It was a chance question by our pediatrician that got us thinking about Vitamin D: How many cups of milk does your little guy drink a day? Probably two cups a day if we’re lucky.

But it turns out that could be a problem. If your child doesn’t eat huge amounts of fish or cod liver oil (which ours don’t), two cups of milk a day isn’t going to do it for Vitamin D. In 2008, the American Academy of Pediatrics upped its Vitamin D recommendation to 400 international units per day (up from 200 per day), the amount you would get from four cups of Vitamin D- fortified milk. (Yes, you get Vitamin D from sunshine, but it is hard to quantify, and many kids don’t spend enough time in the sun to make up the difference.)

So in other words, unless you’re a big milk drinker, it’s hard to get enough Vitamin D in your diet. In fact, one recent study found that at least one in five kids don’t get enough. And a lack of Vitamin D can put you at risk for weak bones, rickets, or even some cancers and other serious diseases.

On the flip side, you often hear that kids shouldn’t have too much milk, because it can interfere with iron absorption. So you need to get to that magical four-cup-a-day goal… but not an ounce over, or else you’re headed for problems in another direction. We’re doing research on Vitamin D supplements now, and now that the issue is on our radar, we’re going to keep an eye out for more Vitamin D studies. Now pass the cod liver oil.