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How Does Stomach Acid Break down Food?

By Brendan McGuigan
Updated May 21, 2024
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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. Sometimes, though, there's an overproduction of acid which can cause discomfort. This can be addressed with certain green foods or natural alternatives.

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. So the more packed with nutrients a certain food or supplement is, the more the body will absorb even when taking what goes to waste into account. Then, once processed as completely as it can be, the remainder is passed out of the body as waste. This entire process is one of the basis used by manufacturers of green foods and organic supplements to make sure that consumers will be able to maximize the benefits of these products.

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. Some of these processes may cause gas, bloating, or a general abdominal discomfort. 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. This can lead to a weaker immune system which can cause all sorts of problems. 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.

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Discussion Comments
By anon243251 — On Jan 26, 2012

How do you know if you have Heliocobacter Pylori bacteria present in your stomach (and taking prilosec isn't working either)?

By Pimiento — On Oct 11, 2010

@plaid - Yes, it is important to maintain healthy stomach acid PH. Overactive stomach acid symptoms are usually coupled with severe abdominal pain which could also be connected with several other symptoms. Occasionally eating too much food will result in that nasty burping of stomach acid that many people associate with throwing up a little bit. It is gross, but it does happen a lot more than you think.

If you stay away from consuming too many spicy or complicated foods like fried foods and such, your stomach will be a healthy environment that will help you get all the essentials you need and are looking for from your food.

By plaid — On Oct 11, 2010

@WalrusTusk - The good thing is that there are several over the counter medications for stomach acid reflux now. They are worth trying if the standard milk thing doesn't work for you. But you are definitely right in that excess acid effects the way your stomach breaks down food. For instance, it might over process the food or process it much too quickly and you risk losing essential nutrients and/or vitamins you might have otherwise normally gotten.

By WalrusTusk — On Oct 11, 2010

@empanadas - Have excess stomach acid like that is a very bad thing. Too much acid can eat away at the lining of your stomach which can, obviously, effect the way that your stomach breaks down food.

With the same thought in mind, this is also how acid reflux works - meaning it can eat away at the lining in your throat and at your gums. I know that sounds gross, but it happens to a very large percentage of people believe it or not.

By empanadas — On Oct 11, 2010

A lot of foods can cause severe stomach acid problems which can lead to acid reflux and even gum disease. It is not pleasant or pretty, but it does happen... especially with food that are rich in spices like cumin, red pepper, or chili powder.

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