Positive energy imbalance occurs when energy intake exceeds energy expenditure. In modern society, increased consumption of caloric-dense food and reduced energy expenditure due to low physical activity and sedentary lifestyle drive positive energy imbalance. Manifested as weight gain, positive energy imbalance is implicated in the development and progression of several cancers. This dissertation focuses on reduced energy expenditure, sedentary behavior and weight gain in relation to cancer risk.
In chapter 1, we prospectively evaluated the relationship between physical activity and incident cancers of the digestive system encompassing the digestive tract (mouth, throat, esophagus, stomach, small intestine, and colorectum) and digestive accessory organs (pancreas, gallbladder, and liver) among men in the Health Professionals Follow-Up Study. In men, a higher level of physical activity, regardless of its intensity, was associated with a decreased risk of digestive system cancers, particularly digestive tract cancers. The association was independent of obesity and diabetes.
In chapter 2, we examined whether sedentary lifestyle, as indicated by time spent sitting watching TV, has an effect on colorectal cancer risk independent of physical activity. The relationship was prospectively investigated among women in the Nurses’ Health Study and men in the Health Professionals Follow-Up Study. In the combined population of women and men, prolonged sitting watching TV was an independent risk factor for colorectal cancer. The elevated risk associated with prolonged sitting was not completely offset by participation in physical activity.
In chapter 3, we assessed the strength and shape of the dose-response relationships between adult weight gain and risk of adiposity-related cancers (e.g., cancers of the breast, endometrium, ovary, prostate, colon, pancreas) using dose-response meta-analysis. Adult weight gain was linearly associated with risk of breast, endometrial, and ovarian cancers among postmenopausal women not using hormone therapy; with risk of colon cancer, particularly among men.
Results of this dissertation suggest that increasing the overall amount of physical activity, reducing sedentary time, particularly sitting watching TV, and avoiding adult weight gain should be widely encouraged for the prevention of several major cancers.
