by Michael Smith

If you look at the world map, the shapes of South America and Africa look as though they should fit together. Maybe they were once together and then drifted apart.

That theory was first put forward in the late 1500s by a Dutch map maker, but it wasn’t until the 1960s that the discovery of plate tectonics finally put the idea on a — well, solid is the wrong word here — on a rational footing.

Now consider the flu. The seasonal variety appears in the fall and dies away in the spring and summer. In the middle of the last century, Robert Edgar Hope-Simpson, an English GP, proposed that some sort of seasonal stimulus was responsible for the waxing and waning of flu.

But what could that be? What else has that pattern? Why, sunlight, of course! As the summer ends, we get less and less sunlight, which means less and less vitamin D. And vitamin D is intimately connected with the immune system.

Presto! Simple explanation. And several people have put it forward, notably John Cannell, MD, of Atascadero State Hospital in Atascadero, Calif., and colleagues.

But like the theory of continental drift, solid evidence is required.

Some of that is perhaps now emerging. Earlier this month, in the American Journal of Clinical Nutrition, Japanese researchers reported a randomized clinical trial in schoolchildren that seems to support the idea.

In the journal, Mistuyoshi Urashima and colleagues said they had shown a 42% reduction in the risk of influenza A among children taking 1,200 IU of vitamin D₃ supplements a day, compared with those getting a placebo. The effect was even more marked when children who were not taking other vitamin D supplements were considered — a 64% reduction in risk. Both reductions were significant, at P=0.04 and P=0.006, respectively.

So, case closed, right?

Well, perhaps not quite. There was no effect on influenza B, which may imply that something is missing in the theory. Or it could just be that influenza B is less common than A.

Either way, though, it’s evidence that boosting the immune system with vitamin D has beneficial effects. And it may mean that the flu takes advantage of a seasonal deficiency in the vitamin — a kind of seasonal drift.

Michael Smith is the North American Correspondent at MedPage Today and blogs at In Other Words, the MedPage Today staff blog.

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