Alcohol Breakdown in the Body – How the Body Detoxifies Alcohol

When an adult drinks one liter of beer with a typical alcohol content of 5% by volume, about 50 milliliters of pure alcohol enter the body. That equals roughly 40 grams of ethanol. The process of alcohol breakdown in the body follows clear, predictable steps.

Absorption and Distribution #

A small amount of alcohol is already absorbed through the mucous membranes in the mouth. The largest portion passes into the stomach and then mainly into the small intestine. The emptier the stomach, the faster this happens. Carbonation speeds up stomach emptying even more. Some ethanol is already broken down in the stomach wall and in the liver before it enters the major bloodstream — this is called the first-pass effect. It usually accounts for 5–20% of the alcohol, and tends to be lower in women.

Once alcohol reaches the bloodstream, it spreads throughout the body’s water compartments. The blood alcohol concentration rises quickly and — on an empty stomach — usually peaks after 20–40 minutes. When alcohol is consumed with food, it can take over an hour. Mathematically, 40 grams of ethanol produce about 0.7‰ (per mille) in an 80-kg (176 lb) man, and roughly 1.2‰ in a 60-kg (132 lb) woman.

Alcohol Breakdown in the Body — Hard Work for the Liver #

Approximately 90–95% of ethanol is metabolized in the liver.

1. Step 1: Alcohol dehydrogenase (ADH) converts ethanol into the highly toxic acetaldehyde.
2. Step 2: Aldehyde dehydrogenase (ALDH) transforms acetaldehyde into acetate.
3. Step 3: Acetate is converted in muscle tissue and the heart into acetyl-CoA and fed into the energy metabolism.

A small remainder is processed by the microsomal enzyme system CYP2E1, which becomes more active with frequent drinking and produces more oxygen radicals. The enzyme catalase in peroxisomes plays only a minor role in breakdown.

The elimination rate is almost constant: On average, the body removes 7–10 grams of ethanol per hour. For most adults this corresponds to a blood alcohol decline of roughly 0.1–0.15‰ per hour.

Metabolic Effects #

Both dehydrogenase enzymes generate a great deal of NADH, which shifts the cellular environment. Gluconeogenesis is slowed, which can lower blood sugar. At the same time, lactate levels rise, favoring muscle cramps and elevated uric acid. Fat burning is inhibited while fat synthesis is promoted — an important mechanism that contributes to fatty liver during regular alcohol consumption.

The intermediate product acetaldehyde is highly reactive and responsible for many acute symptoms such as headache, nausea, and facial flushing. People with genetically slow aldehyde dehydrogenase are especially sensitive.

Excretion and Fluid Balance #

A small portion of alcohol (about 2–5%) is excreted unchanged through breath, sweat, and urine. The rest is completely metabolized into carbon dioxide and water.

At the same time, alcohol suppresses the release of antidiuretic hormone (ADH). As a result, the kidneys excrete more water — explaining the well-known frequent urination and overall fluid loss. In addition, alcohol dilates skin blood vessels, which increases heat loss and can heighten the feeling of cold.

Time Course #

In the first few minutes alcohol acts stimulating and disinhibiting. After 20–40 minutes the peak blood level is usually reached, then it declines linearly. Someone who reaches 0.8‰ typically needs 6–8 hours to return to zero. However, residual alcohol and reduced reaction ability can persist noticeably longer.

Practical example:

One liter of beer (≈ 40 g ethanol) produces about 0.7‰ in an 80-kg (176 lb) man. After one hour roughly 0.55‰ remain; after five to seven hours the level drops to zero. In women the peak value is usually higher, while the elimination time remains similar.

Effects on Sleep #

Although alcohol makes falling asleep easier, it severely disrupts the second half of the night. Sleep becomes shallow and restless, with frequent awakenings. Fatigue and poor concentration the next day are very typical.

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