That is a good question. Researchers are finding that it could be quite early, even during adolescence. While other studies looking at adults didn’t find any connection between dietary fiber intake and cancer, a recent study found that youth who ate foods high in fiber had a lower risk of breast cancer later in life, than those who didn’t.
What we eat when young builds the foundation for our health later in life. So to get the greatest benefit of dietary fiber, start eating foods that are naturally high in fiber as young as possible. This includes: fruits, vegetables, whole grains and legumes.
Willet W, Chen W, Liao X, Cho E, Eliassen H, Farvid M. Dietary Fiber Intake in Young Adults and Breast Cancer Risk. Pediatrics. 2016.