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No Bones About Benefits of Exercise

Health: A jumping program, combined with the hormones of early puberty, can strengthen a child's skeleton, researchers discover.


WASHINGTON — Something as simple as jumping off and onto low platforms can make a child's bones stronger, and it can be accomplished in as little as 10 minutes three times a week, researchers say.

The exercise program requires nothing more high-tech than platforms 4 to 20 inches in height.

"It can be done very easily and it can be done at low cost," said researcher Heather McKay of the University of British Columbia. "Ten minutes, three times a week, isn't too long."

McKay and her colleagues looked at the interaction of exercise and the hormonal torrents that start around puberty. Either exercise or hormones can trigger bone growth. But the researchers say the combination creates optimum conditions -- a window of opportunity -- to soak up calcium and lay down denser bone.

The researchers focused on the years in which human growth hormone and sex hormones surge. This happens in girls typically from ages 10 through 12, with growth hormone levels rising first; in boys, who mature later, the same process takes place from ages 12 through 14.

The gush of hormones creates the growth spurt in which bones get longer and thicker. The benefits of exercise also peak during the spurt years and taper off as the hormone surge winds down, researchers say.

McKay and her colleagues offered a jumping program to 68 children in some schools in British Columbia and summarized the benefits in the August issue of the British Journal of Sports Medicine.

After seven months, girls in early puberty gained about 3% more bone in the lower spine than similar girls who did not participate in the program, the study found. The data on boys have not yet been analyzed.

Because her program required only 10 minutes three times a week, the exercise could be done in any classroom. "We wouldn't even need a gym," McKay said.

About a quarter of the adult skeleton develops during the two-year hormonal window. The additional benefit of exercise can add to bone strength that might someday retard the development of osteoporosis in the older years, she said.

However, bone that is not stressed in exercise can lose density, so the children will have to make exercise a lifetime habit to keep the benefit, McKay said.

Evidence is mounting that exercise in early-to mid-puberty gives a greater gain than the same type and amount of activity would in later years, said Dr. Laura K. Bachrach, a researcher at Stanford University who was not part of the McKay team.

McKay is on the right track, but her program of 10 minutes of jumping three times a week should be tested more broadly, Bachrach said. McKay hopes that schools in British Columbia will try it.

And it takes more than hormones and exercise to build bone strength; it takes calcium, said Babette Zemel, director of the nutrition and growth laboratory at the Children's Hospital of Philadelphia.

Children do not get enough calcium. "When they need the most calcium, ages 9 to 14, that's when calcium intake is decreasing," she said.

Boys and girls ages 9 to 13 should get about 1,300 milligrams of calcium a day, but they get around 800, Zemel said. Children should drink more milk, which has about 300 mg of calcium in a glass, or eat more yogurt, with 200 to 250 mg in a four-ounce container.

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