Enzymes facilitate chemical reactions; those required to help metabolize food are called digestive enzymes. Digestive enzymes break down carbohydrates, protein and fat into smaller components so your body can absorb and use them. Your stomach produces some digestive enzymes, although they are also found in your mouth, secreted from salivary glands and intestines, which are mainly secreted from the pancreas gland. The digestive enzymes secreted by your stomach are specifically referred to as gastric enzymes and their function is essential for good health.
Enzymes rely on other substances to be effective. The stomach secretes a variety of substances that provide an acidic environment so that potential pathogens are destroyed and food can be efficiently digested by gastric enzymes and prepared for the additional digestion that occurs in the small intestine. Hydrochloric acid, or stomach acid, is produced in the stomach by parietal cells. In terms of digestion, the main purpose of stomach acid is to denature or change the protein within food and activate the main gastric enzyme, which is called pepsinogen. Additionally, to protect the stomach lining from the strong acid, a thick substance called mucin is secreted by goblet cells. Furthermore, the stomach produces intrinsic factor and a hormone called gastrin. Intrinsic factor helps with vitamin B12 absorption and gastrin stimulates the release of hydrochloric acid.
Pepsinogen is the main gastric enzyme produced within the stomach. It’s secreted by stomach cells called chief cells, but it’s quickly activated by hydrochloric acid to become pepsin. Pepsin is the active form of pepsinogen and it breaks down or metabolizes protein into smaller building blocks called amino acids and peptide fragments. Consequently, protein digestion begins in the stomach. In contrast, carbohydrate and fat digestion starts in the mouth. Pepsin is most effective in very acidic stomach acid, at least a pH rating of 4, but preferably closer to a pH of 2. Healthy people usually secrete about 80 milligrams of pepsinogen with each meal, which is quickly activated to become pepsin.
Alpha-amylase is found in saliva and metabolizes starch carbohydrate into simpler sugars. Alpha-amylase enters the stomach with swallowed food, but it’s quickly inactivated by gastric acid if the pH is 3.5 or lower. Once in the stomach, starch is further broken down by gastric amylase before it enters the small intestine.
Lingual lipase is also found in saliva and is responsible for beginning the metabolism of fat, although the majority of fat digestion occurs in the small intestine. Lingual lipase also typically becomes inactivated by gastric acid because the enzyme only works at a pH between 4.5 and 5.4. Gastric lipase is present in the stomach and works best on breaking down fat.
Factors that Affect Pepsinogen
For pepsinogen to be activated into pepsin, the stomach must be fairly acidic, so any factors that increase alkalinity in the stomach affect enzyme activity and protein digestion. For example, taking antacid tablets for indigestion reduces the function of pepsin. Furthermore, some prescription medications alter stomach pH and decrease pepsinogen secretion. Protein digestion continues in the small intestine with the help of proteases secreted by the pancreas, but the initial changes to protein that occur in the stomach are important for efficient digestion and health.
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
- Human Physiology: An Integrated Approach; Dee Silverthorn and William Ober
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.