Studies that randomly assign people to different diets, such as low-fat eating or adding fiber, have not shown a clear reduction in breast or colorectal cancer rates, even though observational studies have suggested a possible link.

We analyzed the available evidence and found that randomized trials have not shown a clear reduction in breast or colorectal cancer risk from low-fat diets or fiber supplements, even though other types of studies have suggested a possible connection [1]. What we’ve found so far is that when people are randomly assigned to eat less fat or take fiber supplements over time, their rates of these cancers do not appear to change in a way that stands out from those who didn’t make those changes. This doesn’t mean these habits are useless — it just means the best-controlled studies so far haven’t detected a strong effect on cancer rates. The evidence we’ve reviewed leans toward the idea that these specific dietary changes, as tested in trials, may not lower cancer risk as much as earlier observational data hinted. We don’t have any studies in our review that contradict this finding, but we also don’t have enough strong evidence to say these approaches reliably prevent cancer. The difference between observational studies — which watch what people naturally do — and randomized trials — which assign people to specific habits — is important. Observational studies can be influenced by other lifestyle factors, while randomized trials aim to isolate the effect of the diet itself. Our current analysis shows that if low-fat eating or fiber supplements do help reduce cancer risk, the effect is likely small or not detectable in the studies we’ve reviewed. For now, if you’re considering these changes for cancer prevention, it’s worth remembering that they may support overall health in other ways — but based on what we’ve seen, they shouldn’t be relied on as a clear way to lower breast or colorectal cancer risk.