Ethanol in beer: Production, absorption and metabolism

Beer is brewed using malted grain (usually barley), water, hops and yeast. The fermentation of malted barley during the brewing process generates ethanol. After caffeine, ethanol is the most commonly used recreational drug worldwide and drinking beer is a very popular way of ingesting ethanol. Although non-alcoholic beers exist, the ethanol content of beer usually varies between 3% and 9% alcohol by volume. However, the concentration of ethanol in some specialist brews can be significantly higher. When beer is consumed, ethanol is absorbed from the gastrointestinal tract and rapidly distributed around the body in the blood before entering tissues. Some ethanol is metabolized to acetaldehyde in the stomach and liver before reaching the systemic circulation (first-pass metabolism). The average rate of oxidation of ethanol to acetaldehyde is approximately 15 mg/dl blood/hour. Acetaldehyde is highly toxic and binds cellular constituents generating harmful acetaldehyde adducts. Acetaldehyde is further oxidized to acetate. The metabolism of acetate and its significance in the effects of ethanol are less well understood. In one study, consumption of 5 units of alcohol (40 g ethanol) as 1,000 ml of beer with an evening meal resulted in a mean blood ethanol concentration (BEC) of 6.8 mmol/l after 1 h and just below 2 mmol/l after 3 h. However, the relationship between BEC and the effects of ethanol is complex and varies between individuals and also with drinking habits. Many effects correlate with the peak BEC or peak ethanol concentrations within organs during a drinking session. Other effects are due to products of metabolism and the total dose of ethanol ingested over a period of time. The effects of gender, the ethanol content of the beverage consumed and the coincident consumption of food on the absorption, distribution and metabolism of ethanol are reviewed.