Because the food basically completes the digestion process after chemical digestion of pancreatic juice, bile and intestinal juice in the small intestine and mechanical digestion of small intestine movement, and nutrients are absorbed by the intestinal mucosa. Food stays in the small intestine for a long time, usually 3 to 8 hours, which provides sufficient absorption time. The small intestine is the longest part of the digestive tract, the small intestine is the main absorption organ, and the villi of the small intestine is the main part to absorb nutrients.