Why that's bad: Vitamin D helps the body absorb calcium, so without enough of it, bones may not develop properly. Researchers are also finding strong links between low D levels and other conditions, most recently Type 1 diabetes, the second most common chronic disease in children.

What's going on: Ordinarily, most of the vitamin D we get would come from sun exposure. But the sunscreen we slather on (for good reason!) completely blocks the UVB rays that our bodies convert into D, says James Dowd, M.D., associate professor of medicine at Michigan State University in East Lansing. That leaves us to get D from food, and there aren't many kid-friendly sources. Milk and some cereals, OJs, and yogurts are fortified with D, but not all kids down enough of these. Finally, breast milk has a very low concentration of vitamin D, putting babies who are exclusively breastfed at particular risk.

The solution: Some experts are in favor of letting kids older than 6 months soak up some rays sans sunscreen -- 10 to 15 minutes three times a week at midday, except during winter, when the level of UVB rays is insufficient. That said, not all doctors or parents are comfortable with the strategy, given possible risks of sun damage.

In that case, supplementation is the other way to go. The American Academy of Pediatrics advises that kids get 400 IU of D per day, and specifically recommends a supplement for:

• exclusively and partially breastfed babies until they're drinking at least 32 ounces of milk or formula a day
• bottle-fed babies who drink fewer than 32 ounces of formula per day
• older kids who don't get 400 IU per day through other fortified foods. (To help you do the math, there are 100 IU of D in a cup of milk.) Still not sure if your child is at risk for a D deficiency? Your pediatrician can easily help you figure it out.

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