Boning up on calcium shouldn't be sporadic.

Calcium supplements can preserve and even build bones in the large share of the population that steadfastly consumes diets deficient in the mineral. Yet two new federal analyses indicate that the gains can quickly disappear when people stop taking extra calcium.

Building strong bones in childhood and maintaining them through adulthood offer the best protection against osteoporosis and debilitating fractures in old age, explains endocrinologist Bess Dawson-Hughes of the Agriculture Department's Human Nutrition Research Center on Aging at Tufts University in Boston.

Data from children and the elderly, however, show a consistent and disconcerting picture, she noted this week in San Diego at the Experimental Biology 2000 meeting, where she chaired a symposium on calcium supplementation.

Even in people over age 50, who typically are losing bone, only some 60 percent of men--and less than half of women--consume the recommended daily allowance, or RDA, of calcium, she says. For teens, who need extra calcium for growth spurts, the situation is far worse. Only about one in four boys, and a mere one in 10 girls, consumes the RDA.

At the meeting, pediatrician Steven A. Abrams of the Department of Agriculture Children's Nutrition Research Center in Houston reviewed 10 studies in which scientists gave children supplemental calcium. "Without exception," Abrams says, those getting supplements built more bone than did their unsupplemented counterparts, who in one study were their twins.

Four of the projects followed children for up to 2 years after they stopped taking calcium supplements. In three of those studies, Abrams notes, the increased bone density due to supplementation diminished or disappeared. Though bone benefits persisted in the fourth study, the follow-up period was short, he notes, so it's uncertain how long gains were maintained.

Dawson-Hughes unveiled a similar follow-up for 295 men and women whom she has been following since 1992. They were part of a group of people, all 65 or older, who had been randomly assigned to consume daily for 3 years either a tablet containing 500 milligrams of calcium and 700 international units of vitamin D or a look-alike placebo.

By the end of the study, the supplemented men and women retained significantly more bone than did those getting the placebo. As importantly, Dawson-Hughes says, the supplements cut the bone's turnover--the rate at which old and potentially damaged bone was replaced or remodeled. This turnover fell by 9 percent in men and 14 percent in women.

Remodeling weakens bone, much as home renovations that replace supporting beams temporarily compromise a building's structural integrity. An area of bone undergoing remodeling doesn't get back to full strength for some 40 to 60 weeks, notes Robert P. Heaney of Creighton University in Omaha, Neb.

Indeed, Dawson-Hughes suspects that the lower fracture incidence among the supplemented participants--roughly half that of the others--stems primarily from their reduced bone remodeling.

Unfortunately, she says, even after learning of the supplements' benefits, 35 percent of the volunteers who had received the tablets stopped taking extra calcium once the study ended. By comparing their bone mass and turnover with those of the study's unsupplemented participants, researchers found that a clear picture emerged. Women retained no residual bone benefit within a year of stopping supplements, Dawson-Hughes reports. Men lost their benefits within 2 years of stopping.

She says these findings, along with those from studies of children, reinforce the view that "meeting the calcium requirement should be a lifetime commitment."

Yet data from a third new Agriculture Department study, also reported at the meeting in San Diego, suggest that today's children may be getting off to a substantially worse start in this regard than were youngsters 20 years ago.

Alanna Moshfegh and her team at the Beltsville (Md.) Human Nutrition Research Center analyzed beverage-drinking trends in three large national surveys. Between 1977 and 1998, the share of children ages 6 to 9 years who drank any milk daily fell nearly 10 percent--to 84 percent--the researchers found. Moreover, these youngsters' average daily milk consumption fell 25 percent, to 12 ounces, while juice intake rose nearly 50 percent, to 8 oz.

Most troubling, the latest data show that although 72 percent of the children who drank milk with lunch went on to meet the RDA for calcium during the day, only 40 percent of kids drinking juice or soft drinks did.

Worries Moshfegh, "If you don't develop the habit of consuming good sources of calcium when you're young, you probably won't when you're 20 or 30, either."