1 How alcohol causes short- and long-term harmful effects
1.1 The biological effects of ethanol
This section investigates some of the harmful effects that a high level of blood-ethanol can have on the body: both short-term problems such as ‘hangover’, and long-term health problems that are associated with regular heavy drinking. Whilst this section is primarily about the biological effects that ethanol has on various organs of the body, it is important to remember that the socioeconomic effects of heavy drinking are also very serious (Paton, 2005). Figure 1 summarises some of the increased risks a drinker faces as the ethanol concentration in their blood increases.
The main acute effects of ethanol are on the nervous system, causing increased confidence and heightened mood leading to increased risk of accidents, violence and socially inappropriate behaviour. Alcohol also disrupts the ability of the brain to coordinate between sensory input and motor output which leads to slurred speech, less controlled movement, delayed reaction times and errors of judgement. Tests on drivers have shown that steering problems begin at blood-ethanol concentrations (BAC) of about 20 mg/100 ml, and the risk of being involved in a traffic accident more than doubles at BAC of 80 mg/100 ml, which is the legal limit for driving in the UK (Table 1). If this concentration doubles to 160 mg/100 ml, the risk of being involved in an accident increases more than 10-fold.
Experiments have shown that experienced drivers misjudged gaps with a concentration of 50 mg ethanol in 100 ml of blood (BAC 50 mg/100 ml). From Table 1, in which countries would it be legal for drivers to exceed this amount?
Table 1 European drink-drive blood-alcohol limits (Source: Safe Travel, 2006)
|Country||Limit (mg/100 ml)||Country||Limit (mg/100 ml)|
|Finland||50||Serbia and Montenegro||50|
Drink-drive limits are set at more than 50 mg/100 ml in Cyprus (South), Ireland, Luxembourg, Switzerland and the UK.
If BAC levels increase to around 200 mg/100 ml then speech becomes slurred and coordination of movement is impaired. Coma (loss of consciousness) is likely to occur above 300 mg/100 ml, and concentrations above 400 mg/100 ml are likely to be fatal as a result of disruption of heart and lung function, or inhalation of vomit.
There are variations in the way that people respond to alcohol and some of these are described in Box 1.
Box 1 (Explanation) Variations in alcohol tolerance
In some individuals, the main acute effects of drinking an alcoholic drink are rather different from those just described – they rapidly become quite unwell and experience what is known as a ‘flush’ reaction. The symptoms of this reaction are redness of the face and chest, rapid heart rate, dizziness, nausea, nasal congestion and pulsating headaches. More severe reactions can cause breathing problems and low blood pressure. Individuals who experience these effects accumulate a higher level of acetaldehyde in their blood than those who do not experience these effects.
The enzyme alcohol dehydrogenase (ADH), which converts ethanol into acetaldehyde, and the enzyme aldehyde dehydrogenase (ALDH), which converts acetaldehyde into acetic acid, can each exist in a number of different isoforms which work at different rates. The particular combination of isoforms that individuals have, varies according to the population to which they belong. The production of all proteins in the body, including enzymes, is controlled by genes, individual units of the inherited genetic material, DNA. Genes are inherited from the parents, and everyone inherits a complete set of genes, including those that direct the production of ADH and ALDH. However, people from different populations have slightly different variants of each gene. This is why certain ethnic groups have a relatively high proportion of people who become quite ill after drinking small amounts of alcohol – because of increased levels of acetaldehyde in their blood. More than 75% of Japanese people who drink report flushing, compared with 5–10% of Caucasians (Arnon et al., 1995).
If unusually high levels of acetaldehyde accumulate following an alcoholic drink, how could this have been caused by (a) the rate of reaction of an ADH isoform, and (b) the rate of reaction of an ALDH isoform?
ADH promotes acetaldehyde production, so an ADH isoform with a high rate of reaction would cause more acetaldehyde to accumulate than one with a slower rate. An ALDH isoform with a slow rate of reaction removes acetaldehyde slowly, allowing high levels of acetaldehyde to accumulate.
The overall effects of ethanol metabolism depends on many factors and ranges from a decline in BAC of less than 10 mg/100 ml per hour to over 40 mg/100 ml per hour for different people.