Resisting another crisp from the bag is not just a matter of overcoming greed – scientists have now hit on a theory which explains why it is impossible to have “just one more”.

They found that fats in certain foods trigger a biological mechanism which causes the body to produce natural marijuana-like chemicals, driving gluttonous behaviour.

This means that snacks such as crisps or French fries become effectively irresistible.

The study by researchers at the University of California, Irvine, discovered that when rats tasted something fatty, cells in their upper gut started producing the chemicals, called endocannabinoids.

Louise Turner Arnold, the university’s chair in neurosciences and professor of pharmacology, said: “This is the first demonstration that endocannabinoid signalling in the gut plays an important role in regulating fat intake.”

The team, led by Daniele Piomelli, found the process starts on the tongue, where fats in food generate a signal that travels first to the brain and then to the intestines.

The signal stimulates the production of the chemicals, which initiates a surge that prompts the wanton intake of fatty foods, probably by releasing digestive chemicals linked to hunger, Dr Piomelli said.

Dr Piomelli, director of the university’s centre for drug discovery and development, said: “From an evolutionary standpoint, there’s a compelling need for animals to consume fats, which are scarce in nature but crucial for proper cell functioning.

“In contemporary human society, however, fats are readily available, and the innate drive to eat fatty foods leads to obesity, diabetes and cancer.”

The findings suggest it might be possible to curb this tendency by obstructing endocannabinoid activity – for example, by using drugs that clog receptors.

The study results appear this week in the online edition of Proceedings of the National Academy of Sciences.