The findings, from a large, long-running study of US nurses, showed that women with diets high in plant foods – but low in red meat, sodium and processed carbohydrates – tended to have a lower risk of developing certain breast tumours.
Specifically, they were less likely than other women to develop breast tumours that lack receptors for the hormone oestrogen. Those oestrogen receptor-negative tumours account for about one-quarter of breast cancers.
Of more than 86,000 women the study followed for 26 years, slightly less than one percent developed ER-negative breast cancer.
The risk, researchers found, was lower among women whose diets most closely resembled the Dietary Approaches to Stop Hypertension (DASH) diet – an eating plan experts recommended for lowering blood pressure. It emphasises vegetables, fruit, fibre-rich grains, legumes and nuts, and low-fat dairy.
The women who, at the outset, had the highest DASH "score" were 20% less likely to develop ER-negative breast cancer than those with the lowest DASH scores.
Are diet habits connected to breast cancer?
When the researchers took a closer look, it seemed to be high vegetable and fruit intake that mainly accounted for the link. The results, reported in the American Journal of Epidemiology, do not prove that a plant-rich diet, itself, cuts breast cancer risk.
And in general, studies have come to mixed conclusions on whether diet habits are connected to breast cancer.
But recent research has been suggesting that the risk of ER-negative breast tumours, in particular, may be related to diet, explained Teresa T. Fung, an associate professor of nutrition at Simmons College in Boston and the lead researcher on the new study.
Why that might be is unclear. But in an interview, Fung speculated that with ER-positive breast tumours – whose growth is fuelled by oestrogen – the hormone's influence may be so important that it "overwhelms" potential dietary benefits. But for now, that's a theory.
The bottom line for women, Fung said, is that the type of eating habits that are generally healthy anyway may also be associated with a lower risk of certain breast cancers. And vegetables may be especially key, the current findings hint.
Link between plant-rich diets and lower breast cancer risk is on hold.
Along with the DASH findings, the study also showed that women with diets high in vegetable protein – from foods like beans, soy and nuts – but low in refined carbs (like white bread and other starchy foods) had a 19%lower risk of ER-negative cancer, versus women with the opposite diet pattern.
Of course, Fung pointed out, "healthy behaviours don't occur in isolation".
But when the researchers accounted for factors like weight, exercise habits and smoking, the link between plant-rich diets and lower breast cancer risk still held. Again, that doesn't prove cause and effect, Fung acknowledged. And the researchers could only look at the risk across large groups of women.
Exactly how much any one woman might benefit from eating more vegetables – at least as far as breast cancer risk – is unclear, according to Fung. In general, the average US woman has about a 12% chance of developing breast cancer in her lifetime, with ER-positive tumours being most common.
Fung suggested that women who have less-than-ideal diets gradually introduce healthier fare to make the change seem less daunting. "Find one item you can work on," Fung said. "Maybe start by adding beans to your diet."
"Any improvement," she noted, "is better than no improvement."
The DASH diet recommends that the average woman should get four to five servings of vegetables and the same amount of fruit each day. It also recommends four to five servings of legumes, nuts and seeds per week.
(Reuters Health, Amy Norton, September 2011)