WASHINGTON — Scientists said Monday they have "vaccinated" rats against becoming obese, causing the animals to produce antibodies against a hormone that stimulates hunger and fat storage.
Although rats that received the vaccine against the hormone, known as ghrelin, did not eat less than unvaccinated control animals, they gained less weight and produced less fat tissue, the scientists reported.
The researchers suggested the work could eventually lead to a vaccine to control obesity in humans, which have the same hormone.
However, reaction among other scientists was tempered by evidence that ghrelin has many other functions, possibly including a role in learning and memory formation that could make it useful in combating Alzheimer's disease.
"Ghrelin's really controversial," Kim Janda, a chemistry professor at Scripps Institute in San Diego, said in a telephone interview. ". . . We'll have to do a number of other basic studies to sort things out before we run off and start treating humans for obesity or anything like that."
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Janda and other researchers at Scripps and Osaka City University in Japan reported on the vaccine experiment Monday in the Proceedings of the National Academy of Sciences.
Many animals produce ghrelin in the digestive tract, and high levels appear to send signals that tell the brain to produce a feeling of hunger.
Rats vaccinated against a synthetic form of ghrelin did not eat less than animals that were not vaccinated, but they gained less weight. When they were autopsied, they had less fat tissue relative to lean tissue, the scientists reported.
"We're not claiming that our study answers the question of obesity treatment once and for all," Janda said. "What we are saying — and what our study confirms — is that this looks like a serious workable solution to the problem."
When ghrelin was first discovered six years ago, scientists speculated that it worked simply as a switch to turn hunger on and off. When a person or animal is full, ghrelin production drops, then goes up when the digestive tract does not contain food.
Since then, studies have shown that when overweight persons diet, the body produces high levels of ghrelin to slow down fat metabolism.
"These initial results are extremely positive," Janda said. "Right now it appears that active vaccination against ghrelin is one avenue that can slow weight gain and fat build-up in the body."
Since it was first linked to hunger, appetite and obesity by researchers at the University of Cincinnati in 1999, ghrelin has been the subject of hundreds of experiments and has become increasingly controversial.
"I'm always suspicious of approaches like this," said Stephen Benoit of the Cincinnati university's Obesity Research Center.
"Our recent paper suggests that hunger might not be the only important function of ghrelin," he said, adding that "eliminating ghrelin may have undesirable side effects."
In March, Benoit and other scientists at the University of Cincinnati and Yale University reported that animals bred without the gene that produces ghrelin performed significantly worse on learning tests.
