vitamins are important dietary nutrients, and assume that a good, well-balanced diet will take care of their vitamin needs. However, vitamin D is not a vitamin at all — and is very hard to obtain dietarily.
The Sunshine Vitamin
Historically, “vitamin D” has been obtained by spending time out in the sun — an important source of vitamin D, since most diets do not naturally provide much (if any) vitamin D. A pro-vitamin D molecule present in the skin or oils of most animals requires only ample UVB radiation and a little body heat to convert into vitamin D3. Vitamin D3 was the primary form of vitamin D for humans up until the 1930s. (You can read more about the forms of vitamin D here).
Vitamin D is often called the “sunshine vitamin,” but this phrase (and the people using it) can be misleading. The UVB radiation in sunshine was the only source of vitamin D needed by our hunting and farming ancestors — especially those not living too far from the equator. Those ancestors had no choice but to spend large amounts of time out in the sun in order to survive. In the modern world, the opposite is true; work keeps us indoors during the day and we avoid getting too much UV radiation for our own good health. Add to those factors issues like pollution, sunscreen, and large population centers located far from the equator, and you end up with a serious health problem.
In fact, the problem was so serious at the turn of the century that many countries, the United States included, took to fortifying their milk products with vitamin D2 as soon as they could. Fortification of milk with very small amounts of vitamin D was all it took for the worst symptoms of vitamin D deficiency to virtually vanish in the population — countless children have been saved from deformity and death thanks to vitamin D fortification. 1
Today, milk is still typically fortified with vitamin D2 in America. However, a glass a day does not meet the modern vitamin D recommendations made by the IOM; one would have to consume six cups of fortified milk a day in order to meet recommendations. Obviously, most people are not going to drink that much milk — and they don’t.
There are other dietary sources of vitamin D, too. Milk is not the only food to be fortified with vitamin D. Many breakfast cereals are, as well. Of course, even a serving or two of both milk and fortified cereal will not generally meet vitamin D requirements. Not that most people are consuming that much milk or breakfast cereal every day, anyway. Other foods may be fortified with vitamin D as well.
Arguably, the best dietary source of vitamin D is oily fish. One serving of wild-caught salmon can give you more than the IOM minimum recommended amount of vitamin D. It’s important to note that things like the type of salmon and how it’s cooked do make a huge difference in how much vitamin D you’ll get though.2 Often, fish that are considered to be high in vitamin D simply aren’t — so it’s important to do your research, rather than just counting on your love of seafood to get you the vitamin D you need.
Not that fatty fish are a large portion of most American’s diets.
For the sake of being thorough, I’ll mention a few other dietary sources of vitamin D. Unfortified whole milk tends to have small quantities of vitamin D in it, varying with the season. Other dairy products may have some small amount of vitamin D (around 1%) as well. Eggs may contain a notable amount of vitamin D, depending mainly on the diet of the hens. Certain kinds of irradiated mushrooms can also be a substantial source of vitamin D,3 as can some organ meats.
Listed that way, it may sound as though vitamin D is actually abundant in foods and easily obtained dietarily. However, that’s simply not true. Just look at the studies. The Dietary Guidelines Advisory Committee submitted a report this year in which they stated concern for vitamin D underconsumption and published data showing vitamin D as the most under-consumed nutrient in American diets.4 An NHANES study found that over 30% of Americans have vitamin D blood levels low enough to be at risk of vitamin D inadequacy5. One long-term study found incidents of rickets, a serious condition in children caused by vitamin D deficiency, to be increasing.6
Clearly, vitamin D fortification and occasional seafood consumption isn’t cutting it.
2: Lu, Z., Chen, T. C., Zhang, A., Persons, K. S., Kohn, N., Berkowitz, R., … & Holick, M. F. (2007). An evaluation of the vitamin D 3 content in fish: is the vitamin D content adequate to satisfy the dietary requirement for vitamin D?. The Journal of steroid biochemistry and molecular biology, 103(3), 642-644.
6: Thacher, T. D., Fischer, P. R., Tebben, P. J., Singh, R. J., Cha, S. S., Maxson, J. A., & Yawn, B. P. (2013, February). Increasing incidence of nutritional rickets: a population-based study in Olmsted County, Minnesota. In Mayo Clinic Proceedings (Vol. 88, No. 2, pp. 176-183). Elsevier.