How the body handles alcohol

Ethanol is a small, water-soluble molecule that crosses membranes readily and needs no digestion. Some is absorbed through the stomach lining, but most passes into the small intestine, where the much larger surface area means absorption is rapid.

How fast this happens depends on several things. Food in the stomach slows gastric emptying and therefore slows absorption, which is why the same drink affects a person differently on an empty stomach. Carbonation tends to speed absorption. Very high concentrations can irritate the stomach and delay emptying, while moderate concentrations are absorbed fastest.

Once absorbed, ethanol distributes into body water. This is the reason body composition matters: a person with a larger volume of body water dilutes the same quantity of alcohol into a lower concentration. Differences in average body water between individuals, and on average between men and women, are the main reason the same drink produces different blood alcohol concentrations in different people.

A warm drink is absorbed faster than a cold one, which is one of several reasons mulled cider deserves the caution given elsewhere in this section.

The great majority of ethanol is metabolised in the liver. Alcohol dehydrogenase converts it to acetaldehyde, a toxic and reactive compound; aldehyde dehydrogenase then converts acetaldehyde to acetate, which is metabolised further. A small proportion of alcohol leaves unchanged in breath, urine and sweat, which is the basis of breath testing.

The critical property is that the main enzyme pathway becomes saturated at low concentrations. Above that point, elimination proceeds at a roughly constant rate rather than in proportion to how much is present. The body removes approximately one UK unit per hour in many adults, but this figure varies considerably between individuals and is an average rather than a rule.

Because elimination is close to constant and intake is not, alcohol accumulates whenever drinking outpaces it. Three units drunk in an hour are not eliminated in an hour, and the excess carries forward. This is also why the morning after a heavy evening a person may still be over a legal driving limit, having slept.

Nothing accelerates the process. Caffeine counteracts drowsiness without reducing blood alcohol, which produces an alert person who is still impaired. Eating after drinking does not reduce alcohol already absorbed. Exercise, cold water and fresh air do not clear alcohol. Only time does.

The category spans a wider strength range than beer or wine. Someone who has learned that "a pint" means about 2.3 units from ordinary beer will be substantially wrong about a pint of traditional cider at 7.5 per cent, which is nearly twice that.

Cider is often served in beer measures. A pint is a larger serving than a wine glass, and applying it to a drink that can be as strong as a light wine produces a large quantity of alcohol in a single glass.

Sweetness masks alcohol. Residual sugar, fruit aroma and carbonation all reduce the perception of alcoholic strength, so a strong sweet cider does not taste as strong as it is. This is a known route to unintentionally high intake.

Draught and traditional cider frequently has no ABV shown at the point of service, and farm-produced cider varies between batches. Where the strength is not known, the intake genuinely cannot be counted.

Elimination rate varies between individuals by a substantial factor, and it is influenced by liver function, genetics, regular intake, medication and illness. Genetic variants affecting aldehyde dehydrogenase activity are common in some populations and alter the response to alcohol markedly.

Tolerance is a real phenomenon and a misleading one. Someone who drinks regularly may feel less impaired at a given blood alcohol concentration, but the concentration is the same, the legal position is the same, and the effect on the body is not reduced by having got used to it.

The general consequence is that no calculator, chart or rule of thumb can tell a specific person what their blood alcohol concentration is. Where the answer matters — driving above all — the only safe assumption is that the estimate is wrong.