Stories about: energy expenditure

A mutation and a mystery: Weight gain without a change in energy use

Why did the mouse at left gain so much weight?

Two mice scurry around in an enclosure crossed through with light beams. The beams track their movement to measure their energy expenditure, along with the amount of oxygen they breathe in and carbon dioxide they exhale. The mice, who are siblings, are equally active and are held to the same diet, but there’s one critical difference: One mouse is noticeably heavier than the other.

“These [heavier] mice aren’t burning the fat,” says Joseph Majzoub, MD, chief of endocrinology at Boston Children’s Hospital. “They’re somehow holding onto it.”

In fact, the mice have to be underfed by 10 to 15 percent just to stay as slim as their siblings. Their experiences seem to parallel those of people who complain of gaining weight even when they don’t eat more than others. When allowed to eat as much as they want, the mice quickly begin to eat three to four times as much as the others and balloon to more than twice their size.

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