Triglyceride is the main type of lipid found in the human diet, which is present in oils and fats. Most of these triglycerides consist of long-chain fatty acids (carbon chains > 12 carbons) attached to the glycerol backbone. Additional lipids, such as phospholipids and cholesterol, are also obtained from cell membranes. It's worth noting that the intestine receives not only dietary lipid but also lipid from the liver through biliary secretions. In fact, the cholesterol provided in bile is greater than what is obtained from the diet on a daily basis, except for those who consume a large amount of eggs. Additionally, although they are present in small amounts, the fat-soluble vitamins (A, D, E, K) are crucial nutrients that must be obtained through the diet to prevent diseases. These substances are highly insoluble in water and therefore require special mechanisms to be absorbed by the body.

To facilitate the absorption of lipids, they undergo emulsification and solubilization. When a fatty meal is consumed, the lipid becomes liquid at body temperature and forms a layer on the surface of the gastric contents. This limits the contact between the aqueous and lipid phases, making it difficult for enzymes to break down the lipid for absorption. These lipolytic enzymes, being proteins, are present in the aqueous phase. Thus, emulsification is an important early step in lipid assimilation. The churning action of the stomach mixes the dietary lipid, forming a suspension of small droplets that greatly increases the surface area of the lipid phase.

Bile acids, present in biliary secretions, aid in the formation of micellar solutions, which further facilitate lipid absorption. The details of this process will be discussed later.

The digestion of lipids begins in the stomach. Gastric lipase, released in large quantities by gastric chief cells, attaches to the surface of fat droplets in the gastric contents and breaks down triglycerides into diglycerides and free fatty acids. However, due to the acidic pH of the stomach, little lipid absorption occurs in this organ. The acidic environment causes the free fatty acids released by gastric lipase to become protonated, preventing their assimilation. Additionally, gastric lipase is unable to fully break down the triglyceride ester by hydrolyzing the second position, resulting in incomplete lipolysis. Cholesterol esters and the esters of fat-soluble vitamins also experience little to no breakdown in the stomach. In healthy individuals, gastric lipolysis is not essential due to the presence of abundant pancreatic enzymes.

The majority of lipolysis takes place in the small intestine. Pancreatic juice contains three important lipolytic enzymes optimized for activity at a neutral pH. The first enzyme is pancreatic lipase, which is capable of hydrolyzing both the 1 and 2 positions of triglycerides, producing a significant amount of free fatty acids and monoglycerides. At neutral pH, the charged head groups of the free fatty acids migrate to the surface of the oil droplets. Interestingly, lipase is inhibited by bile acids, which are also present in the small intestine. Bile acids attach to the surface of the oil droplets, causing lipase to dissociate. However, lipase activity is sustained by colipase, an important cofactor supplied in pancreatic juice. Colipase acts as a bridge between bile acids and lipase, anchoring lipase to the oil droplet even in the presence of bile acids.
The predominant form of lipid in the human diet is triglyceride, found in oils and other fats. The majority of these triglycerides have long-chain fatty acids (carbon chains > 12 carbons) esterified to the glycerol backbone. Additional lipid is supplied in the form of phospholipids and cholesterol, mostly arising from cell membranes. It is also important to consider that the intestine is presented daily not only with dietary lipid but also with lipid originating from the liver in biliary secretions, as described in more detail in . Indeed, the cholesterol supplied in bile exceeds that provided in the diet on a daily basis in all but the most egg-loving individuals. Finally, though present in only trace amounts, the fat-soluble vitamins (A, D, E, K) are essential nutrients that should be supplied in the diet to avoid disease. These substances are almost entirely insoluble in water and thus require special handling to promote their uptake into the body.
Emulsification and Solubilization of Lipids
When a fatty meal is ingested, the lipid becomes liquefied at body temperature and floats on the surface of the gastric contents. This would limit the area of the interface between the aqueous and lipid phases of the gastric contents and thus restrict access of enzymes capable of breaking down the lipid to forms that can be absorbed. This is because the lipolytic enzymes, as proteins, reside in the aqueous phase. Therefore an early stage in the assimilation of lipid is its emulsification. The mixing action of the stomach churns the dietary lipid into a suspension of fine droplets, which vastly increases the surface area of the lipid phase.
Lipid absorption is also facilitated by formation of a micellar solution with the aid of bile acids supplied in biliary secretions. Details of this process will be discussed subsequently.
Digestion of Lipids
Lipid digestion begins in the stomach. Gastric lipase is released in large quantities from gastric chief cells; it adsorbs to the surface of fat droplets dispersed in the gastric contents and hydrolyzes component triglycerides to diglycerides and free fatty acids. However, little lipid assimilation can take place in the stomach because of the acidic pH of the lumen, which results in protonation of the free fatty acids released by gastric lipase. Lipolysis is also incomplete in the stomach because gastric lipase, despite its optimum catalytic activity at acidic pH, is not capable of hydrolyzing the second position of the triglyceride ester, which means that the molecule cannot be fully broken down into components that can be absorbed into the body. There is also little if any breakdown of cholesterol esters or the esters of fat-soluble vitamins. Indeed, gastric lipolysis is dispensable in healthy individuals because of the marked excess of pancreatic enzymes.
The majority of lipolysis takes place in the small intestine in health. Pancreatic juice contains three important lipolytic enzymes that are optimized for activity at neutral pH. The first of these is pancreatic lipase. This enzyme differs from the stomach enzyme in that it is capable of hydrolyzing both the 1 and 2 positions of triglyceride to yield a large quantity of free fatty acids and monoglycerides. At neutral pH, the head groups of the free fatty acids are charged, and thus these molecules migrate to the surface of the oil droplets. Lipase also displays an apparent paradox in that it is inhibited by bile acids, which also form part of the small intestinal contents. Bile acids adsorb to the surface of the oil droplets and cause lipase to dissociate. However, lipase activity is sustained by an important cofactor, colipase, which is also supplied in pancreatic juice. Colipase is a bridging molecule that binds to bile acids and to lipase; it anchors lipase to the oil droplet even in the presence of bile acids.