Too little milk, sunshine and exercise: It's an anti-bone trifecta. And for some kids, shockingly, it's leading to rickets, the soft-bone scourge of the 19th century. But cases of full-blown rickets are just the red flag: Bone specialists say possibly millions of seemingly healthy children aren't building as much strong bone as they should - a gap that may leave them more vulnerable to bone-cracking osteoporosis later in life than their grandparents are. "This potentially is a time-bomb," says Dr. Laura Tosi, bone health chief at Children's National Medical Center in Washington. Now scientists are taking the first steps to track kids' bone quality and learn just how big a problem the anti-bone trio is causing, thanks to new research that finally shows just what "normal" bone density is for children of different ages. Dr. Heidi Kalkwarf of the Cincinnati Children's Hospital led a national study that gave bone scans to 1,500 healthy children ages 6 to 17 to see how bone mass is accumulated. The result, published last summer: The first bone-growth guide, just like height-and-weight charts, for pediatricians treating children at high risk of bone problems. Next, the government-funded study is tracking those 1,500 children for seven more years, to see how their bones turn out. Say a 7-year-old is in the 50th percentile for bone growth. Does she tend to stay at that level by age 14, or catch up to kids with denser bones? If not, if she more prone to fractures? Ultimately, the question is what level is cause for concern. "I don't know if we're raising a population that's going to be at risk" for osteoporosis, Kalkwarf cautions. "It's really hard to know what the cutoff is, how low is too low." But almost half of peak bone mass develops during adolescence, and the concern is that missing out on the strongest possible bones in childhood could haunt people decades later. By the 30s, bone is broken down faster than it's rebuilt. Then it's a race to maintain bone and avoid the thin bones of osteoporosis in old age. "There's some early data showing that even a 10 percent deficit in your bone mass when you finish your adolescent years can increase your potential risk of having osteoporosis and fractures as much as 50 percent," says Dr. James Beaty, president of the American Academy of Orthopaedic Surgeons. Already there's evidence that U.S. children break their arms more often today than four decades ago - girls 56 percent more, and boys 32 percent more, according to a Mayo Clinic study. Kalkwarf's hospital recently found that kids who break an arm have lower bone density than their playmates who don't. That suggests the fracture rise isn't due solely to newer forms of risky play, like inline skates. And last year, government researchers found overweight children were more likely to suffer a fracture, even though theoretically their bones should be hardier from carrying more weight. Maybe they have poorer balance; maybe they fall harder. Kalkwarf says there even are hints that fat itself may produce bone-harming substances. Doctors have long known that less than a quarter of adolescents get enough calcium. But strong bones require more than calcium alone. Exercise is at least as important. Consider: The dominant arm of a tennis player has 35 percent more bone than the non-dominant arm. And Canadian researchers recently reported that postmenopausal women who had exercised more as teens had 8 percent stronger bone decades later than their more sedentary counterparts. Yet childhood exercise is dropping as obesity rises. Likewise, the body can't absorb calcium and harden bones without vitamin D. By some estimates, 30 percent of teens get too little. It's not just that they don't drink fortified milk. Bodies make vitamin D with sunlight. With teen computer use, urban youngsters without safe places to play outdoors and less school P.E., it's no wonder D levels are low. Because skin pigment alters sun absorption, black children are particularly at risk. Rickets marks the worst deficiency, where bones become so soft that legs literally bow. Rickets was once thought to have been eradicated with milk fortification, but "I am now treating rickets in a way that I never treated it 20 years ago," says Tosi, who diagnoses rickets or super-low D levels in children every month at a bone clinic she runs for mostly inner-city children. Doctors who've never seen rickets can miss it. Charlene Bullock repeatedly asked her 5-year-old's doctor why his leg was bending inward and he could no longer run with his playmates. It took a trip to Tosi's special clinic to learn Na-shun had rickets - the once energetic child had quit running because his bones ached like an old man's. Fortunately, rickets caught early is easily cured with high-dose infusions of vitamin D and calcium, and Bullock's son quickly rebounded. "He's doing everything with that little leg." It's the kids whose low vitamin D hasn't gotten quite bad enough for symptoms that Tosi most worries about. They may never get treated.