Those who shuddered at the prospect of daily school milk but diligently drank it anyway may have significantly reduced their risk of bowel cancer, a University of Otago study suggests.
The study, just published in the American Journal of Epidemiology, showed the risk of bowel cancer was 30% lower in people who drank school milk daily.
One of the researchers, Associate Prof Brian Cox, said he was "pretty excited" about the results, although a further, similar-sized study would be needed to confirm them. Funding was being sought for this.
A third study, with a completely different design, supporting the findings would "really stitch it", he said.
The reduction in risk was greatest for those who drank 1200 or more half-pint (about 300ml) bottles of milk in their school years.
Studies of adults have suggested calcium intake may reduce bowel cancer risk but few studies of the effect of consumption in childhood have been done.
The study, by Prof Cox and Dr Mary Jane Sneyd, of the Hugh Adam Cancer Epidemiology Unit, looked at the school milk-drinking habits of 562 people aged 30 to 69 diagnosed with bowel cancer and compared them with 571 people of similar age without the disease.
Questions covered other aspects of diet, including the frequency dairy products were eaten during childhood, but the amounts were not sought, so calcium intake from these sources was not calculated. No dietary or alcohol consumption data for adulthood was sought.
Participants and interviewers were not advised of the study's hypothesis.
Although New Zealand has one of the highest rates of colorectal cancer in the world, data shows that the incidence in those born from about 1938 to 1953 is about 50% lower than in those of earlier generations.
The researchers said this suggested there might be environmental factors acting during pregnancy, childhood or adolescence that affected cancer risk.
School milk was provided free in many schools from 1937 to 1967 and Prof Cox said that before that time, while people consumed milk in other ways, it was not commonly considered a drink, even on farms.
In Southland, where many schools withdrew from the programme in 1950, researchers noted that those who went to school solely in Southland had about twice the risk of the rest of the country.
Southland has the highest incidence of bowel cancer in the country, but Prof Cox cautioned against assuming this was entirely related to school milk consumption. Research into the regional aspects of rates of the disease and reasons for geographical variations was yet to be completed.
The study noted its results were contrary to United Kingdom research that found an increased risk of colorectal cancer with increased household dairy intake in children, although that did not incorporate school milk consumption.
Eating food with milk could affect calcium absorption and school milk consumption "might have been advantageous because it was often consumed without food".
Pre-cancerous growths were thought to develop in the colon fairly early in life and it was possible the calcium in school milk reduced the development of that early phase of the disease, thereby lowering the lifetime risk, Prof Cox said.
The researchers suggested further studies into the causes of the disease should examine the effects of childhood milk consumption or calcium intake and childhood diet overall, in addition to the adult diet.
The biological mechanisms by which children's milk consumption might reduce risk of colorectal cancer should also be explored. It was possible that milk components other than calcium, or possibly fermentation of the milk by lactobacillus, might have contributed to the lowered risk of bowel cancer, the researchers suggested.
They also suggested comparing the New Zealand findings with those of similar studies in other countries with school milk programmes.
