Mood hormone may affect fat, U.S. study finds (Reuters)

WASHINGTON (Reuters) - A brain chemical strongly linked tomood and appetite may also directly affect fat gain, U.S.researchers reported on Tuesday.

They said levels of serotonin, the nerve-signaling chemicaltargeted by many antidepressants, may also direct the body toput down fat regardless of how much food is eaten.

"It may be one reason diets fail," metabolism expert KavehAshrafi of the University of California, San Francisco, who ledthe study, said in a telephone interview.

The findings, published in the journal Cell Metabolism,could lead to better diet drugs and treatments for diseaseslike diabetes.

Serotonin may help the body decide whether to burn offexcess calories, or store them as fat, Ashrafi said.

He worked with roundworms for his experiment but said thefindings may relate to humans. "These worms, although they aremicroscopic, they have around 20,000 genes ... and if youcompare them side by side they are about 50 percent similar tous," he said.

Genes controlling appetite, fat storage and metabolism areespecially similar, he said. The tiny worms can be manipulatedto see changes to their metabolism, appetite and weight gain.

"It has been known for a long time that increasingserotonin causes fat reduction," Ashrafi said.

"At the molecular level we are trying to understand what isthe mechanism that allows that to happen. What we discovered inthe worm is that those mechanisms can be separated from themechanisms that mediate the effects of serotonin on appetite."

The research found serotonin levels affected the worms'appetite, but they also affected how much fat the wormsaccumulated, and this was via a separate process.

If the worms detect a food shortage, their metabolismsshift and they store more fat. This could explain why somepeople get fat more easily than others -- and why dieting cancause more weight gain later.

"Different people may have similar diets, may have similarrates of physical activity, but may have very different bodyweights," Ashrafi said. "Appetite is only part of it."

But for now the remedy for excess body fat remains obvious."Nothing in our study says that good nutrition and physicalactivity are not good for you," Ashrafi said.

Simply raising serotonin levels can have seriousside-effects. The diet drug fenfluramine, which has the effectof raising serotonin levels, was pulled off the market in 1997after it caused sometimes deadly heart valve damage.