Alcohol (ethanol) is a foreign compound the body rapidly identifies and must process and eliminate. Unlike food, ethanol does not require digestion and is treated more like a toxin that needs immediate chemical breakdown. Metabolism is the biochemical process that converts alcohol into less harmful substances, primarily occurring in the liver. This detoxification is necessary because ethanol interferes with normal cellular functions.
Absorption and Distribution in the Body
Ethanol’s journey begins with rapid absorption into the bloodstream. A small amount of alcohol is absorbed through the lining of the stomach, but the majority passes into the small intestine where absorption is much faster due to the large surface area available. Since alcohol is a small, highly water-soluble molecule, it easily diffuses through the gastrointestinal tract lining into the blood without needing active transport mechanisms.
Once in the bloodstream, alcohol is quickly distributed throughout the body’s total water content. Organs with a high blood flow, such as the brain, liver, and lungs, are affected rapidly. This widespread distribution is why the physical and cognitive effects of alcohol are felt so soon after consumption. The concentration of alcohol in the blood, known as the blood alcohol concentration (BAC), is a direct measure of how much ethanol is circulating.
The Primary Metabolic Pathway
The liver is the central site for alcohol metabolism, handling over 90% of ingested ethanol. The main detoxification process is a two-step oxidative pathway involving two specific enzymes. This pathway is designed to neutralize ethanol and its toxic byproduct, acetaldehyde.
The first step involves the enzyme Alcohol Dehydrogenase (ADH), which resides in the liver cell’s cytosol. ADH converts ethanol into acetaldehyde, a highly reactive and damaging compound. Acetaldehyde is considerably more toxic than ethanol and is responsible for negative effects, including facial flushing, nausea, and hangovers.
The second step quickly follows and is performed by Aldehyde Dehydrogenase (ALDH), which is primarily located in the mitochondria of liver cells. ALDH rapidly converts the toxic acetaldehyde into a relatively harmless substance called acetate (acetic acid). The speed of this second step is crucial because it prevents acetaldehyde from accumulating and causing widespread cellular damage.
Factors Influencing Processing Speed
The rate at which the body processes alcohol varies significantly due to several biological factors. Genetic variations in the genes that encode the ADH and ALDH enzymes are one of the most powerful influences. For instance, certain variants of the ALDH gene, particularly the ALDH22 allele common in some East Asian populations, result in a less efficient enzyme. This inefficiency causes a rapid buildup of toxic acetaldehyde, leading to an intense “alcohol flush” response that deters heavy drinking.
A secondary metabolic pathway, the Microsomal Ethanol Oxidizing System (MEOS), uses the enzyme Cytochrome P450 2E1 (CYP2E1). While the ADH pathway handles low to moderate amounts of alcohol, the MEOS system becomes more active when alcohol concentrations are high. Chronic, heavy alcohol consumption can “induce” or increase the expression of CYP2E1. This induction accelerates processing and contributes to increased alcohol tolerance in heavy drinkers, requiring them to consume more for the same effect.
Other physiological differences also impact processing speed. Women generally have lower levels of the ADH enzyme in their stomach compared to men, meaning less alcohol is metabolized before entering the bloodstream. They also typically have a smaller volume of distribution for alcohol, resulting in higher peak blood alcohol concentrations. Finally, the presence of food in the stomach significantly slows the overall rate of absorption by delaying the alcohol’s passage into the small intestine.
Final Products and Elimination
The final product of the primary metabolic pathway is acetate. This acetate is released from the liver into the bloodstream and circulated to peripheral tissues, such as the heart, brain, and skeletal muscle. These tissues then metabolize the acetate further, converting it into acetyl-CoA.
Acetyl-CoA is fully oxidized in the tricarboxylic acid (TCA) cycle to produce carbon dioxide and water, generating energy. The vast majority of alcohol is eliminated this way. Only a minor fraction (typically two to ten percent) is excreted unchanged through the breath, urine, and sweat. Excretion via breath is the principle behind breathalyzer tests.