Eating more whole grains, fiber, fruits, and vegetables is linked to a lower chance of developing colorectal cancer and a specific type of breast cancer, but studies that randomly assign people to different diets have not proven this link with statistical certainty.

We’ve found so far that eating more whole grains, fiber, fruits, and vegetables is linked to a lower chance of developing colorectal cancer and estrogen receptor-negative breast cancer, based on two supporting observations [1]. However, studies that randomly assigned people to different diets have not shown this link with statistical certainty, meaning the connection isn’t confirmed through the strongest type of trial. The evidence we’ve reviewed leans toward a possible benefit, but we don’t have enough high-quality experimental data to say this pattern is caused by diet alone. Observational studies — which track what people eat and later check for cancer — often show this association, but they can’t rule out other lifestyle factors like physical activity, smoking, or overall calorie intake that might also play a role. Without randomized trials proving the effect, we can’t be sure whether changing your diet directly lowers risk or if it’s just part of a broader healthy pattern. We also note that the evidence is limited — only two assertions were analyzed, and no studies contradicted the link. That doesn’t mean the link is real, but it also doesn’t mean it’s false. More research, especially long-term randomized trials, would help clarify whether these foods have a direct influence on cancer risk. For now, if you’re looking to eat in a way that aligns with current findings, adding more whole grains, fiber, fruits, and vegetables to your meals is a low-risk choice that may support overall health — even if we can’t say yet whether it directly lowers cancer risk.