Eating just one chip or crisp without then devouring the rest is almost impossible for most people to do.

Now scientists have worked out why it's so difficult to walk away from tasty but unhealthy food.

The fats in these snacks make trigger a surprising biological mechanism that likely drives our gluttonous behaviour.

The culprit is natural marijuana-like chemicals in the body called endocannabinoids, researchers from University of California, Irvine found.

They discovered that when rats tasted something fatty, cells in their upper gut started producing endocannabinoids. Sugars and proteins, the researchers noted, did not have this effect

The process starts on the tongue, where fats in food generate a signal that travels first to the brain and then through a nerve bundle called the vagus to the intestines.

There, the signal stimulates the production of endocannabinoids, which initiates a surge in cell signalling that prompts the wanton intake of fatty foods, lead researcher Daniele Piomelli said.

This most likely occurs by initiating the release of digestive chemicals linked to hunger and satiety that compel us to eat more.

Professor Piomelli said that 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' cannabinoid receptors.

Since these drugs wouldn't need to enter the brain, they shouldn't cause the central side effects - anxiety and depression - seen when endocannabinoid signalling is blocked in the brain, Professor Piomelli added.

The study is published in the journal Proceedings of the National Academy of Sciences