Heartburn happens when stomach acid flows backward into the esophagus, a tube that isn’t built to handle it. Your stomach produces acid with a pH below 4, strong enough to break down food, and a muscular valve at the bottom of the esophagus normally keeps that acid where it belongs. When that valve fails, even briefly, acid contacts tissue that has almost no protective barrier, and the result is that familiar burning sensation behind your breastbone. Around 10 to 20 percent of adults in Western countries experience it regularly.
The Valve That Keeps Acid in Place
At the junction where your esophagus meets your stomach sits a ring of muscle that acts like a one-way gate. It stays squeezed shut at a resting pressure of about 15 to 30 mmHg, which is enough force to hold back the contents of your stomach even when you bend over or lie down. When you swallow food or liquid, the muscle briefly relaxes to let everything pass through, then tightens again.
This valve doesn’t work alone. Your diaphragm, the sheet of muscle you use to breathe, wraps around the same spot and acts like an external clamp, reinforcing the seal. Together, these two structures form a barrier that opens on command and stays closed the rest of the time.
What Goes Wrong
The most common cause of acid escaping upward is something called a transient relaxation: the valve opens spontaneously without a swallow triggering it. Everyone experiences these random relaxations, but in people with frequent heartburn, they happen more often or allow more acid through when they occur. This is different from a permanently weak valve. For most people, the muscle works fine most of the time and simply misfires at the wrong moments.
A hiatal hernia can make the problem structural. In this condition, the upper part of the stomach pushes up through the opening in the diaphragm, separating the valve from the diaphragm’s reinforcing squeeze. This reduces the valve’s effective length and pressure, removes the backup support of the diaphragm, and creates a small pocket above the diaphragm where acid can pool and reflux into the esophagus during swallowing.
Increased pressure inside the abdomen can also force acid upward. Excess body weight compresses the stomach and shifts the pressure gradient across the valve, making it easier for contents to push through. Pregnancy creates the same effect as the uterus grows and presses against the diaphragm. Even temporary strains like heavy lifting, coughing, or vomiting can spike abdominal pressure enough to overwhelm the valve.
Why the Esophagus Can’t Handle Acid
Your stomach lining has a thick mucus coat and cells that secrete bicarbonate, a natural acid neutralizer, directly onto its surface. Damage to the stomach wall heals remarkably fast because cells at the edges of a wound can migrate over the injured area in under an hour. The esophagus has none of these advantages.
The esophagus lacks a true protective mucus layer. Its surface cells don’t secrete bicarbonate. The only mucus that reaches it comes from saliva and small glands in the esophageal wall, and this mucus is the soluble type: good for lubrication, unable to form a fixed protective barrier like the stomach’s lining. When the esophageal lining does get damaged, it can’t patch itself quickly. Instead of cell migration, it relies on cell replication, a process that takes days to weeks.
The esophagus does have a few defenses. Gravity and the rhythmic contractions that move food downward help clear acid that splashes up. Swallowed saliva delivers bicarbonate to the esophageal surface, neutralizing small amounts of acid. Cells lining the esophagus have tight junctions that slow acid penetration, and blood flow beneath the surface delivers additional bicarbonate to buffer acid that gets through. But these defenses are designed to handle occasional, brief exposures. When reflux becomes frequent or prolonged, the damage outpaces what these systems can repair.
How Food and Drink Trigger It
Certain foods directly cause the esophageal valve to relax and slow stomach emptying, which means food sits in the stomach longer and produces more opportunities for reflux. The biggest offenders are foods high in fat, salt, or spice: fried food, fast food, pizza, fatty meats like bacon and sausage, cheese, and processed snacks like potato chips. High-fat meals are particularly effective at loosening the valve because fat triggers hormonal signals that reduce the muscle’s tone.
Other common triggers work through different mechanisms. Chocolate and peppermint contain compounds that directly relax smooth muscle, including the esophageal valve. Tomato-based sauces and citrus fruits are acidic themselves, so they add to the acid load in the stomach. Carbonated beverages introduce gas that distends the stomach, increasing pressure and provoking those spontaneous valve relaxations. Caffeine can also loosen the valve, though sensitivity varies from person to person.
What Heartburn Actually Feels Like
The hallmark sensation is a burning feeling in the center of the chest, sometimes extending into the upper abdomen. It tends to show up after eating, or when you lie down or bend over, because both positions make it easier for acid to flow backward. Many people notice a sour or bitter taste in the back of the throat, and some experience a small amount of stomach contents rising up, which is regurgitation rather than vomiting. Heartburn that wakes you from sleep is common if you ate within a couple of hours of going to bed. Antacids typically relieve the symptoms within minutes.
Heartburn vs. Heart Attack Pain
Heartburn and heart-related chest pain can feel strikingly similar. Even experienced physicians can’t always distinguish between the two based on symptoms alone. There are patterns that help, though.
Heartburn is tied to meals, relieved by antacids, and often accompanied by that sour taste or regurgitation. Heart attack pain tends to feel more like pressure, tightness, or squeezing in the chest or arms, and it may radiate to the neck, jaw, or back. Heart attacks often come with shortness of breath, cold sweats, lightheadedness, or sudden fatigue. Women are more likely than men to experience jaw pain, back pain, nausea, or shortness of breath as their primary symptoms rather than classic crushing chest pain.
If chest pain comes with any of those additional symptoms, especially shortness of breath or pain spreading to the jaw or arm, treat it as a cardiac emergency. The burning from reflux is unpleasant but not dangerous in the moment. The consequences of mistaking a heart attack for heartburn are far more serious than the reverse.