Diet, physical inactivity, and obesity are related to approximately 30–35% of cancer cases.^[1][15] In the United States excess body weight is associated with the development of many types of cancer and is a factor in 14–20% of all cancer deaths.^[15] Physical inactivity is believed to contribute to cancer risk not only through its effect on body weight but also through negative effects on immune system and endocrine system.^[15]

Diets that are low in vegetables, fruits and whole grains, and high in processed or red meats are linked with a number of cancers.^[15] A high salt diet is linked to gastric cancer, aflatoxin B1, a frequent food contaminate, with liver cancer, and Betel nut chewing with oral cancer.^[16] This may partly explain differences in cancer incidence in different countries for example gastric cancer is more common in Japan with its high salt diet^[17] and colon cancer is more common in the United States. Immigrants develop the risk of their new country, often within one generation, suggesting a substantial link between diet and cancer.^[18]