Call it Montezuma's Revenge, traveler's trot or just a real pain, diarrhoea may do at least one good deed: protecting people against colon cancer, researchers report.

The study, published in this week's issue of the journal, Proceedings of the National Academy of Sciences, offers a possible explanation for why people in developing countries seem less prone to colon cancer, the fourth leading cause of cancer in the world and the third biggest in the United States.

Dr Giovanni Pitari and colleagues at Thomas Jefferson University in Philadelphia, USA, focussed on one of many bacteria that can cause diarrhoea. Worldwide the ailment kills two million children a year, but adults develop a partial immunity. And while colon cancer is common in the developed world, it is rarer in poorer countries.

"Over half a million patients suffer from colorectal cancer in industrialised nations, yet this disease exhibits a low incidence in underdeveloped countries," Pitari wrote.

The toxin produced by the bacteria - a nasty strain of the otherwise common Escherichia coli bug - irritates the lining of the bowel, but may also interact with cells in a way that prevents them from becoming cancerous, the researchers said.

The researchers compared the incidence of 'traveler's diarrhoea' against that of colon cancer, and showed that in countries where 'Bali belly' is most common, including the Indian subcontinent, Africa and Mexico, colon cancer was increasingly less common.

Where colon cancer is highest - Australia, New Zealand, the United States and western Europe - traveller's diarrhoea is rare.

Pitari's team analyzed the toxins produced by E. coli, first adding them to a dish full of dividing cancer cells. The cells, which like most cancer cells had been growing and dividing rapidly, slowed their pace to a crawl.

They then tested the toxin's precise action on cells and found it acted like guanylin and uroguanylin - two compounds that naturally limit the growth of colon cancer cells.

The finding not only helps explain why colon cancer is a disease of rich nations, it may also offer new approaches to treating or preventing the disease, Pitari's team said.

Other approaches are known to help prevent colon cancer - such as a diet rich in fruits and vegetables, regular exercise and not smoking. Eating fruits and vegetables helps prevent damage to DNA in cells, while smoking can damage DNA and help cells become cancerous.

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