Lipids are large molecules and are generally not water-soluble. Like carbohydrates and proteins, lipids are broken down into smaller components for absorption. Since most of our digestive enzymes are water-based, how does the body break down fat and make it available for the various functions it must perform in the human body?

From the Mouth to the Stomach

The first step in the digestion of triglycerides and phospholipids begins in the mouth as lipids encounter saliva, which contains lingual lipase, an enzyme that serves to break apart triglycerides (although it plays a minimal role in lipid digestion). Due to the temperature in the mouth, some harder lipids also turn into liquid. Next, the physical action of chewing coupled with the action of emulsifiers enables digestive enzymes to do their tasks. These actions cause the fats to become more accessible to the digestive enzymes.

In the stomach, gastric lipase starts to break down triglycerides into diglycerides and fatty acids. Within two to four hours after eating a meal, roughly 30 percent of the triglycerides are converted into diglycerides and fatty acids. The stomach’s churning and contractions help to disperse the fat molecules, while the diglycerides derived in this process act as further emulsifiers. However, amid these activities, very little fat digestion occurs in the stomach.

Going to the Bloodstream

As stomach contents enter the small intestine, the digestive system sets out to manage a small hurdle to combine the separated fats with its own watery fluids. The solution to this hurdle is bile, which is produced in the liver but stored in the gallbladder. When chyme enters the small intestine, it stimulates the secretion of cholecystokinin into the duodenum. The presence of cholecystokinin signals the gallbladder to contract and release bile into the duodenum, via the common bile duct. Bile contains bile salts, lecithin, and substances derived from cholesterol so it acts as an emulsifier. It attracts and holds onto fat while it is simultaneously attracted to and held on to by water. Emulsification increases the surface area of lipids over a thousand-fold, making them more accessible to digestive enzymes.

Once emulsified, fat-breaking enzymes work on the triglycerides and diglycerides to sever fatty acids from their glycerol foundations. Cholecystokinin signals the pancreas to contract and release pancreatic juices into the duodenum, via the pancreatic duct. The pancreas also secretes bicarbonate to help neutralize the acidic chyme, allowing the digestive enzymes to function and the lining of the small intestine to remain intact. As pancreatic lipase enters the small intestine, it breaks down the fats into free fatty acids and monoglycerides. Yet again, another hurdle presents itself. How will the fats pass through the watery layer of mucus that coats the absorptive lining of the digestive tract? As before, the answer is bile. Bile salts envelop the fatty acids and monoglycerides to form micelles. Micelles have a fatty acid core with a water-soluble exterior. This allows efficient transportation of fats to the intestinal microvillus. Here, the fat components are released and disseminated into the cells of the digestive tract lining.

Just as lipids require special handling in the digestive tract to move within a water-based environment, they require similar handling to travel in the bloodstream. Inside the intestinal cells, the monoglycerides and fatty acids reassemble themselves into triglycerides. Triglycerides, cholesterol, and phospholipids form lipoproteins when joined with a protein carrier. Lipoproteins have an inner core primarily made up of triglycerides and cholesterol esters (a cholesterol ester is a cholesterol linked to a fatty acid). The outer envelope is made up of phospholipids interspersed with proteins and cholesterol. Together, they form a chylomicron, a large lipoprotein that now enters the lymphatic system and will soon be released into the bloodstream via the thoracic duct/ jugular vein in the neck. Chylomicrons transport food fats perfectly through the body’s water-based environment to specific destinations such as the liver and other body tissues.

Cholesterols are poorly absorbed when compared to phospholipids and triglycerides. Cholesterol absorption is aided by an increase in dietary fat components and is hindered by high fibre content. This is the reason that a high intake of fibre is recommended to decrease blood cholesterol. Foods high in fiber such as fresh fruits, vegetables, and oats can bind bile salts and cholesterol, preventing their absorption and carrying them out of the colon.

If fats are not absorbed properly as seen in some medical conditions, a person’s stool will contain high amounts of fat. If fat malabsorption persists, a condition known as steatorrhea occurs. Steatorrhea can result from diseases that affect absorption, such as Crohn’s disease and cystic fibrosis.

Once in the bloodstream

Once in the bloodstream, triglycerides can be taken up into the body and used for energy (e.g., in muscles), to make lipid-containing compounds, or stored in the muscle or adipose tissue for later use.

Storing and Using Body Fat

In a similar manner, much of the triglycerides the body receives from food are transported to fat storehouses within the body if not used for energy production.

After a meal, the triglycerides in the chylomicrons must be moved to where they are needed or stored, such as adipose or muscle cells. But how do the triglycerides get out of the chylomicron? Capillary walls contain an enzyme called lipoprotein-lipase that dismantles the triglycerides in the lipoproteins into fatty acids and glycerol, thus enabling them to enter the adipose cells. Once inside the adipose cells, the fatty acids and glycerol are reassembled into triglycerides and stored for later use. Muscle cells may also take up the fatty acids and use them for muscular work and energy production. When a person’s energy requirements exceed the amount of fuel available from a recent meal, or extended physical activity has exhausted glycogen energy reserves, fat reserves are retrieved for energy utilization.

Another way the body stores fat was previously touched upon in the Carbohydrates chapter. The body transforms carbohydrates into glycogen, which is stored in the muscles for energy. When the muscles reach their capacity for glycogen storage, the excess is returned to the liver, where it is converted into triglycerides and subsequently stored as fat.

As the body calls for additional energy, the adipose tissue responds by dismantling its triglycerides and dispensing glycerol and fatty acids directly into the blood. Upon receiving these substances, the energy-hungry cells break them down further into tiny fragments. These fragments go through a series of chemical reactions that yield energy, carbon dioxide, and water.