Researchers at the Scripps Research Institute in the USA say they have developed a vaccine that – if results in animals are reproduced in humans - could prevent obesity by raising people's metabolic rate, even if they do not alter their food intake.

The vaccine raises antibodies against ghrelin, a hormone that slows down metabolism and stimulates hunger.

In the study, vaccinated rats put on less body weight than control rats that were fed exactly the same diet, with the effect in proportion to the level of antibody expression. Both groups seemed to eat and drink the same amount, which the researchers believe is evidence that the vaccine is working by boosting metabolism, and the reduction in weight gain resulted from less fat being laid down.

“To have an impact on appetite and weight gain, ghrelin first has to move from the bloodstream into the brain where, over long periods, it stimulates the retention of a level of stored energy as fat,” said study leader Jim Kanda.

“Our study is the first published evidence proving that preventing ghrelin from reaching the central nervous system can produce a desired reduction in weight gain.”

The team stressed that much more work must be done before this immunopharmacological effect will be a viable proposition to test in humans, but added that this is the ultimate aim.

One issue identified by the team is that the rats were fed a low-enery. Low-fat diet and were fairly lean. “Whether active immunisation against ghrelin would help prevent the development of obesity caused by … high-fat ‘Western’ diets, or would facilitate weight loss once obesity is established” remains uncertain, they noted.

The ghrelin vaccine produced by Scripps Research scientists is not the only one being tested. Cytos, a Swiss-based biotechnology company, is testing a ghrelin-based vaccine in a combined Phase I/II study with 112 obese patients.

The Cytos vaccine produces antibodies that inhibit the uptake of ghrelin by the brain, but the Scripps team maintains that its approach, which targets only the active form of the hormone and is also more water-soluble which should make formulation easier, could be a better candidate.

Meanwhile, other companies are developing non-vaccine based approaches to blocking ghrelin. For example, Tranzyme Pharmaceuticals, which is developing small-molecule blockers of the hormone for obesity, all of which are in early-stage drug discovery.