Stomach acid, or gastric juice, is mostly made up of hydrochloric acid (HCl), with a good amount of sodium chloride and potassium chloride as well. This acid helps break down food by dissolving some of the bonds in protein molecules, then activates enzymes that further separate these compounds, allowing them to be used by the body. Other nutrients, like carbohydrates and fats, are primarily broken down in the intestines, not the stomach.
The stomach contains oxyntic 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 food to arrive. Once in the stomach, chemicals in the food cause more gastric juice to be produced, as does the stretching the stomach wall. When food leaves the stomach, new signals are sent to stop more acid from being released.
Breakdown of Food
Hydrochloric acid denatures the proteins in food, which means that it breaks the bonds that allows the molecules to hold their shapes. This exposes the peptide bonds that hold together the amino acid units that make up the protein molecules. At the same time, HCl activates an important enzyme, pepsinogen, by turning it into pepsin. The 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 to break down food, HCl also acts as a sort of safety mechanism to help protect the body against 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 the bulk of intruders before the immune system even has to get involved. Although not a perfect defense, it helps reduce the workload on the body’s later defenses.
Once the stomach acid has done its job of breaking down the proteins in food, the resulting material is sent onward. Additional digestive juices are secreted from the pancreas and liver into the intestines, where they break down carbohydrates and fats. The small and large intestines take this material and absorb all of the vital nutrients that they can from it. Then, once processed as completely as it can be, the remainder is passed out of 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 is meant to stop juice from the stomach from making its way in. Sometimes, however, this valve does not do its job properly, and fails to keep all of the acid out, allowing some to leak into the esophageal tissue. When this happens, the HCl in the tissue creates a burning sensation known as heartburn, and sometimes an acidic taste in the back of the throat.
Since gastric juice is so strong, the lining of the stomach has to have a defense mechanism to protect itself from damage. It produces mucus that is high in bicarbonate, an alkaline substance that coats the lining of the stomach, neutralizing any acid that comes into contact with it. Sometimes, this mechanism doesn’t work effectively for a number of different reasons, such as overproduction of HCl, lack of sufficient blood supply, or the bacterium Helicobacter pylori, which can infect the stomach’s mucus layer. When this protective function is impaired, the acid can damage the lining, which may result in a gastric ulcer.
Underproduction of HCl can also be a problem. Many essential vitamins are tightly bound to proteins, and if these cannot be broken down effectively, a person may develop a vitamin deficiency, even with a diet that includes enough of them. A lack of acid also undermines the body’s defenses, since bacteria and other dangerous agents might not be destroyed completely. People with low acid production may be more likely to suffer from gastrointestinal infections and illnesses.