Absorption
Absorption of ethyl alcohol
Ethanol is absorbed by simple diffusion in the stomach and small intestine.
The amount absorbed depends solely on the volume ingested, therefore - within the absorption capacity of our body - all the alcohol consumed can easily pass through the walls of the digestive tract and pass into the blood.
The presence of food in the stomach, slowing gastric emptying, reduces the rate of alcohol absorption . For this reason it is advisable to consume wine at meals and not on an empty stomach, so as to spread the intoxicating effects of the drink over time.
Not all the alcohol consumed is oxidized, since a small percentage, varying from 5 to 15%, is eliminated with the breath, the sweat and with the urine; most, instead, is metabolized in the liver.
Metabolism
Metabolic fate of ethyl alcohol
In the liver, ethanol is oxidized to acetaldehyde, which is in turn oxidized to acetate. After these first two passages the acetate exits the liver and is conveyed by the blood to other tissues, where, especially at the cardiac level, it is activated with acetyl-CoA.
In turn, acetyl-CoA can enter the Krebs cycle and produce energy - or, if present in excess due to a drunkenness - be transformed into ketone bodies. Starting from acetyl-CoA, fatty acids can also form which, combined with glycerol, will form triglycerides to be deposited as an energy reserve.
From ethanol to acetaldehyde
The oxidation of ethanol to give acetaldehyde can take place in different ways.
Enzyme alcholdehydrogenase (ADH)
The first system is mediated by an enzyme called alkolde hydrogenase (ADH), present not only in the liver but also in the stomach, especially in men. ADH is also able to oxidize other alcohols, including small amounts of methanol (in 1986 a batch of wines adulterated with this substance caused the death of 19 Italians).
Microsomal ethanol oxidation system (MEOS)
Alcohol can also be oxidized by MEOS (microsomal system of ethanol oxidation). This enzyme is present at the level of the smooth endoplasmic reticulum of hepatocytes, which is hypertrophic in alcoholics.
Enzyme catalase
Finally, there is a third metabolic pathway mediated by the enzyme catalase. However, this is a mechanism of marginal importance compared to the other two.
From acetaldehyde to acetate
Acetaldehyde dehydrogenase (ALD)
In the oxidation from acetaldehyde to acetate another enzyme, called acetaldehyde dehydrogenase (ALD), intervenes. However, when the alcohol intake is high, the liver cannot dispose of the excess ethanol and also circulates acetaldehyde, a toxic substance for the whole organism.
Tolerance
Can alcohol tolerance be trained?
It depends. There are alcohol metabolic pathways that exploit genetically determined enzymes and others that change substantially depending on the frequency and extent of ethanol intake.
Non-modifiable genetic component
It is interesting to note that there are several isoenzymes of both alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALD). Even more important is the fact that the equipment of these isoenzymes is genetically dictated. Probably, the predisposition to drink and the alcohol tolerance of the individual and of some populations (the Orientals generally tolerate alcohol less well than Westerners) can be explained precisely on the basis of this genetic influence.
Meos
MEOS: increases with alcohol intake
MEOS is an inducible system by ethanol; in other words this enzyme complex becomes more and more powerful the higher the alcohol intake. In fact, hypertrophy of smooth endoplasmic reticulum in hepatocytes is one of the first morphological changes taking place in alcoholics.
MEOS, alcohol and drugs
It should also be borne in mind that MEOS is a non-specific system, capable of metabolizing many medicines as well. For this reason, the alcoholic, when sober, requires a higher dosage of drugs to achieve the same therapeutic effect. On the contrary, when a large drinker takes drugs while intoxicated, the active ingredient is metabolized more slowly and its effects, at the same dose, are significantly higher (since medicines and ethanol compete for the same enzyme system). Furthermore, the combined effects of the two substances in the brain can cause considerable damage. For all these reasons, as reported on all the package inserts, the simultaneous intake of alcohol and drugs should be avoided .
