SCIENTISTS have vaccinated rats against a weight-gain hormone, allowing them to eat freely without getting fat, in a development that raises hopes for treatments for overweight and obese humans.
The vaccine operates against ghrelin - a recently discovered hormone among several which appear to regulate metabolism.
Ghrelin is thought to be implicated in a problem that frustrates many dieters: the tendency for weight to be regained even after successful dieting.
Researchers believe the phenomenon would have conferred an evolutionary advantage among early humans, whose food supply was irregular - ensuring maximum use of available calories during times of famine and programming the body to return quickly to its highest weight.
But for modern people it may mean that if the body is overfed it resets itself to remain that way.
The scientists, from the Skaggs Institute for Chemical Biology in California, vaccinated male rats against several forms of ghrelin, which is produced in the stomach. Two of the three vaccines they tried prevented weight gain in the rats, which were allowed to eat freely and consumed just as much as unimmunised equivalents.
As well, the vaccinated rats retained a higher level of muscle mass compared with fat - suggesting the vaccine was directly affecting the animals’ metabolism rather than just their appetite.
The study leader, Dr Kim Janda, said the rats that mounted the strongest immune response to the vaccine were those who later had the lowest amounts of ghrelin in their brain, suggesting weight can be controlled if ghrelin can be kept out of the central nervous system.
“The results demonstrate a proof of the principle that active immunisation against ghrelin can be used to control weight gain and adiposity in mammals,” Dr Janda wrote in the journal Proceedings of the National Academy of Sciences.
Professor John Shine, the executive director of Sydney’s Garvan Institute of Medical Research, said the US study was a significant advance, but it was unlikely that immunising against a single hormone would prove a long-term solution to weight gain.
“The whole issue of regulation of appetite and obesity is obviously a very complex jigsaw puzzle, involving complex feedback mechanisms,” he said. If ghrelin were blocked in humans, other weight-gain chemicals might become dominant in its place.
Professor Shine, whose institute is also investigating obesity hormones, said it would be dangerous to trial anti-ghrelin vaccines in humans when the hormone’s role in the body was incompletely understood.
But the knowledge gained from the rat study was more likely to translate into short-acting pharmaceuticals than into a vaccine for humans.