Acid reflux happens when stomach contents flow backward into the esophagus, and the causes range from structural problems to everyday habits. Roughly 825 million people worldwide had gastroesophageal reflux disease (GERD) in 2021, affecting close to 10% of adults in North America alone. Understanding what triggers reflux helps explain why it can feel random, and what you can actually change.
How the Anti-Reflux Barrier Works
At the bottom of your esophagus sits a ring of smooth muscle that acts as a one-way valve. This muscle maintains a steady baseline tone, keeping your stomach sealed off between swallows. When you eat or drink, it relaxes briefly to let food pass through, then tightens again. Reflux occurs when this valve either relaxes at the wrong time, stays too loose, or gets overwhelmed by upward pressure from the stomach.
Just above where the esophagus meets the stomach, there’s a sharp angle (called the angle of His) that works like a flap, adding a second layer of protection against backflow. When this angle is disrupted, whether by a hernia or other structural change, one of your body’s key defenses against reflux disappears.
Hiatal Hernia
A hiatal hernia is one of the most common structural causes of reflux. It occurs when the upper part of your stomach pushes up through the diaphragm, the muscle wall that normally helps keep the esophageal valve in place. When this happens, the sharp angle between the stomach and esophagus is lost, making it much easier for acid to travel upward. Many people have small hiatal hernias without symptoms, but larger ones significantly increase the frequency and severity of reflux episodes.
How Body Weight Drives Reflux
Excess weight, particularly around the abdomen, is one of the strongest and most well-documented causes of reflux. Extra abdominal fat increases the pressure inside your abdomen, which pushes against the stomach and forces its contents upward. This pressure also increases the rate of inappropriate valve relaxations, those moments when the esophageal valve opens for no reason, letting acid escape.
Interestingly, total body weight matters less than where the fat sits. The pressure gradient that promotes reflux correlates with abdominal fat distribution rather than BMI alone. This is why two people at the same weight can have very different reflux experiences, and why someone carrying weight around the midsection is at higher risk than someone with the same BMI but a different body shape.
Food and Drink Triggers
Fatty foods are frequently blamed for reflux, and there’s some basis for this. Some studies show that high-fat meals reduce the pressure of the esophageal valve, though the evidence is inconsistent. Other studies find no significant difference in valve pressure or the amount of acid that backs up after fatty meals. The effect likely varies from person to person, which is why dietary advice for reflux often comes down to tracking your own triggers rather than following a universal list.
Coffee is another commonly cited culprit, but controlled research has found that coffee had no effect on reflux time, the number of reflux episodes, or esophageal valve pressure in either healthy people or those with GERD. That doesn’t mean coffee can’t bother you individually, but the blanket recommendation to quit coffee for reflux isn’t well supported by evidence. Alcohol, citrus, tomato-based foods, chocolate, and spicy foods round out the usual suspect list, though again, individual sensitivity varies widely.
Smoking and Nicotine
Nicotine has a direct, measurable effect on the esophageal valve. Research published in Gastroenterology found that nicotine reduced valve pressure by up to 85% at higher doses. That’s a near-complete collapse of the barrier that keeps stomach acid where it belongs. This applies to cigarettes, vaping, nicotine patches, and any other form of nicotine delivery. If you smoke or vape and experience reflux, the connection is not subtle.
Medications That Weaken the Valve
Several common medications lower esophageal valve pressure as a side effect. Calcium channel blockers (used for high blood pressure), nitrates (used for chest pain), and theophylline (used for asthma) all relax the smooth muscle of the valve, making reflux more likely. Anti-inflammatory painkillers like ibuprofen and aspirin don’t relax the valve but can directly irritate the esophageal lining, making reflux symptoms worse even if the amount of acid exposure hasn’t changed. If you take any of these regularly and have worsening reflux, it’s worth discussing with your prescriber.
Hormonal Changes and Pregnancy
Pregnancy is a perfect storm for reflux. Rising progesterone levels throughout pregnancy progressively decrease esophageal valve pressure, weakening the barrier more and more as the pregnancy advances. At the same time, the growing uterus increases abdominal pressure, pushing stomach contents upward. This combination explains why reflux is mild or absent early in pregnancy but often becomes severe in the third trimester.
Progesterone fluctuations outside pregnancy may also play a role. Some women notice reflux symptoms worsen during specific phases of their menstrual cycle, particularly the luteal phase when progesterone peaks. This hormonal link is one reason reflux patterns can differ between men and women.
Eating Habits and Body Position
When you lie down, gravity no longer helps keep stomach contents in place. Research shows that low valve pressure is a particular contributor to reflux in the supine position. This is why eating close to bedtime is one of the most reliable triggers: your stomach is full, your valve pressure naturally dips during sleep, and you’ve removed gravity from the equation. Large meals compound the problem by distending the stomach, which increases both the volume of acid available and the pressure pushing it upward.
Eating quickly, overeating, and consuming large meals late in the evening all independently increase reflux risk. The timing matters as much as the content. Your stomach typically takes two to four hours to substantially empty after a meal, so lying down within that window keeps a pool of acid in direct contact with the valve.
Delayed Stomach Emptying
It seems intuitive that a slow-emptying stomach would cause more reflux, since food and acid sit around longer. Gastroparesis, a condition where the stomach empties abnormally slowly, has long been assumed to worsen reflux. However, a 2024 study in the American Journal of Gastroenterology directly tested this assumption and found no connection. Patients with delayed gastric emptying were not more likely to have confirmed GERD when measured by esophageal acid exposure. The researchers also found no link between higher stomach retention at one or four hours and reflux severity. This challenges a common clinical assumption and suggests that if you have both gastroparesis and reflux, they may be independent problems rather than one causing the other.
Stress and the Gut-Brain Connection
Stress doesn’t produce more stomach acid, but it does change how you perceive reflux symptoms. People under chronic stress report more severe heartburn even when the actual acid exposure in their esophagus is the same as during calm periods. Stress also promotes behaviors that worsen reflux: eating quickly, choosing comfort foods that tend to be fatty or processed, drinking more alcohol or coffee, and sleeping poorly. The relationship is real, but it works through amplified symptom perception and behavioral changes rather than through a direct increase in acid production.
Age-Related Changes
Reflux becomes more common with age for several overlapping reasons. The esophageal valve gradually loses tone over decades. Saliva production, which normally helps neutralize small amounts of acid that reach the esophagus, decreases. The esophagus itself clears acid more slowly as its muscular contractions weaken. Older adults also tend to take more medications that can relax the valve or irritate the lining, and they’re more likely to have developed a hiatal hernia. None of these changes alone is dramatic, but together they explain why reflux often appears or worsens in middle age and beyond.