It has not been clear how our bodies manage this trick.

Some years ago, scientists did discover one of the likely mechanisms, which involves leptin, a hormone released by fat cells. In broad terms, when people add fat, they produce more leptin, which then jump-starts processes in the brain that reduce appetite and should cause their bodies to drop that new weight.

But obviously this system is not perfect or no one would hold on to added pounds.

So for the new study, which was published this month in Proceedings of the National Academy of Sciences, an international group of researchers began to wonder whether there might be other processes at work.

To find out, they first gathered groups of mice and rats. They chose both species, hoping that, if any results were common to each, this might indicate that they also could occur in other mammals, including, potentially, us.

Then the scientists implanted tiny capsules into each rodent’s abdomen. Some contained weights equaling about 15 percent of each animal’s body mass. Others were empty.

In effect, some of the animals had just gained a big chunk of weight.

The scientists then left the rodents alone to deal with these added ounces as they would. And their bodies quickly went to work. Within two days, the animals containing the weighted capsules were eating less and after two weeks, had generally lost almost as much weight as the capsules contained.