Heartburn happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. A ring of muscle at the bottom of the esophagus, called the lower esophageal sphincter (LES), normally keeps 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 the breastbone. Roughly 14% of adults worldwide experience this regularly, making it one of the most common digestive complaints.
How the Sphincter Fails
The lower esophageal sphincter opens briefly every time you swallow, staying relaxed for about 6 to 10 seconds before tightening again. That’s normal. The problem starts with what researchers call transient lower esophageal sphincter relaxations, or TLESRs: spontaneous openings that happen without swallowing. These relaxations are the primary mechanism behind acid reflux, and they occur in everyone, though people with chronic heartburn tend to have more of them.
The most common trigger for these spontaneous relaxations is stomach distension. When your stomach stretches from a large meal or trapped air, it signals the sphincter to open. Nitric oxide, a signaling molecule produced by nerve endings near the sphincter, is the main chemical messenger that tells the muscle to relax. This is the same compound that relaxes blood vessels elsewhere in the body, which is why certain heart and blood pressure medications can worsen reflux as a side effect.
Foods and Drinks That Trigger Reflux
Different foods provoke heartburn through different routes. Fatty foods slow digestion, meaning food sits in your stomach longer and creates more pressure against the sphincter. Fried dishes, cream sauces, cheese, and fatty cuts of meat are common offenders for this reason.
Caffeine and alcohol both increase stomach acid production while simultaneously weakening the sphincter. Chocolate is a triple threat: it contains caffeine, fat, and a compound called theobromine, all three of which relax the sphincter independently. Citrus fruits, tomatoes, and spicy foods don’t necessarily affect the sphincter but can directly irritate an already-sensitive esophageal lining, making existing reflux feel worse.
Carbonated drinks deserve a separate mention. The gas they introduce into the stomach causes distension, which triggers those spontaneous sphincter relaxations. A large carbonated beverage with a heavy meal is one of the most reliable recipes for heartburn.
Body Weight and Abdominal Pressure
Carrying extra weight around the midsection physically squeezes the stomach, pushing acid upward. This isn’t speculation. A meta-analysis in the Annals of Internal Medicine found a clear dose-response relationship: people with a BMI between 25 and 30 (overweight) were 43% more likely to have reflux symptoms than those at a healthy weight. For people with a BMI over 30 (obese), the risk nearly doubled, at 94% higher.
The mechanisms are layered. Excess abdominal fat increases intra-abdominal pressure, impairs gastric emptying, lowers resting sphincter pressure, and increases the frequency of spontaneous sphincter relaxations. Even modest weight loss can reduce reflux episodes, because it relieves pressure on the stomach from multiple directions at once.
Hiatal Hernia
A hiatal hernia occurs when the upper part of the stomach pushes through the diaphragm into the chest cavity. This displaces the lower esophageal sphincter from its normal position, where the diaphragm normally reinforces its grip. Without that backup support, the sphincter can’t seal as effectively. Studies suggest that 50% to 94% of people with chronic reflux disease have a sliding hiatal hernia, making it one of the most significant anatomical contributors to persistent heartburn.
Small hiatal hernias often cause no symptoms. Larger ones allow acid to pool in the herniated portion of the stomach above the diaphragm, creating a reservoir of acid that easily washes into the esophagus when you lie down or bend over.
Pregnancy and Hormonal Changes
Heartburn affects most pregnant women, and it tends to worsen as pregnancy progresses. Two factors are at work. First, rising progesterone levels relax smooth muscle throughout the body, including the lower esophageal sphincter. The muscle becomes too lax to fully seal, allowing acid to seep upward. Second, the growing uterus physically crowds the stomach, pushing acid toward the esophagus. This is why heartburn is most common and most intense in the third trimester, when the uterus is largest and progesterone levels are at their peak.
Medications That Cause or Worsen Heartburn
Some medications irritate the esophageal lining directly. Common pain relievers like ibuprofen and aspirin fall into this category, along with certain antibiotics, iron supplements, and osteoporosis drugs taken by mouth. The burning they cause can feel identical to acid reflux, even though the mechanism is different.
Other medications actually increase reflux by relaxing the sphincter. Calcium channel blockers and nitrates prescribed for blood pressure, opioid painkillers, certain antidepressants, sedatives like benzodiazepines, and progesterone-based hormones all weaken sphincter tone. If heartburn started or worsened after beginning a new medication, that connection is worth exploring with a pharmacist or prescriber.
Slow Stomach Emptying
When the stomach takes too long to empty its contents into the small intestine, food and acid accumulate, increasing pressure and the likelihood of reflux. This delayed emptying can be a standalone condition called gastroparesis, where the stomach muscles don’t contract properly. Diabetes is the most common underlying cause, but opioid medications also slow gastric motility significantly. Even without a formal diagnosis, some people simply empty their stomachs more slowly than average, and those individuals tend to report more frequent heartburn after meals.
Posture, Timing, and Sleep
Gravity is your esophagus’s best friend. When you’re upright, acid naturally stays in the stomach. Lying down removes that advantage, which is why eating within two to three hours of bedtime is one of the most consistent triggers for nighttime heartburn. Bending over after a meal has a similar effect.
Elevating the head of your bed can help. Clinical trials have tested wedge pillows at 20 to 22 degrees of incline and found they reduce esophageal acid exposure, though they don’t necessarily cut the number of reflux episodes. The goal is to keep acid from lingering in the esophagus once it does escape. Stacking regular pillows usually doesn’t work as well because it bends your body at the waist rather than creating a gradual slope. Sleeping on your left side may also help, since it positions the stomach below the esophageal opening, though controlled clinical trials on this specific position are still limited.
Heartburn vs. Heart Attack
Heartburn and heart attacks can feel strikingly similar, and even experienced physicians can’t always tell the difference from symptoms alone. Heartburn typically produces a burning sensation in the chest that starts after eating, responds to antacids, and may come with a sour taste or a small amount of liquid rising into your throat.
Heart attack pain is more often described as pressure, tightness, or squeezing in the chest that may radiate to the neck, jaw, back, or arms. It’s more likely to come with shortness of breath, cold sweats, lightheadedness, or sudden fatigue. Women are more likely than men to experience jaw or back pain, nausea, and shortness of breath as primary symptoms rather than classic chest pain. If chest pain is new, unexplained, or accompanied by any of these additional symptoms, treat it as a cardiac emergency until proven otherwise.