From Scientific American:
New research shows that the answer may lie in serotonin, a neurotransmitter, or chemical messenger produced by nerve cells. Scientists at the University of California, San Francisco, (U.C.S.F.) report in the journal Cell Metabolism that the nerve messenger, a known appetite suppressant, not only controls whether and how much you eat but, independent of that, also plays a role in what the body does with the calories once they’re consumed. “This may mean you could develop therapeutic strategies to manipulate fat metabolism (the rate at which food is turned into energy) independently of what you eat,” says study co-author Kaveh Ashrafi, a U.C.S.F. physiologist.
Many weight-loss drugs now on the market are designed to increase serotonin levels, but they were believed to work by stemming appetite; the new research shows they may also work by speeding metabolism. That means, Ashrafi says, that treatments could be developed that target obesity, which has been linked to a slew of ills from diabetes to cancer, without necessarily suppressing appetite. Ashrafi says he launched the study to determine why some people on diet drugs regained their weight after they stopped popping them, even if they did not increase their caloric intake. “The assumption that body weight is simply a consequence of behavior is not exactly correct,” he says. “It is the combination of behavior and the organism’s propensity for what to do with nutrients it takes in,” whether to store or use them.