The etiology of colorectal cancer is still unclear, and it is widely recognized that the occurrence of colorectal cancer is the result of a combination of factors. The factors related to the occurrence of colorectal cancer are divided into the following categories. Diet: Diet is one of the most important factors causing colorectal cancer. High protein and high fat diet: It is obvious from the observation of colorectal cancer incidence rate that colorectal cancer is closely related to people’s living standard. The incidence of colorectal cancer in developed areas is obviously higher than that in non-developed areas. After undigested protein and fat enter the colon, a large amount of carcinogenic substances are generated by the decomposition of bacteria in the colon, which leads to the occurrence of colorectal cancer. Low-fiber diet: High-fiber diet will reduce the incidence of colorectal cancer. Because of the increase of fiber in food, more stool can dilute carcinogenic substances and stimulate intestinal wall to promote intestinal peristalsis, reducing the time and opportunity for absorption of harmful substances. If a long-term low-fiber diet, the probability of colorectal cancer will be greatly increased. Pickled food: after the vegetables are pickled, the vitamins they contain are lost more, and almost all of the vitamin C is lost. Pickled foods are often contaminated by microorganisms during the pickling process, which can easily induce intestinal lesions. Nitrate in vegetables can be reduced to nitrite by microorganisms, nitrite in the human body when it encounters amines, can generate nitrosamines. Nitrosamines are a carcinogenic substance, so often eat pickled products are prone to cancer. Baked, fried, smoked food: because of repeated high-temperature heating of fats and oils, unsaturated fatty acids produced by high-temperature heating of polymers – dimer, trimer, more toxic. Most fried and baked foods, especially French fries contain high concentrations of acrylamide, a carcinogenic substance. And the cracking products of fish and meat cooked at high temperatures produce mutagens and carcinogens such as dimethylhydrazine, which induces the colon of rats. Lack of essential trace elements in food: trace elements copper, zinc, iron, selenium, potassium, molybdenum and calcium are all useful in preventing colon cancer, among which copper, zinc, iron and selenium are considered to be essential trace elements with anti-cancer effects. Increased intake of copper, zinc, iron and selenium can prevent the occurrence of colorectal cancer, while excessive intake of phosphorus is a risk factor for the development of rectal cancer. Age: colorectal cancer is a disease of the elderly, more than 80% of colorectal cancer occurs in middle-aged and elderly people older than 50 years old, and the higher the age, the higher the risk of colorectal cancer. Therefore, the aging of the population may be an important reason for the current increase in the incidence of colorectal cancer. Family history of colorectal cancer: There are a lot of studies proving that if one person has colorectal cancer, his immediate family members (parents, children, siblings) are 2-3 times more likely to get colorectal cancer than the normal population. Many patients with colorectal cancer present as a cluster of family offspring. In particular, there is a disease called familial adenomatous polyposis, which is an autosomal dominant disease that often appears during adolescence, and its initial manifestation is widespread polyps in the large intestine, which eventually develop into intestinal cancer if left untreated. The offspring of patients have a 50% chance of inheriting this disease, and once the disease occurs, it will be 100% cancerous. Intestinal diseases: Colon polyps are closely related to the occurrence of colorectal cancer, and most colorectal cancers evolve from polyps. However, not all colon polyps will become cancerous, only adenomatous polyps are prone to cancer, while inflammatory polyps and hyperplastic polyps are generally not carcinogenic. Diseases such as ulcerative colitis and colonic schistosomiasis are closely associated with the development of colorectal cancer. One possible reason is that repeated irritation of the intestine by chronic inflammation induces cancerous changes in the intestinal mucosa. Another pathogenesis is the mutation of proliferating cells during repeated damage and repair of body tissues, which in turn evolve into tumor cells (Figure 6-1). Smoking: Tobacco produces a large amount of carcinogenic substances during the burning process, which can increase the incidence of many malignant tumors including lung and colorectal cancers. Previous smoking increases the risk of colorectal cancer. Compared to never smokers, a history of smoking for at least 20 years increases the risk of colorectal cancer by 26%; smoking more than 20 g of tobacco per day increases the risk of colorectal cancer by 30%; smoking for more than 30 years or more than 20 g of tobacco per day increases the risk of colorectal cancer by 48%. Thus, smoking increases the risk of colorectal cancer, and early cessation of smoking is beneficial to the prevention and treatment of colorectal cancer. Alcohol consumption: Some scholars in China have suggested that alcohol abuse is related to the occurrence of bowel cancer, but it is not clear. Foreign reports have not confirmed that alcohol consumption is related to colorectal cancer.