San Diego ? Scientists at Scripps Research Institute in San Diego have developed an experimental anti-obesity vaccine that allowed rats to feast on dry pet chow without getting fat.

The vaccine reduced levels of ghrelin, a hormone found in mammals that regulates the consumption and storage of fat, according to the study published today in the Proceedings of the National Academy of Sciences.

The rats received up to four shots over nine weeks. The vaccine appeared to be long-acting: One week in a rat’s life is the equivalent of one year for humans, the researchers said.

The experiment opens the possibility of a vaccine that could one day help people lose weight.

Since the discovery of the hormone in 1999, scientists have been searching for a potential drug that would block ghrelin to help reduce weight.