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What is Acetaldehyde?

And why does it matter?


Simply put, acetaldehyde is a byproduct of alcohol. It causes some of the worst morning-after effects of drinking. But what is acetaldehyde and what makes it so bad for you? What are the ways your body is exposed to acetaldehyde, and how does your body react to it? This article is meant as your practical guide to acetaldehyde so you can be informed about this unwanted byproduct of alcohol metabolism.

Acetaldehyde: The Basics

Acetaldehyde (pronounced as a-suh-TAL-duh-hide, chemical formula: CH3CHO) – as the name implies – is an aldehyde, which is a highly reactive class of molecule.

The most important thing to know about aldehydes is that they have a double-bonded oxygen (=O group) that is highly reactive, which is what makes aldehydes so toxic. That double-bonded oxygen really wants to react with other molecules in your body, including molecules that you don’t want to be changed, such as your DNA. The main goal of detoxifying acetaldehyde is removing that double-bonded oxygen before it can react with something in your body.

Sources of Exposure

Here we dive into a few interesting factors that affect our perceptions around the effects of alcohol, and there’s a case to be made that it’s as much about psychology as it is about chemistry.

We are inevitably exposed to acetaldehyde in our daily lives, even when we are eating healthy food. Quantitative analysis studies found many foods contain at least some amount of acetaldehyde. It’s often present naturally, but very occasionally it’s used as an additive or byproduct of a manufacturing process. Examples of some of the foods containing acetaldehyde are: yogurt, green tea, and many fruits such as orange, grapefruit, banana, strawberry, mango, pear, apricot, and apple (citation).

Don't worry about eliminating parfait or fruit salad from your diet, though. The human body produces enough enzymes to break down the small amounts of acetaldehyde present in these foods, so none of these food items are significantly toxic.

More significant sources of acetaldehyde include diesel exhaust, tobacco smoke, and alcohol consumption. These sources of acetaldehyde are the most common ways to get harmful levels of acetaldehyde in the body.

Alcohol Consumption and Acetaldehyde

You might be wondering: if your body can break down the small amounts of acetaldehyde in food, how come it can’t do the same after drinking alcohol? It can, but it’s a matter of quantity and location. To give you a clear picture, we should clarify how your body breaks alcohol down.

Alcohol Metabolism and Acetaldehyde

Acetaldehyde formation is part of alcohol metabolism, in which ethanol (the intoxicating chemical in all alcoholic beverages) is converted to metabolites that your body can use. The chemical breakdown of ethanol consists of two stages, as illustrated below:

Each step is catalyzed by a specific enzyme, which speeds up these chemical reactions. The first step involves the oxidation of ethanol by the enzyme alcohol dehydrogenase (ADH). As its name suggests, ADH facilitates ethanol to give up two H atoms and form acetaldehyde, the primary product of the first reaction. In the second step, acetaldehyde is further oxidized to acetate by the enzyme acetaldehyde dehydrogenase (ALDH). ALDH catalyzes this conversion to acetate by adding another oxygen to the molecule, and removing that highly reactive double-bonded oxygen.

Acetaldehyde Formation: Liver vs. gut

This two-step process of ethanol to acetaldehyde to acetate mostly takes place in the liver after the alcohol travels into the gastrointestinal tract, is absorbed into the bloodstream, and subsequently makes its way to the liver (citation).

The acetaldehyde in the liver is short-lived since it is quickly converted to acetate. Compared to acetaldehyde, acetate is harmless, and your liver cells often convert it further into water and carbon dioxide. Approximately 80-90% of ethanol metabolism occurs in the liver.

The question is, what happens to the remaining 10% - 20%?

Scientists have known for a long time that ethanol is also converted into acetaldehyde in the gut. There, it’s both your human gut cells AND your gut microbiome handling the conversion, and that creates an important difference (citation).

Researchers demonstrated that several bacterial strains in your gut microbiome produced ADH enzymes and were also responsible for acetaldehyde production. Although many of these strains also produced ALDH enzymes (the ones that convert acetaldehyde to acetate), it was only in smaller amounts (citation).

In other words, your microbiome is much better at turning ethanol into acetaldehyde than it is at finishing the job by turning acetaldehyde into the non-toxic acetate. The result: acetaldehyde accumulates in the gut. Although the ADH activity in the gut is small in comparison to the liver, the resulting acetaldehyde concentration that is formed in the gut is significantly higher than what is formed in the liver. What’s worse is that the acetaldehyde produced in the gut gets absorbed into your bloodstream, where it can wreak havoc throughout your body on its way to the liver for final oxidation (citation).

Effects of Acetaldehyde

The harmful effects of acetaldehyde stem from its highly reactive nature and specifically its carbonyl group (C=O).

This carbonyl group enables acetaldehyde to react with proteins, lipids, and DNA, which interferes with the functionality of these critical bodily components much like a wrench jams up the gears of a clock.

Accumulation of acetaldehyde also triggers signaling molecules to induce unwanted acute effects. For example, the acetaldehyde-induced release of epinephrine and norepinephrine, normally associated with the body’s fight-or-flight response, often causes cardiovascular symptoms like palpitations (citation). This is why you might feel anxious, sweaty, and have a racing heart rate the day after drinking.

Furthermore, acetaldehyde enhances histamine and bradykinin release, which causes your blood vessels to widen (a.k.a, vasodilation). This often manifests as increased skin temperature and flushing.


Acetaldehyde is not just another molecule that you have to memorize for an organic chemistry exam.

Your body’s exposure to acetaldehyde – particularly with alcohol consumption – can turn your body’s routine upside down. Although your liver is capable of turning alcohol all the way through acetaldehyde into harmless acetate, the same cannot be said for your gut. This is mainly because gut bacteria and your mucosal cells can convert ethanol to acetaldehyde but lack sufficient amounts of the ALDH enzyme to carry out the next step. This seemingly minor site of alcohol metabolism is actually a significant source of disturbance, even in moderate drinking. Not only is it likely to cause misery in your body until full recovery, but it can also have other undesirable effects if you are exposed frequently.