The feeling of being intoxicated after consuming only one alcoholic beverage suggests the body is processing or responding to alcohol differently than the average person. While the feeling of being “drunk” is subjective, the underlying mechanisms are rooted in physiological differences. These variations involve the speed at which alcohol enters the bloodstream, how efficiently the body breaks it down, and how the brain reacts to its presence.
Speed of Alcohol Absorption
The intensity of alcohol’s effects is determined by the rate at which ethanol is absorbed into the bloodstream, causing a rapid spike in Blood Alcohol Concentration (BAC). The stomach and small intestine handle this absorption, and external factors easily modify their function. Drinking on an empty stomach allows alcohol to pass quickly into the small intestine, where most absorption occurs, resulting in a faster increase in BAC.
Conversely, consuming food, especially meals rich in protein and fat, delays the absorption process. Food slows the rate at which the stomach empties into the small intestine, keeping the alcohol contained and allowing some metabolism before it reaches the bloodstream. Carbonated beverages, like sparkling wine or mixed drinks, can also accelerate absorption. They increase pressure in the stomach, which encourages faster passage into the small intestine.
Genetic Variations in Alcohol Metabolism
Genetic variations affecting the enzymes responsible for alcohol metabolism are a key explanation for alcohol sensitivity. The body breaks down alcohol (ethanol) in a two-step process, starting with the enzyme Alcohol Dehydrogenase (ADH). ADH converts ethanol into acetaldehyde, a compound that is toxic to the body.
Acetaldehyde is then quickly processed into harmless acetate by a second enzyme called Aldehyde Dehydrogenase (ALDH). Inherited genetic polymorphisms can cause ADH to be hyperactive or ALDH to be slow or inactive. When ADH is overly efficient or ALDH is impaired, acetaldehyde builds up rapidly in the system.
This accumulation of acetaldehyde causes intensely unpleasant physical symptoms often mistaken for rapid intoxication. These symptoms include facial flushing (the “alcohol flush”), nausea, vomiting, and a rapid heart rate (tachycardia). Certain populations, particularly those of East Asian descent, commonly carry the inactive ALDH22 allele, which reduces the capacity to clear acetaldehyde. This genetic difference acts as a protective factor against alcohol use disorder due to the immediate, aversive reaction caused by consuming even a small amount of alcohol.
Biological Factors Influencing Alcohol Concentration
Fixed biological factors affect how concentrated alcohol becomes once it is distributed throughout the body. Alcohol is a water-soluble molecule, meaning it disperses into the total body water (TBW) of an individual. A person’s body mass and body composition determine the volume of fluid available for dilution.
Individuals with a lower body mass have a smaller volume of distribution, causing the same amount of alcohol to become more concentrated and resulting in a higher peak BAC. Biological sex also plays a role because women generally have a lower percentage of TBW compared to men. A man’s body is composed of approximately 55% to 65% water, while a woman’s body typically contains 45% to 50% water.
This difference means that a woman and a man of the same weight consuming the same amount of alcohol will experience a higher BAC because the alcohol is diluted in less total fluid. Dehydration can also reduce the TBW temporarily. This compounds the lack of fluid available for dilution and causes a higher BAC spike from a single drink.
Neurobiological Sensitivity and External Factors
The final component of feeling “drunk” involves the central nervous system’s reaction to alcohol, which can be heightened regardless of a low BAC. Alcohol acts as a depressant by enhancing the effects of gamma-aminobutyric acid (GABA), the brain’s main inhibitory neurotransmitter. By binding to GABA-A receptors, alcohol increases inhibitory signaling, leading to feelings of sedation, relaxation, and impaired coordination.
Individual neurochemistry, including the composition and sensitivity of GABA receptors, can vary. This variation makes some people’s brains more responsive to alcohol’s inhibitory effects. This heightened neurobiological sensitivity can lead to a strong psychological effect of intoxication even if the blood alcohol level is modest.
The effects of a single drink can be amplified by interactions with certain medications. Many prescription drugs, such as anti-anxiety medications, sedatives, and some pain relievers, already act on the central nervous system and potentiate the effects of GABA. Combining these substances with alcohol increases the depressant effect, creating intense drunkenness or drowsiness from a negligible amount of alcohol. Individuals should consult a healthcare provider regarding potential drug-alcohol interactions, as this combination can depress respiratory function.