Alcohol…
…does it have any redeeming features or is it all bad news? A small section at the end of this post provides information on some of the benefits of alcohol. The rest, which is by no means an extensive account of all the adverse effects of alcohol, is far from positive.
Alcohol metabolism
Alcohol is a term which encompasses a number of compounds with similar properties most of which are not safe to consume. In this context alcoholic beverages contain a type of alcohol called ethanol, an organic compound with a chemical formula C2H5OH. Although not a nutrient, alcohol is very calorie dense. When metabolised it yields 7 kilocalories per gram in energy1, so more than carbohydrate and protein which each yield 4 kilocalories per gram. Given there is no storage capacity for it in the body, alcohol is metabolised as soon as it is consumed2.
Alcohol dehydrogenase (ADH), the main enzyme responsible for metabolising alcohol, is found in the liver, with small amounts in the stomach. Most alcohol consumed will exit the stomach and be readily absorbed into the bloodstream from the small intestine. In the liver alcohol is metabolised by ADH yielding the extremely toxic compound acetaldehyde (see Figure 1 below) which is then quickly converted to acetate (acetic acid) by aldehyde dehydrogenase (ALDH)1. Acetic acid, an important intermediate metabolic substrate found in many tissues, is further oxidised to produce energy.
Figure 1: The alcohol dehydrogenase (ADH) metabolic pathway
Graphic from PsychDB
The average rate at which alcohol is eliminated from the body is around 7g per hour3, so slightly less than one unit of alcohol which is the equivalent of 8g of alcohol (drinkaware.co.uk). Numerous factors can affect this rate of elimination, a couple of which are included below:-
- alcohol is insoluble in fat but readily soluble in water, so where there is a higher percentage of lean tissue to body fat, there is more opportunity for alcohol to diffuse into the lean tissue. Females generally have smaller bodies and less [lean] muscle tissue than males; when alcohol elimination rates are corrected for these differences they are shown to be higher in females than males3.
- certain genetic differences affect the activity of the enzymes in Figure 1 above. For example a high proportion of East Asians are known to have a lower functioning form of ALDH causing a build up of toxic acetaldehyde in the body after the consumption of alcoholic beverages1.
Alcohol consumption and the effect on the brain and central nervous system
With the increase in blood alcohol concentration, effects on the brain and central nervous system (CNS) become more notable. Figure 2 below gives a concise overview of both short and long term effects.
Figure 2: Overview of the effects of alcohol on the brain and CNS
Graphic from Med Case Rep Rev 4
Risk to health
The NHS state that a regular intake of more than 14 units of alcohol per week poses a risk to health. For example:
- as well as alcohol, ADH is also involved in the metabolism of retinol, a form of Vitamin A, converting it to retinal which is important for night vision. Excessive alcohol intake means less ADH is available to convert retinol to retinal and night vision may be compromised1
- excessive alcohol consumption in the long term can result in an accumulation of triacylglycerol in the liver and the development of a condition known as fatty liver; this can be reversed with abstinence but may progress to alcoholic hepatitis or a build up of fibrous scar tissue, known as cirrhosis of the liver5
Moderate alcohol consumption may have benefits
Many research studies have demonstrated a relationship between moderate alcohol consumption of one or two small drinks per day and a reduced risk of cardiovascular disease, although the American Heart Association do not sanction the consumption of alcohol for this purpose1. There has also been much speculation around the benefits of phytonutrients in grapes and wine, in particular the polyphenol resveratrol. It is believed however, that dietary sources of resveratrol would be insufficient to produce any therapeutic effect and further research in this area is still needed6.
Alcohol units – what they mean
From drinkaware.co.uk …
1 unit of alcohol = 8g/10ml of pure alcohol
1 unit of alcohol = a small measure (25ml) of spirits
1.5 units of alcohol = a small glass (125ml) of wine
3 units of alcohol = 1 pint of strong lager (5% ABV) or a large glass (250 ml) of wine
References
- McGuire, M. and Beerman, K.A. (2013). Nutritional Sciences From Fundamentals to Food. 3rd ed. US: Cengage Learning, pp. 301-319
- Lanham-New, S.A., ed., Hill, T.R., ed., Gallagher, A.M., ed., Vorster, H.H., ed. (2020). Introduction to Human Nutrition. 3rd ed. UK: Wiley Blackwell, pp. 126
- Cedarbaum, A.I. Alcohol Metabolism. Clinics in Liver Disease [online]. Elsevier. November 2012, vol. 16(12). 667-685 [viewed 29 March 2021]. Available from: DOI 10.1016/j.cld.2012.08.002
- Griffith, C., La France, B. The Neural Effects of Alcohol. Medical Case Reports and Reviews [online]. Open Access Text. October 2018, vol. 1(3). [viewed 29 March 2021]. Available from DOI 10.15761/MCRR.1000116
- O’Shea, R.S., et al. Alcoholic Liver Disease. Hepatology [online]. AASLD. January 2010, vol. 51(1). 308-328 [viewed 29 March 2021]. Available from: DOI 10.1002/hep.23258
- Weiskirchen, S. and Weiskirchen, R. Resveratrol: How much wine do you have to drink to stay healthy? Advances in Nutrition [online]. ASN. July 2016, vol. 7(4). 706-718 [viewed 30 March 2021]. Available from: DOI 10.3945/an.115.011627
