That lucky friend of yours — the one who barely gains weight when eating the same things you eat, even though your waistline expands — might have a hormone called orexin to thank.

A new study shows that mice deficient in orexin, which is produced in the brain, gained more weight when fed the same high-fat diet as mice that weren't deficient in the hormone.

Supplementing this hormone may be one way to help people lose weight, the researchers suggested, though the new findings are preliminary.

The hormone helps stave off weight gain because it's involved in the body's production of brown fat, which burns calories rather than storing them as white fat does, the study showed.

"Without orexin, mice are permanently programmed to be obese. With it, brown fat is activated and they burn more calories," said study researcher Devanjan Sikder, an assistant professor in Sanford-Burnham Research Institute, in Lake Nona, Fla.

The study is published today (Oct. 4) in the journal Cell Metabolism.

Orexin staves off weight gain, boosts metabolism

 The researchers compared normal mice with mice engineered to lack orexin. When fed a high-fat diet for six weeks, the orexin-deficient mice increased their body weight by 45 percent, while the normal mice plumped up by just 15 percent.

This increased weight gain happened even though the orexin-deficient mice ate less of their food than the normal mice did, the study showed.

The researchers hypothesized that orexin-deficient mice were somehow expending less energy. The found that after eating the high-fat diet, the normal mice's metabolic rate rose 13.5 percent, while the orexin-deficient mice showed no such increase in metabolism.

The researchers collected samples of brown fat from between the mice's shoulder blades, and found that the brown fat of the orexin-deficient mice was immature compared to that of the normal mice. For example, genes whose expression was cranked up in the normal mice's brown fat were tamped down in the orexin-deficient mice.

"Our study provides a possible reason why some people are overweight or obese despite the fact that they don't overeat," Sikder said.

Orexin in people

 Orexin deficiency is known to exist in people with narcolepsy, and previous studies have linked low levels of orexin and obesity in people, the study said.

Measuring the activity of brown fat in people with orexin deficiencies could show whether the same mechanism is at work in people, the researchers wrote.

However, because the hormone also seems to simulate wakefulness, supplementing orexin could make it difficult to sleep, the researchers cautioned.

Pass it on: A deficiency in the brain hormone orexin may contribute to some people's weight problems.