Under The Microscope: Bottom's up!

 

For all practical purposes, it may be assumed that the only  route of absorption of alcohol is by mouth as, although it  can be inhaled, situations where this occurs must be rare  indeed. One case  in which a man suffering from a tumour of the base of the tongue,  probably causing difficulties in swallowing, had instilled  fruit brandy rectally via a plastic tube using a pump, and  dying as the result of acute alcohol poisoning.  Almost as soon as the alcohol is swallowed, it begins  to be absorbed into the blood - and as soon as that blood  reaches the liver, the alcohol begins to be eliminated. Therefore the blood level (and hence the brain concentration) is  a dynamic balance between absorption and elimination,  the peak determining the maximum behaviour effect. Such a  balance is often represented graphically by the 'blood-alcohol  curve' (BAC), which indicates the intensity and duration of  physiological effects.

Ethanol is capable of being absorbed by any part of the gastrointestinal canal, but in practice this is confined to the  stomach and upper small intestine, as only a little alcohol  remains to pass through the wall of the ileum or colon. When  alcohol is taken with food, however, there seems to be a  deficit in the amount absorbed, as some never appears in  the bloodstream. Some of this lost alcohol may be excreted  in the faeces, but when absorption is slow, more may be  destroyed by the liver directly from the portal blood, never surviving to enter the systemic circulation.

Owing to a thinner mucosa, a better blood supply and a  larger surface area, the upper small intestine - the duodenum  and jejunum - has the maximum capacity for absorption,  compared with the gastric mucosa. This has practical implications, as drink taken by mouth will be absorbed more  quickly when:

-          a gastrectomy or gastroenterostomy has been carried  out previously, as the drink will pass rapidly through to the upper small intestine;

-          the stomach is empty, as fluid will pass through the pylorus with almost no delay;

Conversely, when the stomach contains food, the drink will be held up until digestion has proceeded sufficiently for the contents to be released into the duodenum. A fatty meal will slow this process down even more and milk feed also has a marked delaying effect. Obviously some absorption will still occur in the interim via the gastric mucosa, but not at the same high rate as in the duodenum and jejunum. As well as delaying emptying, a full stomach will retard absorption by mixing with the alcohol and physically reducing its access to the gastric lining where transit into the blood takes place. Another factor in the speed of absorption is the concentration of the alcohol. A strength of about 20 per cent is optimum for rapid absorption, which is met (in an empty stomach) by sherry or port wine, or spirits diluted with a 'mixer', such as gin and tonic or whisky and soda. It is also said that carbonated drinks (those containing dissolved carbon dioxide, such as champagne, tonic or soda water, or lemonade) hasten absorption, perhaps because the bubbles greatly increase the surface area carrying alcohol. Dilute drinks, such as beer (with a concentration of about 4 per cent alcohol) will be absorbed much more slowly - probably because the large volume impedes access of the alcohol molecules to the stomach lining. Beer may take twice as long to absorb as stronger drinks, though part of the delay is caused by contained carbohydrates, which is another factor that slows absorption. For example, when whisky is diluted to the same strength as beer, absorption is more rapid and the peak is higher than drinking the same  amount of alcohol in the form of beer.  Very strong drink slows the rate of  more rapid and the peak is higher than drinking the same  amount of alcohol in the form of beer.

Very strong drink slows the rate of transfer into the bloodstream. Neat spirits or liqueurs, which may be in excess of 40 per cent alcohol, cause:

-          pyloric spasm and hence retarded emptying into the duodenum;

-          irritation of the gastric lining, forming a barrier of mucus, which slows absorption;

-          reduced gastric motility, which also retards emptying;

Given an empty stomach and an optimum concentration of alcohol, most of the drug will have entered the bloodstream between 30 and 90 minutes after drinking. It has been calculated that 98 per cent of alcohol drunk  would be absorbed within 10 minutes if it went straight into  the small intestine - most of the delay is due to hold-up in  the stomach. Rates vary greatly among different people and  even in the same person at different times, irrespective of  food being taken, but an acceptable mean time would be  that 60 per cent of the imbibed alcohol would be absorbed  within 60 minutes and 90 per cent within 90 minutes.

Food in the stomach can, however, at least double these  times and a large fatty meal can delay total absorption for  a number of hours. This has an important effect on the  dynamic state between absorption and elimination, as the  rate of the latter is relatively constant (see below) and can  therefore deal with the slow delivery of alcohol from the portal blood so effectively that the peak of the blood-alcohol  curve is low - indeed the curve becomes a long shallow curve  instead of a sharp hillock. Where there is a legal threshold for  blood alcohol in relation to driving, the taking of food can  easily cause one person who has had a large meal to remain well under this limit, whereas another person who drank the same amount at the same time may rapidly exceed the threshold if he drank on an empty stomach. Incidentally, the same effect can occur, notwithstanding food, where a man and woman drink the same amount, as described earlier.

Some drugs will affect absorption rates, by modifying the speed of stomach emptying. Atropine, chlorpromazine, tricyclic antidepressants, procyclidine, amphetamines, morphine, antidiarrhoea compounds, codeine, methadone, heroin, pethidine, etc., will delay gastric transit, whilst the antiemetics cisapride and metoclopramide, as well as the antibiotic erythromycin, will hasten stomach emptying.

Almost all alcohol is detoxified by the liver, only 2-10 per cent being excreted unchanged. This means that a heavy  drinking session places a great metabolic burden upon the  liver and is the cause of hepatic damage after long-standing  drinking. The elimination mechanism is an oxidation of alcohol by liver enzymes, through aceraldehyde to acetate.

The first stage is performed by the enzyme alcohol dehydrogenase but, as the second stage is much more rapid, little nacetaldehyde has time to accumulate. The acetic acid is rapidly oxidized further to carbon dioxide and water. Some may also be broken down by a microsomal oxidase system. The rate of elimination is of crucial importance to the shape of the blood-alcohol curve, the height of its peak and to the duration of the alcoholaemia. It is also central to any attempts at retrospective calculations of blood, breath or urine levels, as discussed later. Whereas the rate of absorption is variable and is affected by a number of factors, the speed of detoxification in the liver is much more constant and relatively independent of external influences. This is not to say that it is fixed and immutable, even in the same person at different times. but it is capable of reasonable approximation. Much experiment and research has been devoted to this topic and the results, though variable, lie within a fairly narrow band.

Acknowledgements:

www.aived.nl    AIVD – @Erik Akerboom ©

www.politie.nl  Politiekorpschef  @Janny Knol©

www.politie.nl WEB Politie - @Henk van Essen©

 

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