Calcium may become a dieter's best friend.

There's encouraging news for people who've been losing the battle of the bulge. Weight loss may be at hand--if that hand begins reaching for a glass of milk, slice of cheese, or dish of yogurt, all low-fat, of course.

At the Experimental Biology 2000 meeting last week in San Diego, scientists from the University of Tennessee in Knoxville reported dramatic findings from a weight-loss study in mice. How much calcium the animals consumed--and its source--greatly affected what share of their meals turned to fat.

Reanalysis of data collected earlier on women supports that finding, another scientist adds.

The Tennessee team used mice that model human patterns of obesity. The animals had been genetically engineered to express in their fat cells a gene called agouti, which normally operates in human but not mouse fat cells. This gene strongly influences whether a fat cell burns energy-containing molecules or converts them to fat.

Michael B. Zemel, who directs the university's Nutrition Institute, and his colleagues put these mice onto a low-calorie diet for 6 weeks. Their meals contained just 70 percent as much energy as the rodents would normally choose to eat. One group received a diet that was also deficient in calcium. Its calcium content, adjusting for species differences, is "almost exactly what American women are consuming," Zemel notes, "about 500 milligrams per day." That's well below the recommended daily allowance of 1,300 mg calcium.

The calorie-restricted mice lost 8 percent of their body fat and 11 percent of their weight.

Zemel's group again restricted the food but boosted calcium intake of another two groups of the mice. Each received the mouse equivalent of a human dose of 1,600 mg calcium per day. Mice getting this as a carbonate supplement lost 42 percent of their body fat and 19 percent of their weight. Those that consumed the extra calcium as nonfat dry milk--substituted for an equal amount of dietary protein--lost 60 percent of their body fat and 25 percent of their weight.

A fourth group, receiving twice as much dairy-derived calcium, showed little extra benefit, Zemel notes.

These differences occurred even though all of the low-calorie groups got the same exercise and mix of dietary fat, protein, and carbohydrates. The results show that varying dietary calcium alters the animals' metabolism, says Zemel. Among the dieting animals, core body temperature--a measure of basal energy use--fell during the low-calcium diet but climbed with the high-calcium chow.

Under low-calcium conditions, the Tennessee scientists find, the agouti gene directs calcium channels to open. "That turns out to be a bad thing," Zemel says, because it activates fat synthesis while suppressing fat breakdown.

Zemel's group is now testing whether a 6-month augmentation of dietary calcium will offer similar weight-loss benefits to obese women.

"I'm impressed by this," says Robert Marcus of the Veterans Affairs Medical Center in Palo Alto, Calif., referring to the mouse data reported last week.

When endocrinologist Robert P. Heaney of Creighton University in Omaha, Neb., first learned of preliminary data by Zemel's group last year, "I thought they made sense--but I still had a degree of skepticism," he says. So, he reanalyzed data from five calcium-supplement trials he had conducted in people over the years.

"And in all five," he says, "we found a significant weight effect that we had ignored." These data, to be published soon, show that women consuming the least calcium weighed the most.

Ironically, Zemel says, among weight-conscious teens, "the first thing they jettison from their diet is dairy." This choice, he suspects, is "moving them farther from their goal, not closer."