Is it true that a low-fat diet protects against breast cancer?

The idea that a low-fat diet protects against breast cancer is a popular assumption, but the evidence for it is weak and partly contradicted. Some reviews find a modest protective effect of eating less fat, but the underlying studies are inconsistent and of varying quality. Total fat intake on its own was not found to have a significant link with breast cancer mortality in more detailed research.

One specific type of fat does stand out, however: women with the highest intake of saturated fat had a 51% greater chance of dying from breast cancer than women with the lowest intake. That is a meaningful difference, although it is an observational association rather than a proven causal relationship. Eating less saturated fat therefore seems more worthwhile than simply eating 'less fat' in general.

It is precisely here that the PREDIMED study, a randomised trial, offers a surprising counterpoint. Participants who followed a Mediterranean diet with extra olive oil, in other words a high-fat diet, developed invasive breast cancer 68% less often than the group eating a low-fat diet. This is a secondary analysis based on only 35 cases, and the result needs confirmation in larger research. It does show, however, that eating less fat is not necessarily the best approach: the type of fat matters.

After a breast cancer diagnosis, a low-fat diet does appear to be beneficial. Several studies combined show that women who ate a low-fat diet after their diagnosis had a 23% lower chance of relapse, and overall mortality was possibly 17% lower. Those figures are encouraging, but here too the research is limited and firm conclusions are premature.