Study adds to healthy diet evidence

A recent study in the Journal of the American Medical Association* adds to the evidence that a healthy diet has a more protective effect against breast cancer than just taking vitamin supplements.

Kellie Bilinski, the research dietitian at the Westmead Breast Cancer Institute (BCI), said the study conducted by Zhang, which was part of the Women's Antioxidant and Folic Acid Cardiovascular Study (WAFACS), evaluated the effect of folic acid, vitamin B6, and vitamin B12 treatment on cancer risk, and found that these vitamins had no major effect on overall risk of invasive cancer or breast cancer.

“Studies such as these add to the evidence that suggest the protective effect of diet in the development of cancer may not be a simple matter of taking a particular supplement but rather, the protective effect of diet is obtained by eating a diet high in lots of fresh fruit and vegetables, wholegrain breads and cereals."The study was conducted in 5442 women over 42 years of age, who were at high risk of cardiovascular disease. Women were randomly given either a daily combination of folic acid, vitamin B6, and vitamin B12 or a matching placebo for 7.3 years. The results showed that of the 379 women who developed invasive cancer, 49% were from the supplemented group and 51% were from the placebo group. Treatment with combined B vitamins had no major effect on risk of cancer.

Despite no overall reduction in cancer risk, a considerably reduced risk in invasive and breast cancer was seen in women aged 65 years or over at the time they entered the study . When younger women (40-64 years old) were observed, no reduction in risk of cancer was seen, which suggests that the combined B vitamin supplement may have a benefit in older women.

There was also no effect when women’s history of cancer, alcohol intake, or other risk factors were taken into account, nor were effects seen when hormone receptor status or other characteristics of breast tumours were accounted for, although a significant reduction was seen in women with oestrogen receptor–positive and progesterone receptor–negative tumours.

The study’s authors said these findings could be generalised to the entire population and that it was possible that lower occurrences of cancer were seen in this study because the participants were female health professionals who had well-balanced diets and greater access to health care and screening. Although, similar to the general population, more than two-thirds of participants were overweight or obese, and therefore at higher risk of heart disease and cancer.

Combined with other studies these results provide reassurance that folate supplements don’t appear to increase the risk of cancer, however, the authors suggest that because cancers can take more than 10 years to develop, the possible harmful or beneficial effects of combined B vitamins may not be captured in a 7-year study.

BCI Executive Director John Boyages said there was still a lot of research needed to confirm convincingly whether any particular nutrient is able to reduce the risk of cancer. “The BCI is commencing a vitamin D research program to help understand its links with breast cancer,” he said.  

*^{_{Zhang, Cook and Albert (JAMA, November 5, 2008—Vol 300, No. 17)}}