WASHINGTON — 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, in which bones become so soft that legs bow, 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,” said 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, was the first bone-growth guide 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 cautioned. “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.

Building strong bones

The body can’t absorb calcium and harden bones without vitamin D.

Calcium: Young children should consume about 800 milligrams of calcium a day. But between ages 9 and 18, when bone growth speeds up, that requirement almost doubles to 1,300 mg. That’s about three cups of fat-free or low-fat milk plus additional calcium-rich foods, such as broccoli, cheese, yogurt, or calcium-fortified orange juice.

Vitamin D: Children and adolescents need at least 200 international units of vitamin D. Milk and orange juice often is fortified with the vitamin. Sunlight is a major source. About 10 to 15 minutes of sun exposure weekly is enough for many children, although skin pigmentation alters sun absorption so black children need more. Babies who are breast-fed only and older children at risk for vitamin D deficiency should receive supplements.

Associated Press