My 3-year-old drinks milk, but not very much of it, and that could be a problem for her.

We live in the Northeast, where the sun's rays are weaker. And now that the days are shorter, my kid arrives at day care a few hours after sunrise and leaves when it's dark. That all made me wonder whether she's getting enough vitamin D, and if not, what we should be doing about it.

Why should I, or anyone else, care about vitamin D? It's essential to building strong bones during childhood and adolescence. Children who are extremely low in the nutrient can develop rickets, which is characterized by soft, weak bones. Some research has tied low levels of vitamin D in active teen girls with higher rates of stress fractures. There have been a host of studies associating low vitamin D levels with other chronic diseases and ailments, but the Institute of Medicine, or IOM, says there's no direct evidence that low levels cause those problems.

It turns out that given my daughter's demographics, there's a good chance she's low on D. She hasn't been tested. The U.S. Preventive Services Task Force last year recommended against routinely screening everyone for vitamin D deficiency, saying there isn't enough evidence to assess the benefits and harms. But there are some data to suggest that a significant percentage of children aren't meeting the floor of 20 nanograms of 25-hydroxyvitamin D per milliliter of blood serum that the IOM says is sufficient for most people to maintain bone health. Some medical groups argue for a higher bar of 30 ng/mL.

A study published in 2008 looked at infants and toddlers visiting a primary care clinic in Boston. Of the 380 kids studied, 12 percent had levels below 20 ng/mL, and 40 percent below 30 ng/mL. Research has shown that an even greater percentage of teens are deficient in vitamin D. That's likely because they drink less milk and spend less time playing outside than younger kids, says Deborah Mitchell, a pediatric endocrinologist at Massachusetts General Hospital.

To maintain sufficient vitamin D levels in the blood, the IOM recommends taking in 400 international units, or IU, of vitamin D per day in the first 12 months of life and 600 IU for older kids. (The American Academy of Pediatrics supports those recommendations.)

But it can be hard to get that much D through diet alone. The vitamin occurs naturally in salmon, mackerel, sardines and shiitake mushrooms, none of which is high on my child's list of favorites. Fortified foods like milk, some yogurts, cheese, some cereals and orange juice are available, but still may not be enough. Each cup of fortified milk or orange juice, for example, contains 100 IU of vitamin D.

Vitamin D is also known as the sunshine vitamin. Indeed, on a clear day between the months of May and October, about five minutes of daily exposure is enough to spur lighter-skinned kids into making enough of the nutrient. Kids with darker skin tones need more sun exposure.

But most parents are told to slather their children in sunscreen every time they leave the house in order to prevent future skin cancers. And even if you do let your kid get a bit more sunshine, during the winter in the northern latitudes, it's not enough. "You could be naked all day in Minnesota or Seattle and you'd make no vitamin D. The sun is just too low in the sky," says Frank Greer, professor emeritus of pediatrics at the University of Wisconsin School of Medicine and Public Health and former chair of the AAP committee on nutrition.

Given all that, "it's probably not a bad idea for anyone, realistically," to take vitamin D supplements, says Mitchell. That doesn't mean that more is always better. The IOM's recommended upper limit of D consumption for children ranges from 1,000 IU for infants to 4,000 IU for teenagers and adults.

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