Alcohol converts to acetaldehyde first, then to acetate, and finally to carbon dioxide and water. This three-step breakdown happens primarily in the liver and involves two key enzymes that work in sequence to neutralize what is essentially a toxin your body wants to eliminate as quickly as possible.
The Three-Step Breakdown
When you drink, your liver does the heavy lifting. About 90 to 95 percent of the alcohol you consume is processed there, with the remaining small fraction leaving your body unchanged through breath, sweat, and urine (which is why a breathalyzer works).
The first enzyme, alcohol dehydrogenase (ADH), converts ethanol into acetaldehyde. This is the critical and potentially dangerous step, because acetaldehyde is a highly toxic substance and a known carcinogen. The second enzyme, aldehyde dehydrogenase (ALDH), quickly converts that acetaldehyde into acetate, a far less harmful compound. Acetate then leaves the liver and gets broken down into carbon dioxide and water in other tissues throughout the body, where it’s easily eliminated.
Think of it as a conveyor belt: ethanol enters, gets stripped down into something dangerous, then that dangerous intermediate gets neutralized into something harmless. The whole system works well as long as the conveyor belt isn’t overwhelmed.
Why Acetaldehyde Matters
Acetaldehyde is the reason alcohol is so damaging at the cellular level. Even though it’s normally short-lived, this intermediate compound causes real harm while it’s present. It directly damages DNA, promotes mutations, and can alter the way genes controlling cancer growth are expressed. These effects are a major reason the International Agency for Research on Cancer classifies acetaldehyde from alcohol consumption as carcinogenic to humans.
Acetaldehyde is also behind many of the unpleasant physical effects of drinking. That flushed feeling, nausea, and rapid heartbeat some people experience are signs of acetaldehyde building up in the bloodstream faster than the body can clear it. Hangovers are partly driven by the same chemistry: lingering acetaldehyde and its downstream effects on inflammation and oxidative stress.
What Happens to the Acetate
Once acetaldehyde is converted to acetate, your body treats it like a simple fuel source. Acetate circulates through the bloodstream and gets used for energy in muscles and other tissues, where it’s ultimately broken down into carbon dioxide (exhaled through your lungs) and water (excreted normally). This is also why alcohol carries calories, roughly 7 per gram, making it nearly as calorie-dense as fat. Your body prioritizes burning alcohol-derived acetate for energy, which means it temporarily puts the brakes on burning fat and carbohydrates from food. This metabolic shift is one reason heavy drinking contributes to weight gain over time.
How Fast Your Liver Can Keep Up
The average person’s liver processes about 7 grams of alcohol per hour, which translates to roughly one standard drink per hour. For a 70-kilogram (about 155-pound) person, total metabolic capacity tops out at around 170 to 240 grams per day. Drink faster than your liver can work, and alcohol accumulates in your bloodstream, raising your blood alcohol concentration and intensifying its effects on your brain and body.
There’s no way to speed this process up. Coffee, cold showers, and food after drinking don’t accelerate the enzymatic breakdown. Your liver works at a fixed pace determined largely by how much ADH and ALDH enzyme you have available.
The Backup System for Heavy Drinking
Your liver has a secondary pathway that kicks in when alcohol levels are high. This system, driven by an enzyme called CYP2E1, also converts ethanol to acetaldehyde, but it works differently and becomes more significant during binge drinking or at high blood alcohol concentrations. Chronic heavy drinkers actually produce more of this enzyme over time, which is part of why regular drinkers develop a higher tolerance and seem to clear alcohol faster.
The tradeoff is significant, though. CYP2E1 generates harmful molecules called reactive oxygen species as a byproduct, which damage cell membranes and create additional toxic compounds. This is one of the key mechanisms behind alcohol-related liver disease. The backup system handles the extra load but inflicts more collateral damage in the process.
Genetics Change the Equation
Not everyone processes alcohol the same way, and genetics play a major role. Two common gene variants affect how quickly this conversion happens and how dangerous it is.
One variant, found frequently in East Asian populations, produces a faster version of the first enzyme (ADH1B). This speeds up the conversion of ethanol to acetaldehyde, flooding the body with the toxic intermediate more quickly. The other variant involves a less functional version of the second enzyme (ALDH2), which impairs the body’s ability to clear acetaldehyde. People who carry this ALDH2 deficiency experience the classic “alcohol flush reaction,” with facial redness, nausea, and a pounding heartbeat even after small amounts of alcohol.
These reactions serve as a natural deterrent, and people with ALDH2 deficiency generally drink less. But those who push through the discomfort and continue drinking face a paradoxically higher risk. Because acetaldehyde lingers longer in their bodies, they have elevated rates of esophageal cancer, stomach cancer, and liver disease compared to people with fully functional enzymes drinking the same amount.