Doctors generally assume that North Americans are so well nourished that they never need to worry about supplements. In fact, they sometimes fuss that people who take vitamin pills could overdose on vitamin D or vitamin A, fat-soluble vitamins that are stored in the body. Is that worry justified?
It turns out that many Americans have low levels of vitamin D circulating in their bodies. This is especially true of African-Americans, since dark skin takes more time to make vitamin D in the presence of sunshine. Those with adequate levels of 25-hydroxyvitamin D, the usual measured metabolite, have generally been taking supplements (Journal of Nutrition, April, 2015). What about people in other countries?
Australians and Vitamin D:
Q. I’m glad you have been writing about vitamin D. Even here in Australia, many people have low levels of vitamin D.
White people are at risk for skin cancer, with 48 percent of skin cancers occurring in people of Celtic descent. (There are a lot in Australia). But we have gone overboard: we work indoors, apply sunscreen and wear hats. No wonder we end up low in vitamin D.
If you are an indigenous Aussie, either Aboriginal or Torres Strait Islander (TSI), and work for the government, you get free vitamin D tablets. A TSI friend is a lawyer and works indoors.
She is very dark-skinned and thus in need of more vitamin D. However, she spends her weekends doing the traditional TSI woman’s work of fishing while wearing very little clothing.
She told me that the government worried about ‘too much vitamin D’ when most of the new reef rangers were TSI people. Is that even possible?
A. If the reef rangers were taking vitamin D pills in addition to working out in the tropical sunshine, they could get too much vitamin D. But it would be from the pills, not from the sun exposure. The body has feedback mechanisms to keep it from making excessive vitamin D (Mayo Clinic Proceedings, April, 2012).
Can You Overdose on Vitamin D?
Excess vitamin D can indeed result in toxicity. This has happened, particularly when an infant or young child is given vitamin drops so they overdose on vitamin D (Journal of Steroid Biochemistry and Molecular Biology, April, 2015). Adults taking high-dose supplements can also overdose on vitamin D (Food and Chemical Toxicology, June, 2012).
You are right that many fair-skinned people in sunny climates can become low or even deficient in vitamin D by spending most of their time indoors. This happens in the southern US as well as in Australia.
We have written about this problem in our Guide to Vitamin D Deficiency. It tells how to tell whether you need vitamin D supplements and what can result if you don’t get enough vitamin D.