A weight-loss vaccine has shown promise in early tests. The shot suppresses the hormone ghrelin, which stimulates appetite and slows metabolism. Both obese mice and mice at healthy weights that received the vaccine had significant decreases in food intake and increases in calorie burning compared with unvaccinated mice. One reason diets fail is that they trigger a rise in ghrelin; the vaccine should block this effect, making it easier to lose weight and keep it off, says study coauthor Mariana P. Monteiro, associate professor of human anatomy at the University of Porto in Portugal. She estimates that human testing will begin in two to three years, and that the vaccine's effects could last about four years. Ghrelin has other functions in the body besides energy regulation—such as stimulating growth hormone, which plays a role in learning and memory—so, for safety, the vaccine is designed to reduce ghrelin but not completely abolish it, Monteiro adds.