BEIJING, Nov. 27 (Xinhuanet) -- It was the dreaded soft-bone disease of the 19th century, but now American bone specialists say possibly millions of seemingly healthy children are not building as much strong bone as they should, which leaves them susceptible to rickets.

    Why? Too little exercise, milk and sunshine that may leave them more vulnerable to bone-cracking osteoporosis later in life than their grandparents are.

    Already there's evidence that U.S. children break their arms more often today than four decades ago ¡ª girls 56 percent more, and boys 32 percent more, according to a Mayo Clinic study.

    "This potentially is a time-bomb," says Dr. Laura Tosi, bone health chief at Children's National Medical Center in Washington.

    Dr. Heidi Kalkwarf of the Cincinnati Children's Hospital led a U.S. 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."

    (Agencies)