Risk is greatest if there is a family history of premenopausal breast cancer.
26 Human studies difficult because of inherent error in dietary assessment. Studies of Native American diets show a change from a traditional diet high in plant sources and lean met and low in fat to a more typical American diet high in fat and low in fruits and vegetables. Migration studies indicate that these factors may not be reflected in cancer rates for decades or even generations. Increased risk may be due to compounds in smoke with estrogenic activity. 35 Many studies have examined the relationship between physical activity and breast cancer risk.