By Michael Holick

Web edition: October 8, 2010
Print edition: October 23, 2010; Vol.178 #9 (p. 32)

It’s known that vitamin D is necessary for proper bone formation and maintenance. But recent decades have seen a torrent of studies suggesting that vitamin D can also affect many other aspects of health; some scientists have come to consider the daily recommended intake of 400 international units of vitamin D far too low. Michael Holick is a biochemist and endocrinologist at Boston University who has spent a career researching the effects of vitamin D (which is actually not a vitamin but a hormone precursor). His new book is The Vitamin D Solution (Hudson Street Press, 2010). Holick recently spoke with Science News biomedicine writer Nathan Seppa.

How much vitamin D do we need?
Children should be taking at least 400 to 1,000 international units of vitamin D as a supplement every day, and adults should take 1,500 to 2,000 IU.

What about pregnant or breast-feeding women?
We tested pregnant women who were taking a prenatal vitamin containing 400 IU of vitamin D each day and drinking two glasses of fortified milk, and found that 76 percent of them — and 81 percent of their newborns — were still vitamin D deficient at the time of giving birth. We also estimate that most breast-feeding women are vitamin D deficient, and they pass along deficient milk to their infants.

Vitamin D deficiency has been linked to increased risks of infectious diseases, cancer, autoimmune diseases, heart disease, cognitive decline, Parkinson’s disease, asthma, mood disorders and even diabetes. Is there biological evidence to show how vitamin D could influence so many conditions?
Sure. For example, we know that immune cells called macrophages activate vitamin D, which causes cells to make defensin proteins that specifically kill infective agents like tuberculosis bacteria. A Japanese study recently found that children receiving 1,200 IU of vitamin D each day reduced their risk of getting the flu by almost 50 percent. Every tissue and every cell in the body has a vitamin D receptor protein. It’s estimated that upwards of 2,000 genes are directly or indirectly regulated by vitamin D.

Have there been clinical trials showing the utility of vitamin D?
Absolutely. For instance, a trial of postmenopausal women showed that taking vitamin D over four years reduced their risk of cancer by 60 percent.

Since we make vitamin D from sun exposure, don’t our levels fluctuate with the seasons?
Only to a small degree, because most people avoid the sun. And sunblock reduces your ability to make vitamin D through your skin. The average levels we found ranged from 22 nanograms per milliliter of blood at the end of winter to about 28 ng/ml by the end of summer, unless you’re a tennis player. Blacks start out at 13 to 15 ng/ml during the winter and go up to 22 ng/ml. Less than 30 ng/ml is insufficient vitamin D, and less than 20 ng/ml is considered a deficiency.

But don’t dermatologists counsel people to limit their sun exposure?
Moderation in all things. Humans evolved in sunlight. Our hunter-gatherer forebears were making thousands of units of vitamin D every day, and our body has adapted to that need. That’s why we think vitamin D research will have sustainability and won’t go up in flames like vitamins C and E.

How do you see vitamin D use changing in the future?
It’s likely that the [National Academies’] Institute of Medicine will come out with recommendations of at least 800 to 1,000 IU a day for adults and also substantially increase the tolerable upper limits. That’s what influences what manufacturers can put in foods. Right now the upper limit is 2,000 IU a day. I would predict that within a couple of years many more foods [besides milk] such as pasta and bread will be fortified with vitamin D. I’ve estimated that across the board you could reduce health care costs by 25 percent by food fortification. Supplements are great, but nobody remembers to take them every day. As for sun exposure, a little bit is great for you, but you can only do it spring, summer and fall. So you really need vitamin D in a place where everyone would be able to get it, and that’s the diet.

To what extent does the medical community agree with you on vitamin D?

I recently gave a talk in Toronto to internal medicine doctors and asked how many of them were “believers,” like it was a religious revival. And 90 percent raised their hands. The test for vitamin D levels is now the most ordered assay by doctors in the United States.