Heartburn happens when stomach acid flows backward into your esophagus, the tube connecting your mouth to your stomach. A ring of muscle at the bottom of that tube, called the lower esophageal sphincter, normally stays closed to keep acid where it belongs. When that muscle relaxes at the wrong time or becomes too weak, acid escapes upward and irritates the esophageal lining, producing that familiar burning sensation behind your breastbone. An estimated 10 to 20% of adults in Western countries experience this regularly, and global cases nearly doubled between 1990 and 2021, reaching over 825 million people.
How the Sphincter Fails
Your lower esophageal sphincter operates under a balance of signals telling it to tighten or relax. Tightening signals come primarily through the same nerve chemical that controls many muscles throughout your body (acetylcholine). Relaxation signals rely mainly on nitric oxide, which suppresses calcium flow into the muscle cells and causes them to loosen.
The most common cause of acid escape is something called a transient relaxation. This is a brief, spontaneous opening of the sphincter that occurs outside of swallowing. It’s actually a protective reflex: when your stomach stretches with food or gas, a nerve signal triggers the sphincter to open and vent excess air. The problem is that acid often hitches a ride with the gas. In people who get frequent heartburn, these transient relaxations happen more often or last longer than they should.
Foods and Drinks That Trigger It
Certain foods directly weaken the sphincter or increase acid production. Chocolate contains methylxanthine, a compound from the cocoa plant that’s chemically similar to caffeine and relaxes the sphincter muscle. Coffee, both caffeinated and decaf, can stimulate extra acid secretion. Peppermint, while soothing for some digestive complaints, also relaxes the sphincter and can make reflux worse.
Fatty and fried foods slow stomach emptying, which keeps the stomach full and stretched for longer. That prolonged stretching triggers more of those transient sphincter relaxations. Acidic foods like tomatoes, citrus, and vinegar don’t necessarily cause reflux on their own, but they intensify the burning when acid is already reaching the esophagus. Spicy foods work similarly, irritating tissue that’s already exposed. Alcohol and carbonated drinks both increase gastric pressure and relax the sphincter, a combination that makes reflux more likely.
Body Weight and Abdominal Pressure
Carrying extra weight around your midsection physically squeezes the stomach. In people with severe obesity, the pressure inside the abdomen can reach levels high enough to overpower the sphincter’s ability to stay shut. A healthy sphincter maintains a resting pressure above 10 mmHg to keep acid contained. Abdominal pressure in morbid obesity has been measured at around 9 mmHg, which gets dangerously close to overwhelming that barrier, especially if the sphincter is already somewhat weakened.
This is why even moderate weight gain can trigger new heartburn symptoms, and why weight loss is one of the most consistently effective lifestyle changes for reducing reflux. The effect is mechanical: less abdominal fat means less upward force on the stomach.
Hiatal Hernia
A hiatal hernia occurs when part of the stomach pushes up through the diaphragm, the flat muscle separating your chest from your abdomen. Normally, the diaphragm wraps around the sphincter and reinforces it like a second clamp. When a hernia displaces the stomach upward, several things go wrong at once: the sphincter gets shorter and weaker, the diaphragm can no longer reinforce it, and the portion of stomach sitting above the diaphragm acts as a reservoir of acid that can splash into the esophagus between swallows.
Not everyone with a hiatal hernia gets heartburn, and not everyone with heartburn has a hernia. But the combination of a hernia with a weakened sphincter creates a setup where reflux becomes much harder to control with lifestyle changes alone.
Pregnancy
Heartburn affects the majority of pregnant women, particularly in the second and third trimesters. Two forces work against you simultaneously. Progesterone, which rises dramatically during pregnancy, relaxes smooth muscle throughout the body, including the esophageal sphincter. It also slows digestion, meaning food sits in the stomach longer. Meanwhile, the growing uterus pushes upward on the stomach, increasing the physical pressure that forces acid through a weakened sphincter. These symptoms typically resolve after delivery.
Medications That Make It Worse
A surprisingly long list of common medications can cause or worsen heartburn. Some relax the sphincter directly: calcium channel blockers and nitrates (used for blood pressure and heart disease), opioid painkillers, sedatives like benzodiazepines, certain antidepressants, and medications for overactive bladder. Others irritate the esophageal lining on the way down, including certain antibiotics, bone density medications (bisphosphonates), iron supplements, and potassium supplements.
Over-the-counter pain relievers like ibuprofen and aspirin occupy a complicated space. These drugs inhibit the production of prostaglandins, which protect the lining of the digestive tract. While there’s theoretical reason to think they might actually help the esophagus (since prostaglandins can relax the sphincter), clinical experience shows they frequently cause esophageal irritation, ulceration, and worsening reflux symptoms. If you take these regularly and have heartburn, that connection is worth exploring.
Eating Habits and Timing
What you eat matters, but when and how you eat plays an equally large role. Large meals stretch the stomach more, triggering more transient sphincter relaxations. Eating within two to three hours of lying down is one of the most reliable triggers because gravity can no longer help keep acid in the stomach. Eating quickly leads to more air swallowing, which increases gastric distension.
Tight clothing around the waist, bending over after meals, and vigorous exercise on a full stomach all increase abdominal pressure in the same way excess body weight does, just temporarily.
Sleep Position Matters
Nighttime heartburn tends to be more damaging because acid stays in contact with the esophagus longer when you’re lying flat. Your sleep position makes a measurable difference. In a study using simultaneous sleep position tracking and esophageal acid monitoring, sleeping on the left side produced significantly less acid exposure (a median of 0.0% of the time) compared to sleeping on the right side (1.2% of the time). This happens because of anatomy: when you lie on your left, the stomach hangs below the esophageal opening, and acid pools away from the sphincter. On your right side, the junction sits in a lower position relative to the acid pool, making reflux easier. Elevating the head of your bed by six to eight inches provides additional gravitational protection.
Low Stomach Acid Can Cause It Too
Most people assume heartburn means too much acid. That’s often the case, but low stomach acid can produce identical symptoms. When the stomach doesn’t produce enough acid, food digests more slowly and sits in the stomach longer, causing the same distension and fermentation that triggers sphincter relaxation. The Cleveland Clinic notes that symptoms commonly blamed on excess acid, including heartburn and reflux, can actually stem from insufficient acid production. This is an underdiagnosed condition, and it matters because acid-suppressing medications would make it worse rather than better.
Stress and Nervous System Activation
Stress doesn’t directly produce stomach acid, but it alters how your digestive system functions. The sphincter’s relaxation reflex runs through the vagus nerve, the same nerve that responds to emotional and physical stress. Stress can increase sensitivity to normal levels of acid exposure, meaning the same amount of reflux that wouldn’t bother you on a calm day produces noticeable burning when you’re anxious. It can also slow digestion and change the pattern of muscle contractions in the esophagus, reducing its ability to clear acid quickly after a reflux episode.