Blood Alcohol Content (BAC) reflects the percentage of alcohol present in a person’s bloodstream. A BAC of 0.10%, for instance, indicates one part alcohol for every 1,000 parts of blood. While the amount consumed is a factor, BAC is ultimately determined by individual physiological characteristics. These differences dictate how the body absorbs, distributes, and processes alcohol, leading certain groups to register higher concentrations than others after consuming the same quantity.
Biological Sex and Total Body Water
A primary physiological factor influencing BAC is the difference in average total body water percentage. Alcohol is water-soluble, distributing easily into the body’s water content. Adult females tend to have a lower percentage of total body water, typically ranging from 45% to 55% of their total body mass.
In contrast, adult males typically have a higher average body water content, which can range from 55% to 65% of their mass. This difference is largely due to the higher average body fat percentage and lower muscle mass in females compared to males. When the same amount of alcohol is introduced into a smaller volume of water, the resulting concentration in the blood is higher.
Furthermore, a difference in the activity of the enzyme gastric alcohol dehydrogenase (ADH) also contributes to a higher BAC in females. This enzyme is responsible for breaking down a small amount of alcohol in the stomach before it ever reaches the bloodstream. Since females generally have lower levels of this enzyme, more of the consumed alcohol passes through the stomach unchanged, leading to greater systemic absorption and a higher peak BAC.
Body Mass and Fat-to-Muscle Ratio
Overall body mass is a primary determinant of the volume into which alcohol is distributed, but the composition of that mass is equally important. A person with a larger overall body size has a greater total volume of body fluid, which dilutes the alcohol more effectively, resulting in a lower BAC than a smaller person who consumed the same amount.
Alcohol does not distribute well into fat tissue because it is low in water content. Muscle tissue, conversely, is higher in water content and acts as a better distribution space for alcohol. Therefore, two individuals of identical total weight can have different BACs if their body compositions differ.
An individual with a higher percentage of body fat and a lower percentage of lean muscle mass will experience a higher BAC. The alcohol is confined to a smaller volume of total body water, which is disproportionately lower than their overall weight suggests. This higher concentration occurs because the fat tissue reduces the effective space where the alcohol can be diluted.
Genetic Variations in Alcohol Metabolism
Beyond initial distribution, inherited genetic variations affect the rate at which alcohol is cleared from the body, influencing how long a high BAC is sustained. Alcohol breakdown involves two primary enzymes: Alcohol Dehydrogenase (ADH) and Aldehyde Dehydrogenase (ALDH). These enzymes are encoded by genes that can have different forms, known as polymorphisms.
Some genetic variations in ADH produce an enzyme that rapidly converts alcohol into the toxic intermediate compound, acetaldehyde. Other polymorphisms in ALDH can result in a slow or inactive enzyme, causing acetaldehyde to build up in the body. This accumulation leads to unpleasant symptoms, such as facial flushing, nausea, and rapid heart rate.
Individuals with these variations, particularly the slow-acting ALDH22 allele common in East Asian populations, may experience a rapid onset of effects that can deter heavy drinking. Conversely, individuals with genetic profiles that produce slower ADH and faster ALDH may clear alcohol less efficiently or sustain intoxicating effects for a longer period, contributing to a prolonged high BAC.