An obesity vaccine has shown promising results in a small animal study, according to a report in today's Proceedings of the National Academy of Sciences. The vaccine targets the hormone ghrelin, which is produced in the stomach and plays a role in hunger and weight gain. Rats given the vaccine, which causes the immune system to attack and break down the hormone, eat the same amount of food as other rats but gain less weight. (Here's the press release from the Scripps Research Institute, which conducted the study.)

While scientists have successfully identified several hormones involved in appetite, targeting those molecules to create successful weight-loss treatments has proven difficult. Ghrelin, for example, has complex effects: mice given it tend to eat more, but those who lack the hormone have normal appetites.

It's unclear if the current results will translate to humans, who likely have more a complex appetite control system. But we may find out soon. Swiss biotech company Cytos is currently testing another type of ghrelin vaccine in humans.  -- By Emily Singer