The Women's Health Initiative (WHI) Dietary Modification Trial enrolled 48,835 post-menopausal women with an average age of 62.3 between 1993 and 1998. The average follow-up time was 8.1 years.

Dietary intervention for 19,541 women entailed cutting down fat intake to less than 20 per cent, eating five servings of fruit and vegetables, and six portions of grains per day. The other 29,294 women ate a normal diet.

The intervention trial, reported in the Journal of the American Medical Association (Vol. 295, pp. 629-642), succeeded in reducing fat intake by 8.2 per cent and increasing vegetable intake by 1.1 servings.

“After approximately eight years of follow-up, breast cancer incidence was nine per cent lower in the dietary intervention group compared with women in the comparison group,” said lead researcher Ross Prentice from the Fred Hutchinson Cancer Research Center.

However, “because incidence rates did not differ between the intervention group and the comparison group at the conventional 0.05 level of significance, chance provides an explanation for the modestly lower breast cancer incidence rates in the intervention group,” said Prentice.

The disagreement between earlier studies and this new intervention study highlight the limitations of the present study. Thirty-five per cent of the volunteers were overweight, based on BMI measures, while over thirty per cent were obese.

A study published in Breast Cancer Research (2005, Vol. 7, pp. R833-R843) reported that young women who lost weight reduced their risk of breast cancer by 50 per cent. However, women who gained weight had a 65 per cent higher risk of breast cancer.

When the breast cancer subtypes in the WHI study were defined, the hazard ratio (a measure of the risk) varied amongst the groups, leading the authors to claim: “Such variation would not be expected if the intervention had no effect on breast cancer risk.”

In an accompanying editorial by Aman Buzdar from the University of Texas M.D. Anderson Cancer Center (JAMA, Vol. 295, pp. 691-692) added: “A diet rich in fruit and vegetables was not associated with breast cancer risk reduction suggesting that increased intake of fruit and vegetables may be more useful in secondary rather than in primary prevention of breast cancer.”

Buzdar said that oncologists were beginning to understand which approaches might be effective for specific subsets of breast cancer patients.

“Additional research is needed to determine the specific dietary elements that may help prevent breast cancer, the optimal time to initiate dietary interventions, and the duration these diets should be followed to achieve the maximum benefit,” said Buzdar.

There are over one million new cases of breast cancer diagnosed every year worldwide, according to the European School of Oncology, with the highest incidence rates found in the Netherlands and the US.