Heartburn happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. Over 825 million people worldwide experience this regularly, making it one of the most common digestive complaints. The burning sensation you feel behind your breastbone isn’t actually about your heart. It’s acid touching tissue that wasn’t designed to handle it.
How the Valve at Your Stomach Fails
At the base of your esophagus sits a ring of muscle that acts like a one-way gate. When you swallow, this muscle relaxes to let food drop into your stomach, then tightens again to keep everything sealed. Heartburn occurs when this muscle relaxes at the wrong time or becomes too weak to close properly, allowing stomach acid to wash upward into the esophagus.
Your stomach lining is built to withstand acid. Your esophagus is not. When acid makes contact with the esophageal lining, it irritates the tissue and produces that familiar burning pain. The longer acid stays in contact with the esophagus, the worse the discomfort. This is why heartburn often feels worse when you’re lying down: gravity is no longer helping drain acid back into the stomach, so it lingers.
Foods and Drinks That Trigger It
Certain foods directly weaken that muscular valve. Chocolate, coffee, alcohol, mint, garlic, and onions all have a relaxing effect on the muscle, especially in larger amounts. When the valve loosens, acid escapes more easily.
Fatty foods are a double problem. They increase the amount of acid your stomach produces while also taking longer to digest. That combination means more acid sitting in your stomach for a longer period, creating more opportunities for it to back up. This is why a heavy, greasy meal is one of the most reliable heartburn triggers. Spicy foods and citrus don’t necessarily weaken the valve, but they can irritate an already-sensitive esophagus and make symptoms feel more intense.
How Body Weight Plays a Role
Carrying extra weight around your midsection puts physical pressure on your stomach. That pressure pushes stomach contents upward toward the valve, making it more likely that acid will be forced into the esophagus. This is a mechanical effect: the more pressure on the stomach, the harder the valve has to work to stay shut. Even moderate weight gain can increase heartburn frequency, and losing weight often reduces symptoms noticeably.
Hiatal Hernia and Structural Changes
Your diaphragm, the flat muscle separating your chest from your abdomen, has a small opening where the esophagus passes through. A hiatal hernia occurs when the top of your stomach pushes up through that opening into your chest cavity. This creates two problems. First, it pulls the valve out of its normal position, weakening the muscles that help keep it closed. Second, the hernia traps a pocket of acid above the stomach that can’t drain away normally. Many people have small hiatal hernias without knowing it, but larger ones are a common cause of persistent heartburn.
Pregnancy and Hormonal Changes
Heartburn affects a large proportion of pregnant women, and the reasons are both hormonal and physical. Progesterone, which rises significantly during pregnancy, relaxes smooth muscle throughout the body, including the valve at the base of the esophagus. This makes it easier for acid to escape. Progesterone also slows digestion, keeping food in the stomach longer and increasing the window for reflux. As pregnancy progresses, the growing uterus pushes upward on the stomach, adding physical pressure that compounds the hormonal effect.
Medications That Weaken the Valve
Several common medications reduce the strength of the esophageal valve as a side effect. Calcium channel blockers (used for high blood pressure), nitrates (used for chest pain), and theophylline (used for asthma) all decrease the valve’s ability to stay tightly shut. If you started a new medication and noticed heartburn appearing or worsening, the drug itself may be the cause. Anti-inflammatory painkillers like ibuprofen can also irritate the stomach lining and esophagus directly, compounding the problem.
Slow Stomach Emptying
A condition called gastroparesis causes the stomach muscles to stop contracting normally, which means food sits in the stomach far longer than it should. The longer food and acid remain in the stomach, the greater the chance that acid backs up into the esophagus. Gastroparesis is most common in people with diabetes but can also develop after certain surgeries or infections. If your heartburn comes with bloating, nausea, and feeling full after just a few bites of food, slow stomach emptying could be a contributing factor.
Why Heartburn Gets Worse at Night
Lying flat eliminates gravity’s help in keeping acid where it belongs. During the day, when you’re upright, any acid that escapes the stomach drains back down relatively quickly. At night, it pools in the esophagus. Research from Harvard Health found that while body position doesn’t change how often acid backs up, it dramatically affects how long the acid stays in contact with the esophagus. Sleeping on your left side clears acid faster than sleeping on your back or right side. Eating within two to three hours of bedtime also worsens nighttime symptoms because your stomach is still actively producing acid to digest that meal.
When Heartburn Becomes GERD
Everyone gets occasional heartburn. It becomes a clinical condition, gastroesophageal reflux disease (GERD), when acid reflux causes symptoms two or more times a week. GERD is also diagnosed when acid exposure has started damaging the tissue lining the esophagus, regardless of symptom frequency. That tissue damage can lead to inflammation, ulcers, or over time, changes in the cells lining the esophagus.
If you’re reaching for over-the-counter antacids more than twice a week, that pattern itself suggests the problem has moved beyond occasional heartburn into something that needs a more targeted approach. Persistent acid exposure isn’t just uncomfortable. It can narrow the esophagus through scar tissue or cause cellular changes that need monitoring.