Lipid digestion mainly occurs in the duodenum, because very little fat digestion happens before chyme enters it. When chyme (partially digested food) enters the duodenum, it stimulates the release of CCK and secretin. CCK signals the pancreas to release pancreatic lipase, and the gallbladder to release bile salts. Bile salts emulsify fats—this means they break big fat droplets into small ones, increasing the surface area for enzyme action. Lipase then breaks triglycerides into glycerol and fatty acids. These products, along with sterols and fat-soluble vitamins, are not water-soluble, so they combine with bile salts to form micelles. Micelles are tiny, water-soluble droplets that can be absorbed by the intestinal epithelial cells.
Inside the cells, micelles release their contents and triglycerides are reformed, then coated with protein, cholesterol, and phospholipids to form chylomicrons, a type of lipoprotein. These chylomicrons are released by exocytosis into the lacteals (tiny lymph vessels in the villi), then travel through the lymphatic system and enter the bloodstream via the thoracic duct, near the vena cava of the heart. From there, chylomicrons distribute lipids to body tissues, transported by LDL and VLDL to where they are stored or used for energy.
Absorption of vitamins happens mainly in the small intestine. Water-soluble vitamins, like vitamin C and B-complex, are absorbed by diffusion—this means they pass through the intestinal membrane with the help of channel proteins, without using energy. Fat-soluble vitamins (A, D, E, K) are absorbed as part of micelles, which are lipid-bile salt droplets formed during fat digestion. These micelles help fat-soluble vitamins move through the watery environment of the intestine and get absorbed. Vitamin B12 is special—it needs a protein called intrinsic factor to be absorbed in the ileum (last part of the small intestine). Without intrinsic factor, vitamin B12 cannot be absorbed properly.
Water absorption also mostly happens in the small intestine. About 9 litres of water enter the digestive system every day—2 L from food and drinks, and 7 L from digestive secretions (like saliva, bile, and pancreatic juice). Of this, about 92% is absorbed in the small intestine, 7% in the large intestine, and only about 1% is lost in faeces. Water moves by osmosis, which means it follows the movement of dissolved substances, and it can move in both directions depending on the concentration of solutes inside the intestines.
Negative feedback is a key mechanism the body uses to keep things in balance. In digestion, at least 18 hormones are released from the intestinal mucosa to stimulate the liver, pancreas, and other systems. Some important hormones include gastrin, secretin, and CCK (cholecystokinin). Gastrin is released in response to peptides and amino acids in the stomach and stimulates the release of HCl and pepsinogen. Secretin is released when acid enters the small intestine and signals the pancreas to release bicarbonate and water. CCK is triggered by fats and proteins and tells the pancreas to release digestive enzymes and the gallbladder to release bile.
Two other hormones, insulin and glucagon, have direct effects on carbohydrate (CHO) metabolism. When blood glucose levels are too high, the pancreas releases insulin, which helps move glucose into liver and muscle cells. As more glucose enters cells, the glucose level in the blood drops. This is negative feedback—as blood sugar rises, insulin rises to bring it back down. Then as glucose drops, insulin secretion also drops. This back-and-forth creates a kind of oscillating pattern - maintain stable glu