Large-scale studies have demonstrated that obesity increases the risk of developing some forms of cancer. The association between obesity and cancer may result from factors such as fat distribution or sex hormone levels. Studies have also shown a relationship between a high-fat, low-fiber diet and cancer risk. High estrogen levels and low progesterone levels are associated with an increased risk of endometrial cancer. Obesity is known to raise estrogen levels and may lower progesterone levels. Obesity may increase the risk of breast cancer, but the evidence is less clear, since factors, such as age, country of origin, body-fat distribution, and family history, also play a major role in determining breast cancer risk. Sex hormones, insulin, and nutritional factors are also involved in the etiology of breast cancer. The incidence of lung cancer is inversely related to body weight.