Heartburn is caused by stomach acid flowing backward into the esophagus, the tube that connects your throat to your stomach. This happens when the ring of muscle at the bottom of your esophagus, called the lower esophageal sphincter, relaxes when it shouldn’t or becomes too weak to stay closed. The esophagus has no protective lining against acid the way your stomach does, so even brief exposure creates that familiar burning sensation behind your breastbone.
How the Valve Between Your Stomach and Esophagus Works
At the junction where your esophagus meets your stomach, a ring of smooth muscle acts like a one-way gate. It opens to let food pass down into the stomach, then squeezes shut to keep everything from coming back up. This muscle is responsible for about 90% of the pressure that holds that junction closed. It doesn’t work alone: muscle fibers from the diaphragm wrap around the same area, adding a second layer of protection.
When this system works properly, acid stays where it belongs. When the sphincter weakens or relaxes at the wrong time, acidic stomach contents wash up into the esophagus. That’s reflux. If it happens often enough or severely enough, it becomes gastroesophageal reflux disease, commonly known as GERD.
Foods That Trigger Heartburn
Certain foods cause heartburn through two pathways: they either relax the sphincter muscle directly, or they slow digestion so food sits in the stomach longer, increasing the window for acid to escape. The most common culprits are foods high in fat, salt, or spice. Fried food, fast food, pizza, fatty meats like bacon and sausage, and cheese all fall into this category. Processed snacks like potato chips are another frequent trigger.
Beyond greasy or heavy foods, several other items are well-known offenders:
- Tomato-based sauces and citrus fruits, which are naturally acidic
- Chocolate, which relaxes the esophageal sphincter
- Peppermint, which has the same relaxing effect on the muscle
- Carbonated beverages, which increase pressure inside the stomach
- Spices like cayenne, black pepper, and chili powder
Not everyone reacts to the same foods. Paying attention to what triggers your symptoms specifically is more useful than avoiding every item on a generic list.
How Excess Weight Increases Acid Reflux
Carrying extra weight, particularly around the abdomen, physically squeezes the stomach and pushes its contents upward. The higher your body mass index, the greater the pressure inside your abdominal cavity. Research shows a strong positive correlation between that internal pressure and the number of obesity-related conditions a person develops, with GERD being one of the most common. Even moderate weight gain can increase the frequency of reflux episodes, and losing weight is one of the most effective non-drug approaches to reducing them.
Smoking and Nicotine’s Effect
Nicotine directly weakens the esophageal sphincter. Studies measuring sphincter pressure after nicotine exposure found dose-dependent reductions, meaning the more nicotine, the weaker the muscle became. At higher doses, sphincter pressure dropped by as much as 85%. Nicotine acts on inhibitory nerve pathways that cause the sphincter to relax, and this effect isn’t blocked by standard nerve-blocking agents, which suggests the mechanism is deeply wired into the muscle’s nerve supply. If you smoke or vape and experience frequent heartburn, nicotine is likely a significant contributor.
Heartburn During Pregnancy
Heartburn affects a large proportion of pregnant women, and it tends to worsen as pregnancy progresses. Progesterone, a hormone that rises dramatically during pregnancy, relaxes smooth muscle throughout the body. That includes the esophageal sphincter. Progesterone also slows digestion, which keeps food in the stomach longer and creates more opportunity for reflux. In the later months, the growing uterus pushes upward against the stomach, compounding the problem. For most women, symptoms resolve after delivery once hormone levels return to normal and the physical pressure is relieved.
When a Hiatal Hernia Is Involved
A hiatal hernia occurs when part of the stomach slides upward through the opening in the diaphragm where the esophagus passes through. Normally, the diaphragm and the esophageal sphincter sit at the same level, working together like two hands gripping a tube. When a hiatal hernia develops, these two structures separate. The sphincter gets displaced into the chest cavity, where it loses the benefit of abdominal pressure that normally helps keep it closed. The natural flap valve at the junction between the esophagus and stomach also becomes distorted. The result is a significantly weakened barrier against reflux. Not everyone with a hiatal hernia has heartburn, but the hernia makes the sphincter’s job much harder.
Medications That Can Cause or Worsen Heartburn
Some medications irritate the esophagus directly as they pass through, creating a burning pain that mimics acid reflux. Common examples include ibuprofen, aspirin, certain antibiotics, iron supplements, and osteoporosis medications taken by mouth. Taking these with a full glass of water and staying upright afterward can reduce the irritation.
A separate group of medications actually increases acid reflux by relaxing the esophageal sphincter or altering stomach acid levels. Blood pressure medications like calcium channel blockers and ACE inhibitors fall into this category, along with certain antidepressants, sedatives, opioid painkillers, and medications for overactive bladder. Progesterone supplements can have the same effect, mirroring what happens naturally during pregnancy. If you started a new medication and noticed heartburn worsening, the drug may be contributing. Switching to an alternative is often possible.
Occasional Heartburn vs. GERD
Almost everyone experiences heartburn occasionally, often after a large meal, a late-night snack, or one too many slices of pizza. This is normal and not a sign of disease. GERD is diagnosed when reflux becomes frequent, typically happening two or more times per week, or when it causes damage to the esophageal lining. Doctors can measure the exact amount of time acid spends in contact with the esophagus using a small monitoring device. An acid exposure time above 6% is considered diagnostic of GERD, while below 4% generally rules it out.
The distinction matters because chronic acid exposure can inflame and erode the esophageal lining over time. If your heartburn is infrequent and tied to obvious triggers, lifestyle adjustments and occasional antacids typically handle it. If it’s persistent, happening at night, or accompanied by difficulty swallowing, that points toward GERD and warrants a closer look.