Does a Mediterranean diet really protect against breast cancer?

‘Mediterranean diet could slash risk of deadly breast cancer by 40 per cent,’ said the headlines in the Telegraph and elsewhere.

It’s certainly highly plausible. Evidence already exists that a Mediterranean diet protects against various other forms of cancer — including colon, gastric and prostate — and appears to lower overall risk of death from cancer.

The new study followed up 63,000 women aged 55 to 69 over 20 years. As a prospective cohort it cannot establish causation. More than 3,500 cases of breast cancer occurred, but analysis was only possible for 2,321 of these. This was then combined with a meta-analysis of previous studies.

Researchers assessed the women’s compliance with a Mediterranean diet, in crude terms classing compliance either as low, moderate or high.

Compliance was calculated through the alternate Mediterranean diet scoring system. Participants were given points for consuming higher amounts of vegetables (excluding potatoes), fruits, nuts, legumes, grains and fish. They got a point, too, if they had a better-than-the-median ratio of monounsaturated fat to saturated fat. Points were also given if they ate lower amounts of unhealthy foods, such as red and processed meat. (Low compliance equalled 0-3 points, moderate was four to five, and high was six to eight points.)

As a basic summary, this might mean fewer sausages and sweets than we tend to consume and more olive oil, fish and nuts.

The analysis excluded alcohol, as alcohol is a risk factor for breast cancer. In small amounts alcohol is usually seen as part of the Mediterranean diet.

Patients with the highest compliance had no statistically significant decrease in total breast cancer over the course of follow up. This was in contrast to the Predimed study, which found a 57 per cent decrease in breast cancer risk among those who ate a Mediterranean diet.

Further analysis looked at the effect of the diet on different subtypes of breast cancer. That we know about these subtypes is down to recent advances in molecular medicine. These have expanded our knowledge of breast cancer exponentially.

The study found that greater dietary compliance was associated, as the headlines say, with a 40 per cent decrease in risk of oestrogen receptor negative (ER-) breast cancer. ER- breast cancer is not stimulated by the hormone oestrogen. It is often harder to treat and more likely to be fatal.

A decreased risk of 30 per cent was seen with progesterone receptor-negative (PR-) cancers, but no statistically significant beneficial effect was seen in the other subtypes.

The molecular mechanism behind the effect has yet to be fully understood, and may relate to inflammation, DNA damage or hormones, but given the benefits the Mediterranean diet has been shown to have on cancer, cancer mortality, cardiovascular disease, obesity and other key outcomes, it may be something to consider, particularly for those at higher risk.

These findings are important. I am not sure how much more evidence is required for the diet to become a mainstream ‘prescription’, but I would certainly suggest that anyone interested in improving their health should consider it.