Digestion is the process in which your body decreases the size of the food particles you eat until they are small enough to be absorbed. Absorption occurs when these small molecules of food particles pass from your digestive system, or gut, into your bloodstream so your body can benefit from the nutrients they contain. Protein and fats, often eaten at the same time, share some similarities in how your body digests and absorbs them, but, for the most part, the processes are different.
When your food protein contacts the acidic environment of your stomach, the tightly folded protein begins to relax. As this happens, a digestive enzyme in your stomach clips the long chain of the protein's amino acids into shorter pieces called peptides. The peptides travel to your small intestine, where different digestive enzymes – secreted from your pancreas – break down the peptides into even shorter chains and eventually into single amino acids. These individual amino acids are now ready to be absorbed by the walls of your small intestine.
Small structures, known as villi, line the walls of your small intestine, while even smaller structures, called microvilli, line your villi. The villi and microvilli are a series of folds that serve to increase the surface area available for absorption. The digested amino acids from your food protein pass from the inside of your small intestine through the epithelial cells of your villi and microvilli and into your capillaries. They travel through your epithelial cells with the help of proteins called amino acid transporters, which shuttle the amino acids from the gut side of your cell to the capillary side of the cell. Once in the capillaries, the amino acids move through your body via your bloodstream.
Fats from foods consist primarily of triglycerides, three fatty acid molecules bound to a glycerol backbone. A small amount of digestion of your food triglycerides begins in your stomach, although most occurs in your small intestine. In your small intestine, the fat molecules mix with a substance called bile, secreted from your gallbladder, that emulsifies the fat particles, or makes them more water-soluble. Your pancreas secretes a digestive enzyme, called lipase, into your small intestine, where it acts on the emulsified triglycerides. Lipase digests each triglyceride into its three individual fatty acids plus a glycerol molecule. These fat components are now small enough to undergo absorption.
The absorption of fat is quite different than that of protein. The fatty acids and glycerol from triglyceride digestion enter the epithelial cells of your small intestine by both passive diffusion and a fatty acid transporter protein. Once inside the epithelial cell, they reform into a triglyceride and then into a transporter package called a chylomicron. Chylomicrons pass into lymphatic vessels located in the villi of your small intestine before entering your bloodstream. Once the chylomicrons reach your blood, they disassemble and their contents move throughout your body.