Alcohol Breakdown in the Body

What is the alcohol that we drink?

In chemistry, “alcohol” can describe any compound that contains a hydroxyl group (an oxygen bonded to a hydrogen, or -OH).  When we talk about drinking alcohol, however, we mostly refer to ethanol, which is CH3CH2OH, or a 2-carbon chain attached to a hydroxyl group.

How do we metabolize alcohol?

When people drink alcohol, their bodies break it down in three stages.  The first stage breaks ethanol into acetaldehyde, which is much more toxic than ethanol and is responsible for a sick feeling after drinking a lot of alcohol.  This happens using an enzyme, which is a protein that helps speed up reactions, called alcohol dehydrogenase.  The second stage breaks the acetaldehyde into acetic acid, and then the third stage eventually breaks that down into carbon dioxide and water, which is then eliminated from the body.

Why can some people drink more alcohol than others?

The enzyme alcohol dehydrogenase has many different forms, and certain forms are more efficient than others.  The people who have more efficient forms of this enzyme tend to feel sick from alcohol quicker, because their body very rapidly converts ethanol into toxic acetaldehyde.  Some evidence that suggests the levels of this enzyme decrease as people age, which could lengthen the period of intoxication.

Why is it bad to drink methanol?

Methanol, an alcohol form that contains only one carbon with an -OH group, CH3OH, is toxic and can cause blindness.  This is because, when methanol is broken down by alcohol dehydrogenase, it breaks down into formaldehyde and then formic acid.  Formic acid can harm the optic nerve, cause blindness, and also impairs the ability of mitochondria to function.  Some scientists suggest that the cure to methanol poisoning, if caught in time, may actually be to ingest ethanol, although it is still very dangerous.  This is because the breakdown of methanol is much slower than ethanol, so if both are ingested alcohol dehydrogenase will break down ethanol first, allowing the body to excrete methanol without breaking it down.

Why shouldn’t you drink alcohol on an empty stomach?

Ethanol is a small molecule, allowing it to easily move through the walls of the stomach and the small intestine, but mostly through the small intestine because there is so much surface area in the small intestine that a lot more ethanol can diffuse there.  When there is food in the stomach, the pyloric sphincter (the muscle that forms the entrance of the small intestine from the stomach and can open and close to allow food into the small intestine) is closed to allow the food in the stomach to digest a bit.  This prevents too much ethanol from going into the bloodstream from the walls of the small intestine because it can’t get into the small intestine.  This delays intoxication because the alcohol cannot get into the blood.  If that sphincter is open, however, lots of alcohol can get into the blood all at once.

Why does alcohol make people feel warm?

Once the alcohol gets into the bloodstream, it causes the blood vessels to widen (also known as vasodilation), which allows a lot of heat to rise to the surface of the skin and dissipate into the air.  This makes people feel really warm, because the heat goes to their skin, but actually it causes a net loss of body heat.

Why do people say that men can drink more than women and reach the same level of intoxication?

While this is definitely a generalization and is different among individuals, in general this can be true.  Ethanol has the very polar -OH group (it is polar because the electrons move towards the electronegative oxygen) and therefore cannot get into completely fatty, nonpolar tissue.  Since women tend to have a higher concentration of fatty tissue than men do, the same amount of alcohol is able to diffuse into fewer cells in women than men, meaning there is more alcohol per cell in women than men.  This can cause women to become intoxicated faster than men.

Why do people say alcohol causes weight gain?

Typically when someone ingests food or drink, some part of that is stored as fat that can be used later if the body needs it.  The rest of the calories from the food or drink are used to sustain the body.  However, the energy from ethanol cannot be stored in the body, so the body uses the energy from ethanol to perform typical functions, giving energy to muscles and the brain to sustain life.  However, since ethanol is being used as energy, any other food or drink ingested that would otherwise be used has to be stored.  This can probably only cause weight gain in large quantities however.

Why do people drink coffee to cure hangovers?

The headache in hangovers can be caused by vasodilation in the head, which is the widening of blood vessels in the head.  Caffeine actually is a vasoconstrictor, meaning it constricts blood vessels, so coffee or tea can reverse the effects of certain hangover headaches. They are not actually overall cures for hangovers, however.

Works Cited

McGee, Harold; On Food and Cooking: The Science and Lore of the Kitchen: Scribner, 2004, 713-720.

Smart, Lesley; Alcohol and Human Health: Oxford University Press, Oxford, 2007.

Elmhurst College, “Alcohol Metabolism Effects,”

Campbell, Mary K. and Farrell, Shawn O., Biochemistry: 6th Edition, Thomson Higher Education, 2009.

PubChem Compound, National Center for Biotechnology Information,

DNA Learning Center, Cold Spring Harbor Laboratory,

National Institutes of Health and National Institute on Alcohol Abuse and Alcoholism, Information about Alcohol,

Montoliu, C., Valles, S., Renau-Piqueras, J., Guerri, C.  Ethanol-Induced Oxygen Radical Formation and Lipid Peroxidation in Rat Brain: Effect of Chronic Alcohol Consumption.  Journal of Neurochemistry.  1994, Vol. 63, Issue 5, 1855-1862.

Lobo, V., Patil, A., Phatak, A., Chandra, N. Free radicals, antioxidants and functional foods: Impact on human health.  Pharmacognosy Review.  2010, 4(8): 118-126.

Zakhari, S., Overview: How is Alcohol Metabolized by the Body?  NIH: NIAAA Publications,

PubChem Compound, National Center for Biotechnology Information,

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Brent, Jeffrey; McMartin, Kenneth; Phillips, Scott; Aaron, Cynthia; Kulig, Ken. Fomepizole for the treatment of methanol poisoning. The New England Journal of Medicine. 2001, 344, 424-429.

Koop, Dennis R., Alcohol Metabolism’s Damaging Effects on the Cell: A Focus on Reactive Oxygen Generation by the Enzyme Cytochrome P450 2E1, NIH: NIAAA Publications, Volume 29, Number 4, 2006,

Dekker, Anthony, What are the effects of alcohol on the brain?  Scientific American, July 26, 1999,


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