Indigestion happens when your stomach struggles to process food normally, whether because of how it moves, how much acid it produces, or how sensitive its nerves are. About 8% of people worldwide experience chronic indigestion, and occasional episodes are far more common than that. The causes range from what you ate for lunch to how your stomach communicates with your brain.
How Your Stomach Normally Processes Food
To understand indigestion, it helps to know what’s supposed to happen. When you swallow food, it passes through a muscular ring at the bottom of your esophagus called the lower esophageal sphincter. This ring stays tightly closed at rest, maintaining enough pressure to keep stomach acid from flowing backward. Once food enters the stomach, muscles in the stomach wall churn and mix it with acid and enzymes, gradually breaking it into a semi-liquid that moves into your small intestine.
Your pancreas produces three key enzymes for this process: one that breaks down carbohydrates, one for fats, and one for proteins. When any part of this system falters, whether it’s the muscular ring, the stomach’s churning motion, or enzyme production, you feel it as indigestion. The symptoms vary depending on which step went wrong: burning in your chest, fullness after a few bites, bloating, nausea, or upper belly pain.
Acid Reflux and the Muscular Ring
One of the most common causes of indigestion is acid escaping upward from your stomach. The muscular ring at the base of your esophagus sometimes relaxes on its own, without any swallowing to trigger it. These spontaneous relaxations last 10 to 45 seconds, long enough for acid to wash into the esophagus. They happen in everyone, but in people prone to reflux, they occur more frequently or cause more damage.
When your stomach is full of food or gas, it stretches, and that stretch triggers these relaxations as a pressure-release valve. This is why large meals, carbonated drinks, and swallowing air can all provoke heartburn. A hiatal hernia, where part of the stomach pushes up through the diaphragm, can also prevent the ring from closing completely, making reflux a recurring problem.
When Your Stomach Moves Too Slowly (or Too Fast)
Your stomach is a muscular organ, and like any muscle, it can underperform. In many people with chronic indigestion, the stomach empties food into the small intestine too slowly. Food sits longer than it should, producing that heavy, overly full sensation after meals. The upper portion of the stomach can also fail to relax and expand properly when food arrives, which is why some people feel uncomfortably full after eating only a small amount.
Less commonly, the stomach empties too quickly or contracts in irregular rhythms. Both can cause nausea and discomfort. These motility problems are a hallmark of functional dyspepsia, the medical term for chronic indigestion that occurs without any visible damage to the stomach lining.
Foods and Drinks That Trigger Symptoms
Certain foods are reliably harder on the stomach. Fatty and greasy foods slow gastric emptying, keeping food in the stomach longer and increasing acid exposure. Coffee stimulates acid production. Carbonated drinks inflate the stomach with gas, triggering those spontaneous relaxations of the esophageal ring. Citrus fruits and juices are acidic on their own and can irritate an already sensitive stomach lining.
Wheat and other grains cause problems for some people, even without celiac disease, possibly because they’re harder to break down or because they ferment in the gut and produce gas. Alcohol relaxes the esophageal sphincter and directly irritates the stomach lining. Eating quickly, eating large portions, or eating close to bedtime all make indigestion more likely because they keep the stomach fuller for longer.
How Stress Affects Your Stomach
The connection between stress and indigestion isn’t just in your head. Your brain and stomach communicate constantly through the vagus nerve, a major nerve highway running from your brainstem to your gut. When you’re under stress, your brain changes how it signals through this nerve, altering stomach acid production and the speed at which your stomach contracts.
Stress also makes the nerves in your stomach wall more sensitive. Normal amounts of gas or stretching that you’d usually never notice can register as pain or discomfort. This heightened sensitivity, where the stomach overreacts to ordinary signals, is one of the main reasons some people experience indigestion without any obvious physical cause. The stomach looks perfectly healthy on examination, but the nervous system is amplifying routine sensations into distressing symptoms.
Medications That Irritate the Stomach
Common pain relievers like ibuprofen and aspirin are a frequent and underappreciated cause of indigestion. These drugs work by blocking enzymes called COX-1 and COX-2 throughout the body. The problem is that those same enzymes help maintain the stomach’s protective mucus layer and regulate blood flow to the stomach lining.
When these enzymes are suppressed, two things happen. First, the stomach loses some of its protective coating, leaving the lining more exposed to acid. Second, the stomach begins contracting more forcefully than normal. These stronger contractions compress the folds of the stomach lining, restricting blood flow to those areas and creating small zones of damage. With repeated use, this can progress from mild irritation to full erosions or ulcers. Taking these medications with food helps but doesn’t eliminate the risk.
Bacterial Infection of the Stomach Lining
A bacterium called H. pylori infects the stomachs of a significant portion of the world’s population, often without causing any symptoms at all. It survives in the harsh stomach environment by burrowing into the mucus layer and producing ammonia, which neutralizes the acid immediately around it. Over time, it triggers chronic inflammation of the stomach lining.
H. pylori contributes to indigestion in several ways: it increases acid production, damages the stomach’s natural defenses, and produces toxins that injure the lining directly. When symptoms do appear, they typically include upper abdominal pain or discomfort and a general sense of indigestion. The infection is treatable with a course of antibiotics, and clearing it often resolves symptoms that have persisted for years.
Enzyme Deficiency and Poor Breakdown
Some people don’t produce enough digestive enzymes, or their pancreas doesn’t release them properly. When the pancreas underperforms, a condition called exocrine pancreatic insufficiency, your body can’t adequately break down carbohydrates, proteins, or fats. The result is food passing through the digestive tract only partially digested, leading to bloating, gas, and general GI discomfort.
This can happen after repeated bouts of pancreatitis, in people with cystic fibrosis, or sometimes without an identifiable cause. It’s less common than the other triggers on this list, but worth considering if you experience persistent bloating and indigestion that doesn’t respond to dietary changes or acid-reducing medications.
What Chronic Indigestion Looks Like
Occasional indigestion after a large or rich meal is normal. Chronic indigestion, or functional dyspepsia, is diagnosed when symptoms persist for three months or longer without an identifiable structural cause. It generally falls into two patterns. In one, the main complaint is uncomfortable fullness during or after meals and feeling full after eating very little. In the other, the primary symptom is burning or pain in the upper abdomen that isn’t tied to meals.
Many people experience overlap between the two. A global analysis of over 250,000 participants across 40 countries found that about 8.4% of the population meets criteria for functional dyspepsia. That number has actually decreased over the past few decades, from about 12% in the early 1990s to around 7% in recent years, possibly reflecting changes in H. pylori infection rates and improved treatment options.
If your indigestion is new, occasional, and clearly tied to specific meals or stressful periods, it’s typically a matter of adjusting what and how you eat. If it’s persistent, worsening, or accompanied by unintentional weight loss, difficulty swallowing, or vomiting, those patterns warrant investigation to rule out structural causes like ulcers or other conditions affecting the stomach lining.