Acid reflux happens when stomach acid flows backward into the esophagus, the tube connecting your mouth to your stomach. It affects roughly 10 to 20% of adults in Western countries, with over 825 million cases worldwide as of 2021. The causes range from a malfunctioning valve at the base of your esophagus to everyday habits like eating too fast or sleeping in the wrong position.
The Valve That Keeps Acid Down
At the bottom of your esophagus sits a ring of muscle called the lower esophageal sphincter (LES). Think of it as a one-way gate: it opens to let food into your stomach, then squeezes shut to keep acid from climbing back up. In its resting state, this muscle maintains a pressure of about 15 to 30 mmHg, which is more than enough to hold the stomach’s contents in place. Anything above 10 mmHg generally prevents backflow. Below 5 mmHg, you’re in trouble.
The single most common cause of acid reflux is something called transient lower esophageal sphincter relaxation. This is when the valve opens spontaneously, without any swallowing to trigger it. It’s actually a normal reflex: when your stomach fills with food or air, your body relaxes the valve briefly to vent excess gas. Everyone experiences these relaxations, but in people with chronic reflux, they happen more frequently or last longer, giving acid a window to escape upward.
Hiatal Hernia and Structural Changes
Your diaphragm, the large breathing muscle that separates your chest from your abdomen, wraps around the esophagus and helps reinforce the LES. A hiatal hernia occurs when the upper portion of the stomach pushes up through the diaphragm’s opening into the chest cavity. This disrupts the anti-reflux barrier in several ways at once: it shortens and weakens the LES, removes the reinforcing squeeze of the diaphragm, widens the junction between the esophagus and stomach, and creates a small pouch above the diaphragm where acid can pool and splash upward during swallowing. Studies using high-resolution imaging show that the greater the separation between the sphincter and the diaphragm, the more reflux episodes a person experiences.
How Body Weight Increases Pressure
Excess abdominal fat physically squeezes the stomach. In people with severe obesity, the pressure inside the abdomen can reach around 9 mmHg, which approaches the threshold needed to overpower a weakened LES. That sustained upward pressure on the stomach makes reflux episodes more frequent and more forceful. The effect is compounded when a hiatal hernia is also present, because the structural barrier is already compromised. This is why weight gain, even modest amounts concentrated around the midsection, often correlates with new or worsening reflux symptoms.
Foods and Drinks That Relax the Valve
Certain foods don’t just irritate the esophagus on the way down. They chemically relax the LES itself, making it easier for acid to travel upward.
- Chocolate contains a compound called methylxanthine (closely related to caffeine) that directly relaxes the sphincter muscle.
- Coffee, both regular and decaf, relaxes the LES. Caffeine plays a role, but other compounds in coffee contribute as well, which is why switching to decaf doesn’t always help.
- Peppermint is a smooth-muscle relaxant, which is great for stomach cramps but counterproductive for keeping acid in the stomach.
- Garlic and onions also promote LES relaxation in many people.
Fatty and fried foods slow stomach emptying, which keeps the stomach fuller for longer and triggers more of those spontaneous valve relaxations. Large meals have the same effect simply through volume: a distended stomach is the primary trigger for the valve to open.
Medications That Weaken the Sphincter
Several common drug classes reduce LES pressure as a side effect. These include calcium channel blockers (used for high blood pressure), nitrates (used for chest pain), benzodiazepines (used for anxiety and sleep), certain asthma inhalers that contain beta-2 agonists, and xanthine-based medications. If you started a new medication and noticed reflux symptoms appearing or worsening shortly after, the timing may not be a coincidence.
Smoking and Nicotine
Nicotine weakens the LES directly. In one study, applying a nicotine patch to nonsmokers reduced their sphincter pressure by about 30% within 12 hours, dropping it from a healthy range into borderline territory. Smoking compounds the problem further by increasing stomach acid production and reducing saliva, which normally helps neutralize acid that reaches the esophagus. The effect isn’t limited to cigarettes; nicotine pouches, patches, and vaping can all contribute.
Pregnancy and Hormonal Changes
Up to two-thirds of pregnant women experience acid reflux, and the causes are both hormonal and mechanical. Rising levels of estrogen and progesterone during pregnancy relax smooth muscle throughout the body, including the LES. Research shows that even in early pregnancy, before the uterus is large enough to press on the stomach, the sphincter’s ability to respond to stimulation is already significantly reduced. As pregnancy progresses, the growing uterus adds physical pressure from below, pushing stomach contents upward. The combination of a weakened valve and increased abdominal pressure makes reflux nearly inevitable for many women in the third trimester.
Slow Stomach Emptying
When the stomach takes too long to move food into the small intestine, a condition called gastroparesis, the prolonged fullness triggers more spontaneous sphincter relaxations. The longer food sits in the stomach, the more opportunities acid has to escape. Gastroparesis can result from diabetes, certain medications, or nerve damage, and reflux is one of its most common overlapping symptoms. Even without a formal diagnosis, anything that slows digestion (very high-fat meals, eating close to bedtime, certain pain medications) can have the same downstream effect.
Why Reflux Gets Worse at Night
Gravity is your esophagus’s best friend during the day. When you’re upright, acid has to travel uphill to reach your throat. Lying down removes that advantage entirely, which is why reflux often feels worst at night or first thing in the morning.
Your sleeping position matters more than you might expect. Lying on your right side places the esophagus below the level of the stomach, essentially creating a downhill path for acid. Sleeping on your left side does the opposite: it positions the esophagus above the stomach, so gravity works in your favor even while you’re horizontal. A systematic review of multiple studies confirmed that left-side sleeping is associated with fewer reflux symptoms and faster acid clearance. Elevating the head of your bed by six inches has a similar effect, keeping the junction between the stomach and esophagus above the level of gastric acid.
When Multiple Causes Stack Up
Acid reflux rarely has a single cause. A person with a small hiatal hernia might manage fine until they gain weight, start a blood pressure medication, and begin eating larger dinners. Each factor chips away at the anti-reflux barrier: the hernia weakens the structural support, the extra weight increases abdominal pressure, the medication lowers sphincter tone, and the big meals trigger more frequent valve relaxations. Understanding which factors apply to you is the first step toward figuring out which ones you can actually change.