Heartburn happens when stomach acid flows backward into the esophagus, the tube that connects your mouth to your stomach. A ring of muscle at the bottom of the esophagus, called the lower esophageal sphincter, normally opens to let food pass into the stomach and then closes to keep acidic digestive juices from rising back up. When that muscle weakens or relaxes at the wrong time, acid escapes upward and irritates the sensitive lining of the esophagus, producing the burning sensation behind your breastbone.
Nearly everyone experiences this occasionally, but the reasons it happens range from what you ate for dinner to how your body is built. Understanding the specific triggers helps explain why some people deal with it rarely while others face it multiple times a week.
How the Sphincter Fails
The lower esophageal sphincter is a complex segment of smooth muscle controlled by nerves and hormones. It works like a one-way gate: food goes down, nothing comes back up. But a wide range of factors can impair that gate. Certain foods, medications, and even signals from the nervous system can cause it to relax when it shouldn’t, or weaken it over time so it no longer seals tightly.
This is the core mechanism behind virtually all heartburn. Whether the trigger is a slice of pizza or a hormonal shift during pregnancy, the end result is the same: the sphincter lets acid through, and the esophagus, which lacks the protective lining the stomach has, burns.
Foods That Trigger It
Some foods cause heartburn by directly relaxing the sphincter muscle or slowing digestion so food sits in the stomach longer, building pressure. The most common culprits include foods high in fat, salt, or spice: fried food, fast food, pizza, bacon, sausage, cheese, potato chips, and processed snacks. Chili powder, black pepper, white pepper, and cayenne are frequent offenders too.
A second group of foods works the same way through different chemistry. Chocolate, peppermint, and carbonated beverages all promote sphincter relaxation. Tomato-based sauces and citrus fruits are a double threat. They can loosen the sphincter and directly irritate the esophageal lining with their acidity. Coffee and alcohol fall into this category as well, both increasing acid production and weakening the barrier that’s supposed to contain it.
Eating a large meal compounds the problem regardless of what’s on the plate. A full stomach puts more pressure on the sphincter, making it easier for acid to push through.
Body Weight and Physical Pressure
Extra weight around the midsection increases pressure on the stomach, which in turn pushes acid upward toward the esophagus. This is one reason heartburn tends to become more frequent as people gain weight, and why losing even a moderate amount of weight often reduces symptoms significantly. Tight clothing around the waist, heavy lifting, and bending over after eating create the same kind of upward pressure, though temporarily.
Hiatal Hernias
Your diaphragm, the large muscle separating your chest from your abdomen, has a small opening called the hiatus that the esophagus passes through on its way to the stomach. In a hiatal hernia, part of the stomach pushes up through that opening into the chest cavity. A small hernia may cause no problems at all, but a larger one allows food and acid to back up into the esophagus much more easily. People with hiatal hernias often experience chronic, recurring heartburn because the normal anatomy that helps keep acid in the stomach is physically disrupted.
Pregnancy and Hormonal Changes
Heartburn is extremely common during pregnancy, and it’s not just because a growing baby presses on the stomach. Rising progesterone levels relax smooth muscle throughout the body, including the lower esophageal sphincter. As progesterone climbs through the second and third trimesters, the sphincter loses its resting pressure and becomes less effective at blocking acid. Research published in the American Journal of Obstetrics and Gynecology found that this hormonal effect, rather than increased abdominal pressure alone, is the primary driver of pregnancy-related heartburn.
Medications That Make It Worse
Several common medications can trigger or worsen heartburn through two different routes. Some irritate the esophageal lining directly as they pass through. Over-the-counter pain relievers like ibuprofen and aspirin are well-known examples. Others work systemically by relaxing the sphincter muscle or increasing acid production. Blood pressure and heart disease medications, including calcium channel blockers and nitrates, fall into this group. If heartburn appeared or worsened after starting a new medication, the timing is likely not coincidental.
Why It Gets Worse at Night
Gravity helps keep stomach acid where it belongs when you’re upright. Lying down removes that advantage, which is why heartburn often flares after going to bed. The position you sleep in matters too. Lying on your left side positions the esophagus and its sphincter higher than the stomach, so acid drains back down more quickly. Sleeping on your right side does the opposite, letting acid pool near the sphincter and increasing the chance it will leak through. Eating within two to three hours of bedtime makes nighttime symptoms more likely because the stomach is still actively producing acid to digest food.
When Heartburn Becomes GERD
Occasional heartburn after a heavy meal is normal. But when acid reflux causes symptoms two or more times a week, it crosses into a chronic condition called gastroesophageal reflux disease, or GERD. The distinction matters because repeated acid exposure can damage the esophageal lining over time, leading to inflammation, narrowing, or cellular changes that need monitoring.
A useful benchmark: if you’re reaching for over-the-counter antacids or acid reducers two or more times a week, you’re likely dealing with GERD rather than occasional heartburn. The underlying mechanisms are identical, but the frequency and persistence mean something different is going on, whether that’s a consistently weak sphincter, an undiagnosed hiatal hernia, a medication side effect, or a combination of factors that keeps the cycle going.