Heartburn happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. A muscular valve at the bottom of the esophagus normally keeps acid where it belongs, but when that valve relaxes at the wrong time or weakens over time, acid escapes upward and irritates the unprotected lining. The result is that familiar burning sensation behind your breastbone. Roughly 10 to 20 percent of adults in Western countries experience this regularly, and the causes range from what you ate for dinner to medications you take every day.
How the Valve at the Bottom of Your Esophagus Works
The lower esophageal sphincter (LES) is a ring of muscle that opens when you swallow and closes immediately after. A normal swallow triggers it to relax for about six to ten seconds, just long enough for food to pass through. The diaphragm, the large breathing muscle that wraps around the esophagus where it meets the stomach, adds a second layer of reinforcement.
Problems start when the LES relaxes outside of swallowing. This is called a transient relaxation, and it’s actually a normal reflex triggered by a full or gas-filled stomach. The valve opens briefly to let excess air escape (this is what produces a belch). But when it happens too often, stomach acid hitches a ride. Anything that weakens the LES, increases pressure below it, or causes it to relax more frequently can produce heartburn.
Foods and Drinks That Trigger Heartburn
Certain foods lower the resting pressure of the LES, making it easier for acid to slip through. Fat is one of the most reliable triggers. Fatty and fried foods sit in the stomach longer, which increases the chance of acid backing up. Chocolate, caffeine, peppermint, and alcohol also reduce LES pressure directly.
Other foods don’t weaken the valve but irritate the esophageal lining on contact. Spicy foods, citrus fruits, tomato sauces, and vinegar fall into this category. Carbonated drinks are a double threat: the gas distends the stomach, triggering more transient valve relaxations, while the acidity of many sodas adds to the irritation. Onions are another common offender, though the mechanism varies from person to person.
Not everyone reacts to the same foods. Keeping a simple log of what you eat and when symptoms appear is the most practical way to identify your personal triggers.
Excess Weight and Abdominal Pressure
Carrying extra weight around your midsection physically squeezes the stomach. Abdominal fat transmits gravitational force into the abdominal cavity, raising the pressure inside. Studies using pressure sensors have confirmed that people with obesity have measurably higher intra-abdominal pressure than those without it. That pressure pushes stomach contents upward against the LES, and over time it can even force part of the stomach through the diaphragm, creating a hiatal hernia.
Central obesity, meaning fat concentrated around the waist, matters more than overall body weight. Someone with a normal BMI but a large waist circumference can still face elevated reflux risk. Even modest weight loss, particularly around the abdomen, tends to reduce heartburn frequency.
Hiatal Hernia
A hiatal hernia occurs when the upper part of the stomach slides upward through the opening in the diaphragm. This displaces the LES from its normal position, where the diaphragm would normally reinforce it. The result is a valve that can’t close completely, reduced sphincter pressure, and more frequent transient relaxations.
A hernia also creates a small pouch of stomach above the diaphragm. Acid and food can pool in this pouch and reflux into the esophagus during subsequent swallows, even when the LES is functioning relatively well. Many people have small hiatal hernias without knowing it, but larger ones are a significant and persistent source of heartburn.
Medications That Worsen Reflux
Some medications cause heartburn by relaxing the LES. Blood pressure drugs called calcium channel blockers are a well-known example. Sedatives like benzodiazepines, opioid painkillers, certain older antidepressants, and progesterone-based hormones all reduce sphincter tone. Theophylline, used for asthma, does the same.
A second group of medications irritates the esophageal lining directly as they pass through. Common pain relievers like ibuprofen and aspirin fall into this category, along with certain antibiotics, oral bone-density drugs, iron supplements, and potassium supplements. These pills can cause a burning pain that mimics classic reflux even without excess acid production.
If heartburn started or worsened after beginning a new medication, that connection is worth exploring with whoever prescribed it. Adjusting timing, switching formulations, or taking pills with plenty of water can sometimes resolve the issue.
Smoking
Smokers have significantly lower LES pressure compared to nonsmokers. Research shows that smoking provokes acid reflux directly and may cause a long-lasting reduction in sphincter pressure that persists beyond the act of smoking itself. Nicotine also reduces saliva production, and saliva is one of the body’s natural defenses against esophageal acid exposure because it neutralizes acid and helps wash it back down.
Pregnancy
Heartburn is one of the most common complaints during pregnancy, and it has two overlapping causes. In the first trimester, rising levels of estrogen and progesterone begin to alter how the LES responds to stimulation. Research on women in early pregnancy found that their sphincter pressure responses to meals and other stimuli were already diminished, even before the uterus was large enough to exert physical pressure. This hormonal effect explains why some women experience reflux well before they “show.”
As pregnancy progresses, the growing uterus pushes the stomach upward and increases abdominal pressure, compounding the hormonal weakening of the valve. Symptoms typically peak in the third trimester and resolve after delivery.
Eating Habits and Body Position
Large meals distend the stomach, which is one of the primary triggers for transient LES relaxations. Eating close to bedtime compounds the problem because lying down removes gravity’s help in keeping acid in the stomach. When you’re upright, acid has to travel uphill to reach the esophagus. When you’re flat, it doesn’t.
Sleep position matters more than most people realize. Lying on your right side positions the esophagus below the stomach, essentially creating a downhill path for acid. Sleeping on your left side does the opposite, placing the esophagus above the level of the stomach. A systematic review found that left-side sleeping significantly decreased both acid exposure time and the time it took to clear acid from the esophagus compared to right-side or back sleeping. Elevating the head of your bed by six to eight inches (using a wedge or bed risers, not just extra pillows) and combining that with left-side sleeping provides the strongest nighttime relief.
Stress and Other Contributing Factors
Stress doesn’t directly increase acid production, but it heightens your perception of symptoms. People under chronic stress report more severe heartburn even when their measurable acid exposure hasn’t changed. Stress can also lead to behaviors that promote reflux: eating quickly, choosing comfort foods high in fat, drinking more alcohol or coffee, and smoking.
Tight clothing around the waist, heavy lifting, and vigorous exercise (especially running or high-impact activities) can all temporarily increase abdominal pressure enough to trigger reflux episodes. Even chronic coughing or straining from constipation adds upward force on the stomach.
Warning Signs That Need Attention
Occasional heartburn after a large or spicy meal is extremely common and not a sign of anything serious. But certain symptoms alongside heartburn signal conditions that need prompt evaluation: difficulty swallowing, pain when swallowing, unintentional weight loss, loss of appetite, vomiting blood or material that looks like coffee grounds, and persistent vomiting. These are considered alarm symptoms and typically lead to an endoscopy to rule out complications like esophageal narrowing, ulcers, or precancerous changes in the esophageal lining.