Adults who drank plenty of the free milk supplied at schools when they were young have a reduced risk of bowel cancer, a study has found.

A free half-pint bottle of full cream milk - 284ml - was given daily to children at school between 1937 and 1967.

The mid-morning drink was paid for by the state.

Not all children drank it. Some loved it; others hated it, especially after it had warmed in the summer sun. Now researchers at Otago University, comparing people who have bowel cancer with a non-cancer control group, have found an apparently protective effect in the school milk programme.

Writing in the American Journal of Epidemiology, Associate Professor Brian Cox and Dr Mary Jane Sneyd report they found a 30 per cent reduced risk of bowel cancer for those who took part in school milk programmes.

And the more school milk people drank, the lower their bowel cancer risk, culminating in a 38 per cent reduction for the biggest drinkers.

The researchers suggest calcium - a nutrient in milk more commonly associated with healthy bone development - may affect the growth of bowel adenomas, benign tumours that can become malignant.

They say the daily bottle of milk would have provided around a half to two-thirds of the children's estimated calcium requirement.

"Although calcium supplementation in adults has been shown to reduce the risk of recurrent adenoma," the journal paper says, "the effect of childhood dietary calcium on their initial development is not known."

Dr Cox said: "The results of this study, if confirmed, would provide a means of reducing the very high rates of bowel cancer in New Zealand.

"The research team is currently planning further research which, if funding can be obtained, could confirm that the provision of milk at school can significantly reduce the risk of bowel cancer in future generations."

New Zealand has one of the world's highest bowel cancer rates. Each year around 2800 people are diagnosed with the disease and it kills more than 1200.

But the journal paper says that for successive generations of New Zealanders born from about 1937, the incidence rate decreased.

Dr Cox said yesterday it was too soon to know whether that trend had been reversed since the end of school milk programmes.

The 2002 National Children's Nutrition Survey found that while most children aged 5 to 10 were consuming an adequate amount of calcium, more than a quarter of those aged 11 to 14 were not getting enough.

Dr Cox said: "Whatever the main cause of colo-rectal cancer is in New Zealand, it seems to be something we are exposed to before the age of 25 and establishes your lifetime risk.

"It's not clear how much changing your diet in adulthood would change your risk of bowel cancer."

* Otago University researchers, with no funding from dairy industry.

* Postal questionnaire completed by 562 people with bowel cancer, and a randomly selected control group of 571 people.

* Those who took part in school milk programme were 30 per cent less likely to develop bowel cancer.