Scientists are developing an infant formula designed to be given to babies to prevent them growing up fat, it was revealed today.

The controversial research aims to supplement baby milk with a hormone that suppresses hunger.

Animal studies suggest early exposure to the hormone leptin can programme the brain to prevent over-eating throughout life.

But fears have been raised about the safety of tampering with the brains of babies in such a fundamental way.

The infant formula, together with other "child-friendly" leptin preparations, is being developed by Professor Mike Cawthorne, director of metabolic research at the Clore Laboratory, University of Buckingham.

Prof Cawthorne has already shown that giving rats leptin supplements early in life provides permanent protection against obesity and diabetes.

His study, published in the American Journal of Physiology, found even adult animals fed a high-fat diet remained slim.

Leptin is naturally produced throughout life. But the research suggests that in infancy it "hardwires" the body's energy balance settings.

It may even determine whether someone is fat or thin before birth.

Feeding the hormone to pregnant rats seems to have a life-long impact on their offspring. Animals born of leptin-treated mothers remain lean, despite being fed a fat-laden diet. In contrast those whose mothers were untreated gain weight and develop diabetes.

Prof Cawthorne argues he is only giving babies what they would normally get anyway from their mothers' milk.

"The supplemented milks are simply adding back something that was originally present," he told Chemistry & Industry magazine, which reported on the research today. "Breast milk contains leptin and formula feeds don't."

Previous studies looking at the ability of leptin to reduce hunger in human volunteers have proved disappointing.

Prof Cawthorne believes this is because they involved adults, rather than infants. Leptin was only likely to leave its stamp on the malleable brains of babies.

"You would only take this for a short time, very early in life," said Prof Cawthorne.

However other experts remain to be convinced, and have expressed concerns about safety.

Dr Nick Finer, clinical director of the Wellcome Clinical Research Facility, at Addenbrookes Hospital, Cambridge, said: "The leap to a functional food being effective or safe is enormous. All obesities result from an excess of energy intake over energy expenditure, so any treatment must have effects on either or both arms of the equation.

"Even if this approach is shown to be valid in humans, extensive clinical trials would be needed to allow the right dose to be chosen and to show that the approach was safe. The concept that adding something to a food that could permanently alter brain development is exciting but at the same time so scary that it would mean a wholly new approach about how such treatments can be tested and approved for use. Would the first trials be in newly born children?"

Dr Ian Campbell, medical director of the charity Weight Concern, said: "Without evidence that this works in humans, it is pure flight of fancy that those consuming leptin from infancy will never get fat. I'd be surprised if this product could be advertised or marketed with these extraordinary claims.

"To date, leptin has proved to be a great disappointment. Most of us have plenty and true deficiencies are rare. In fact, obese people tend to have higher than normal levels."

Professor Jonathan Seckl, a hormone expert at the University of Edinburgh, said: "We need to know whether leptin is acting pre and post-natally, figure out how it works, and dissect the possible side effects before this becomes a potential approach in humans."

Prof Cawthorne told the Press Association the infant formula work was in the "very early stages".

"It's something we're in the process of looking at," he said. "There's potential there because we know that breast-fed offspring have less of a tendency towards obesity in adult life.

"I'm not in the least suggesting that it will cure world wide obesity, but it's something that could make a difference.

"There are always safety concerns, and whenever you do anything there tend to be unexpected events. But one could argue that giving formula feeds to babies that are different from breast milk might itself be changing their programming."

He compared giving young children leptin with getting them to take cod liver oil - although he accepted that hormones were powerful substances.

Prof Cawthorne agreed that leptin was not the only natural ingredient missing from formula milk, and other factors also influenced obesity.

Whether or not infant formula with leptin should be classified as a food or medicine was a question that would have to be resolved, he said.

"It is still a grey area," he added. "One could argue that as you're replacing something that should be there, it's not pharmaceutical."

A spokesman for the Food Standards Agency said: "If you make a functional food using ingredients that are already on the market, then you wouldn't have to go through a safety assessment. But if it includes new ingredients then you would, and it depends on the sort of ingredients."

Claire Williamson, from the British Nutrition Foundation, said: "Leptin is a hormone that is synthesised in adipose (fat) tissue which appears to signal how much fat is stored, thus helping to regulate appetite.

"Researchers in the past found that when leptin was injected into specially bred mice, the mice ate less, burned food faster and lost significant amounts of weight.

"However, the precise details of the leptin system and its physiological significance in human weight control still remain to be fully understood.

"The research in rats supplemented with leptin is in its very early stages and needs to be replicated by further studies. Furthermore, results from animal studies cannot be assumed to apply to humans, so it is much too early to say whether this could be a possible way to prevent obesity in humans.

"Obesity is a complex disorder with a diverse range of causal factors, which makes it a complex and challenging disorder to tackle.

"Both genetic and environmental factors come into play in the causes of obesity. However, one incontrovertible fact is that for an individual to become obese, energy intake (i.e. energy from food) must exceed energy expenditure (i.e. through physical activity) for an extended period of time."