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It plays a critical role in building strong bodies, but 40% of American women are deficient in vitamin D. And if they’re pregnant or breast feeding, their babies might be, too.

“I do take the supplement …”

It’s what OBGYN Dr. Julie Levitt wants to hear from her patients, especially those who breast feed.

Dr. Julie Levitt: “Breast milk is relatively poor in vitamin D content, and unless moms who are breast feeding are supplementing their vitamin D by taking their vitamin or taking additional vitamin D, then, in turn, those newborns become vitamin D deficient.”

It’s a risk seven-week old Mac won’t have to worry about. His mother is taking a supplement to make sure her son doesn’t suffer the health effects that come with a vitamin D deficiency.

Dr. Julie Levitt: “The lack of good bone growth and their relative risk of developing blood pressure issues and glucose control problems, asthma and other infectious conditions, does persist.”

Sunlight is one way to get your dose – just 15 minutes a day does the trick — but that source is scarce in the winter. This time of year, supplements are key.

Dr. Julie Levitt: “600 to 800 international units (IU) a day. If they contain 400 up to 1000 then that’s fine.”

And so is diet. The best food sources are oily fishes like salmon and sardines — cod liver oil, too.

Dr. Julie Levitt: “Vitamin D is involved with absorbing calcium from our blood stream. What we eat that contains calcium is brought into our circulation by vitamin D, so it’s a necessary cofactor for building bones but also keeping them strong.”

Researchers at Northwestern are studying the impact of vitamin D deficiency in infants with the help of a grant from the Friends of Prentice – a volunteer fundraising group.