Stomach issues stem from a wide range of causes, from the food you eat to infections, medications, and chronic conditions. In the U.S. alone, 60 to 70 million people are affected by digestive diseases, and roughly one in five adults experiences acid reflux symptoms on a weekly basis. Understanding the most likely culprits can help you narrow down what’s going on in your own gut.
Foods That Trigger Digestive Discomfort
Diet is the most immediate and controllable cause of stomach problems. A group of short-chain carbohydrates called FODMAPs are among the most common triggers. These are sugars that your small intestine doesn’t absorb well. They’re found in everyday foods: wheat, onions, garlic, legumes, dairy, apples, honey, and certain artificial sweeteners like sorbitol. When these carbohydrates reach your large intestine, gut bacteria ferment them and produce gas. At the same time, they draw extra water into the intestinal tract. The combination of gas and water stretches the intestinal wall, causing bloating, cramping, and pain.
For people with sensitive guts, that stretching produces exaggerated sensations of discomfort compared to what someone without a sensitive digestive system would feel eating the same meal. Fatty and fried foods, alcohol, and coffee can also provoke symptoms by relaxing the valve between your stomach and esophagus or by stimulating excess acid production. Large meals and eating late at night compound the problem.
Food Intolerances and Sensitivities
A food intolerance is different from a food allergy. Allergies involve the immune system and can be life-threatening. Intolerances typically mean your body lacks the enzyme to break down a specific substance, or your gut reacts to it in a way that causes discomfort rather than danger.
Lactose intolerance is the most well-known example. People who are lactose intolerant don’t produce enough of the enzyme that breaks down the sugar in milk and dairy products. Undigested lactose passes into the large intestine, where bacteria ferment it, producing gas, bloating, cramps, and diarrhea. Gluten sensitivity is another common culprit, though it’s harder to pin down. Unlike celiac disease, which causes measurable intestinal damage, non-celiac gluten sensitivity produces similar symptoms (bloating, abdominal pain, fatigue) without the same immune markers. Symptoms can appear anywhere from hours to days after eating gluten, which makes it tricky to connect the dots without a structured elimination diet.
Acid Reflux and GERD
About 20 percent of the U.S. population experiences reflux symptoms at least once a week. Acid reflux happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. At the bottom of that tube sits a ring of muscle called the lower esophageal sphincter. It’s supposed to open when you swallow and then close tightly. When this muscle weakens or relaxes at the wrong time, acid escapes upward, causing the burning sensation most people call heartburn.
Several things increase the risk. Obesity puts extra pressure on the abdomen. Pregnancy does the same. A hiatal hernia, where the upper part of the stomach pushes up through the diaphragm into the chest cavity, physically undermines the sphincter’s ability to stay closed. Smoking weakens the sphincter over time. When reflux becomes frequent and persistent, it crosses into gastroesophageal reflux disease, or GERD, which can damage the lining of the esophagus if left untreated.
Bacterial Infections
Infections are a massive source of stomach trouble. Nonfoodborne gastroenteritis affects an estimated 135 million Americans, while foodborne illness accounts for another 76 million cases. Most of these are short-lived bouts of nausea, vomiting, and diarrhea caused by viruses or bacteria picked up from contaminated food, water, or close contact with an infected person.
One bacterial infection stands out for its long-term impact: H. pylori. This bacterium burrows into the protective mucus lining of your stomach. Once there, it produces an enzyme that neutralizes the surrounding stomach acid, creating a pocket where the bacteria can thrive. The problem is that this process also weakens the stomach’s protective barrier, leaving the underlying cells exposed to acid and digestive enzymes. The result is inflammation that can progress to ulcers. About 15.5 million Americans have peptic ulcer disease. H. pylori spreads through mouth-to-mouth contact, contaminated water, and food that wasn’t properly cleaned or cooked.
Medications That Damage the Stomach
Over-the-counter pain relievers are one of the most overlooked causes of stomach problems. Ibuprofen, aspirin, and naproxen all belong to a class of drugs that reduce pain and inflammation but simultaneously compromise your stomach’s defenses. These medications interact with the fatty layer that lines your stomach wall, essentially stripping away its waterproof coating. They also reduce blood flow to the stomach lining by blocking enzymes your body needs to maintain that blood supply. With less blood flow and a weakened barrier, your stomach becomes vulnerable to its own acid. The result can range from mild irritation to erosions and full-blown ulcers, especially with regular or long-term use.
Irritable Bowel Syndrome
IBS affects over 15 million Americans and is one of the most common reasons people seek help for chronic stomach and bowel problems. It’s classified as a functional disorder, meaning the digestive tract looks structurally normal but doesn’t behave normally. The hallmark is recurrent abdominal pain at least one day per week for three months, tied to changes in how often you go to the bathroom or what your stool looks like. Symptoms must have started at least six months before a diagnosis is considered.
The exact cause of IBS isn’t fully understood, but it involves a combination of gut sensitivity, disrupted communication between the brain and the digestive system, and changes in how quickly food moves through the intestines. Stress, certain foods (particularly high-FODMAP foods), and hormonal shifts can all trigger flare-ups. People with IBS often have a gut that overreacts to normal amounts of gas and stretching, which is why a meal that doesn’t bother one person can cause significant pain in someone with the condition.
Stress and the Gut-Brain Connection
Your brain and your digestive system are in constant communication through a dense network of nerves. Stress, anxiety, and emotional upheaval can directly alter how your stomach and intestines function. Stress hormones slow digestion in some people and speed it up in others, producing symptoms that range from nausea and loss of appetite to cramping and diarrhea. This isn’t imaginary. The gut has its own nervous system with more nerve cells than the spinal cord, and it responds to psychological stress just as it responds to a bad meal. For people with IBS or other functional gut disorders, stress is one of the most reliable triggers for symptom flare-ups.
Other Common Causes
Several other conditions round out the picture. Gallstones affect an estimated 20 million Americans and typically cause pain in the upper right abdomen after eating, especially fatty meals. Constipation is remarkably common, affecting 63 million people, and can produce bloating, cramping, and a general sense of abdominal discomfort that many people describe simply as “stomach issues.” Diverticular disease, where small pouches form in the wall of the large intestine and sometimes become inflamed, affects over 2 million people. Pancreatitis, inflammation of the organ that produces digestive enzymes, causes severe upper abdominal pain and affects about 1.1 million Americans.
Symptoms That Need Urgent Attention
Most stomach issues are uncomfortable but not dangerous. A few warning signs, however, suggest something more serious is going on. Blood in your stool or vomit, severe abdominal pain that makes it hard to move or eat, sudden onset of intense pain, and high fever alongside stomach symptoms all warrant an emergency room visit rather than a wait-and-see approach. Unexplained weight loss combined with ongoing digestive symptoms also raises concern for conditions that need prompt evaluation. It’s worth knowing that heart problems, including heart attacks, can sometimes present as severe nausea or pain in the upper abdomen beneath the rib cage, particularly in women and older adults.