A hormone-like substance could be added to babies’ milk or given to their mothers during pregnancy to stop them becoming obese as adults.

Scientists gave pregnant and lactating rats food laced with leptin and found that their offspring did not put on weight no matter what they ate.

The theory is that leptin given at this crucial stage in life “hard-wires” the body’s energy-balance settings.

The more leptin they are given the more inefficient the infant rats’ bodies are at turning calories into fat. Instead, they burn it up metabolically.

Leptin was greeted as the cure for obesity when it was discovered a decade ago. Produced by fat, it tells the brain when fat deposits are adequate, and thus discourages eating. However, leptin injections benefit very few obese people.

Dr Mike Cawthorne of the University of Buckingham said that breast milk contained leptin but formula feed did not, and foods fortified with leptin should be available soon.

But Professor Steve O’Rahilly, a leptin expert from Cambridge, said: “There is no evidence that this ‘early life’ imprinting effect of leptin is at all relevant for humans.”

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