Connections Between Diet and Cancer
The relationship between diet and cancer is complex and so the extent to which a vegan diet protects against cancer is difficult to establish. However, overweight/obesity clearly increases the risks for certain cancers and vegans have a lower BMI and body fat percentage (see post on "Effects of a Vegan Diet on Cholesterol, Blood Pressure and BMI").
Moreover, case studies have shown that fruits and vegetables lower the risk of oral cavity, esophagus, stomach, and colorectal cancer. Jack Norris, Registered Dietitian and the President and Executive Director of Vegan Outreach, presents the data on cancer rates in vegetarians (very little data exists for vegans).
Additionally, the Physicians Committee for Responsible Medicine (PCRM) argues that a low-fat, fiber-rich vegan diet containing whole grains, vegetables, fruits, and beans, without added fats, can contribute to both cancer prevention and survival.