For any dieter prone to snack attacks and a growling stomach, a shot in the arm might soon be the answer. Cytos Biotechnology, based in Switzerland, is testing a vaccine for obesity that stimulates antibodies to the hormone ghrelin, which is released in the stomach and travels to the brain, where it raises the "Feed me!" alarm. Vaccinated mice gained 15 percent less weight than untreated littermates when they were all given a high-fat diet. Now the shot's being tried on 112 obese humans. Ghrelin levels usually increase after successful dieting, so if the vaccine works, it may be particularly useful to people who lose weight but have trouble keeping it off.