Digestion of lipids, or fats, and proteins enables your body to use these nutrients to support your metabolic functions. Fat digestion begins in your mouth and protein digestion begins in your stomach. The most extensive breakdown of these nutrients, however, occurs in your small intestine, with assistance from your pancreas and gallbladder. Different groups of chemicals are responsible for the digestion lipids and proteins.
Liquefaction and Denaturation
Liquefaction and protein denaturation are preliminary digestive processes that occur in your stomach. Gastric parietal cells produce and secrete highly concentrated hydrochloric acid with a pH of approximately 0.8, roughly the same as battery acid. Stomach acid causes protein denaturation, the breakdown of chemical bonds that hold proteins together. This process increases the digestibility of dietary proteins because it makes them more susceptible to digestive enzymes.
Your stomach is also the site of food liquefaction, the transformation of solid food into a particle-containing liquid. Think of your stomach as your body's food blender, mixing stomach juices into swallowed food while breaking it into pieces small enough to pass into your small bowel.
Proteases are enzymes that digest proteins, breaking them into smaller subunits that can be absorbed into your bloodstream. Your stomach produces the enzyme pepsin, which begins the process of protein digestion after denaturation. Digestive fluid from your pancreas, which empties into the first portion of your small bowel, contains the proteases trypsin and chymotrypsin, which further digest dietary proteins. Peptidases produced by the lining cells of your small intestine accomplish the final enzymatic breakdown of dietary proteins, leaving them small enough for absorption into your circulation.
Your liver produces bile acids from cholesterol and other chemicals. These substances are stored in your gallbladder until they are needed for lipid digestion, when they are released into your small bowel. Bile acids are chemical detergents, breaking down large, dietary fat globules into small droplets ready for enzymatic digestion. In addition to its role in lipid digestion, bile serves as the only mechanism by which your body can rid itself of excess cholesterol.
Lipases and Cholesterol Esterase
Lipases are fat digesting enzymes. Your stomach, pancreas and salivary glands near your tongue produce different forms of lipases. Although the process begins in your mouth, the majority of fat digestion occurs in your small intestine through the action of pancreatic lipase. Your pancreas secretes large amounts of bicarbonate with lipase because the enzyme requires an alkaline environment to function. Pancreatic lipase acts on dietary triglycerides, breaking them into fatty acids and monoglycerides for subsequent absorption. Another pancreatic enzyme called cholesterol esterase digests dietary fats liberating cholesterol for absorption into your bloodstream.