In Health Watch: Two new studies show millions of children across the US have low levels of Vitamin D.That means those kids are at higher risk of developing heart and bone problems.As Drew Levinson reports.. The advice from experts.. get out in the sun.Stephanie Mitzner just found out she'll miss the start of cross-country season."In terms of running, i just don't think it's safe, especially cross country the terrain is too variable"Ok"The 17-year old runner has three stress fractures.Doctors say she's not getting enough Vitamin D.Dr. Shevaun Doyle/Hopsital for Special Surgery says,"Some of the vitamin D deficient children may take longer to heal their fractures or may re-fracture and have other fractures down the road."Stephanie is one of millions of youngsters who have this problem. in fact 7 in 10 American kids don't have enough vitamin D.Weakened bones is just one of the effects.Two new studies in the Journal "Pediatrics" say these kids also have an increased risk of cardiovascular disease and diabetes.Drew Levinson, CBS news says,"Researchers blame a number of factors.. Including children not getting out in the sun and not drinking enough milk."A child would need to drink a whole *quart* a day to get enough of the vitamin. Other foods have Vitamin D, but its hard to get children to eat them.Dr. Michal Melamed, Albert Einstein Medical college says,"Things like fish have a lot of vitamin D, but I don't know about other people's kids, but my kids are not a big fan of fish. Sardines, and other types of things are not so popular."AND Lifestyles too, have to change.Experts say kids need to leave the TV and computer screens... and get outside.Dr. Michal Melamed, Albert Einstein Medical College says,"10 to 15 minutes a day for a light skinned individual is all you need to get enough vitamin D."But for darker completed kids like Stephanie it can be up to two hours and sometimes that's not enough.Ironically sunblock can limit your ability to process vitamin D.Researchers recommend small doses of sun without sunscreen every day.Drew Levinson, CBS News, New York.