How does stomach acid digest food?

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Stomach acid, or gastric juice, consists mainly of hydrochloric acid (HCl), with also a good amount of sodium chloride and potassium chloride. This acid helps break down food by dissolving some of the bonds in protein molecules, then activates enzymes that further break these compounds apart, allowing them to be used by the body. Other nutrients, such as carbohydrates and fats, are primarily broken down in the intestines, not the stomach.

Production

The stomach contains oxynt cells (also called parietal cells), which release hydrochloric acid in response to a number of different factors. Seeing, smelling, tasting, or even thinking about food, for example, causes the brain to send signals to the stomach to prepare it for the food to arrive. Once in the stomach, the chemicals in the food cause more gastric juice to be produced, as well as the stomach wall to stretch. As food leaves the stomach, new signals are sent to stop more acid from being released.

Breakdown of food

Hydrochloric acid denatures the proteins in foods, meaning it breaks down the bonds that allow the molecules to hold their shape. This exposes the peptide bonds that hold together the amino acid units that make up protein molecules. At the same time, HCl activates an important enzyme, pepsinogen, turning it into pepsin. Pepsin then breaks the peptide bonds in proteins, freeing the amino acids and allowing them to be absorbed by the body.

In addition to helping break down food, HCl also acts as a sort of safety mechanism to help protect the body from dangerous bacteria that may have been ingested with food or water. Its pH is typically between 1 and 2, which is very strong. The highly acidic environment is deadly to the vast majority of harmful bacteria and other microorganisms, helping to wipe out most intruders before the immune system even needs to be involved. While not a perfect defense, it does help reduce the workload on the body’s subsequent defenses.

Once the stomach acid has done its job of breaking down the proteins in the food, the resulting material is sent onward. Additional digestive juices are secreted by the pancreas and liver into the intestines, where they break down carbohydrates and fats. The small and large intestines take in this material and absorb all the vital nutrients they can from it. Then, once processed as completely as possible, the rest is eliminated from the body as waste.

Problems caused by stomach acid
When food is swallowed, it goes down a long tube called the esophagus, which has strong muscles at both ends and a valve at the bottom that’s meant to keep stomach juice from getting in. Sometimes, however, this valve does not do its job properly and fails to keep all the acid out, allowing some to seep into the esophageal tissue. When this happens, the HCl in the tissue creates a burning sensation known as heartburn and sometimes a sour taste in the back of the throat.
Because gastric juice is so strong, the stomach lining must have a defense mechanism to protect itself from damage. It produces mucus that is high in bicarbonate, an alkaline substance that coats the stomach lining, neutralizing any acid that comes in contact with it. Sometimes, this mechanism doesn’t work effectively for a variety of different reasons, such as overproduction of HCl, lack of sufficient blood supply, or the bacteria Helicobacter pylori, which can infect the mucus layer of the stomach. When this protective function is compromised, the acid can damage the lining, which can lead to a stomach ulcer.

Underproduction of HCl can also be a problem. Many essential vitamins are closely related to protein, and if protein cannot be broken down effectively, a person can develop a vitamin deficiency, even with a diet that includes enough. A lack of acid also undermines the body’s defenses, as bacteria and other harmful agents may not be destroyed completely. People with low acid production may be more likely to suffer from gastrointestinal infections and illnesses.

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