Why Do Some People Turn Red When They Drink Alcohol, and Others Don't?
― The Culprit Isn't the Alcohol Itself, but a Substance Made Along the Way
One glass of beer turns some people bright red, while others can drink glass after glass without changing colour. The difference isn't "getting used to it" or "willpower". Your body breaks alcohol down in two steps. The strength of the enzyme in charge of the second step differs from person to person, from birth.
It's a year-end get-together, and you've drunk about half of the toast beer. Your friend next to you already has a bright red face and neck.
They laugh and say, "I always go red." But their heart is pounding, and they seem to have a bit of a headache too.
Meanwhile, the person across from you is on their third glass without any change in colour. They're drinking the same alcohol. Why is the difference so big?
There are just two reasons
In the liver, the alcohol you drink first turns into a substance called acetaldehyde. This widens your blood vessels, reddens your face, and causes the pounding heart and headache.
How well the enzyme that clears acetaldehyde works is set by your genes. In people with the weak version, the poison builds up easily, and even a small amount makes them red.
So a red face is less a sign that "the alcohol has hit you" and more a sign that "clearing it can't keep up". Let's go through it step by step.
In the body, alcohol is cleared in two steps
Alcohol taken by mouth is absorbed in the stomach and small intestine, then carried by the blood to the liver. In the liver, two enzymes work like a relay team.
The first enzyme turns alcohol into acetaldehyde. The second enzyme turns that into acetic acid, the same substance that makes vinegar sour. In the end, it becomes water and carbon dioxide throughout the body and leaves as breath and urine.
Look at Figure 1. The top row is a person with a wide second pathway, and the bottom row is a person with a narrow one. The first step runs at the same speed, but when the second is narrow, the acetaldehyde in the middle backs up like a traffic jam. The built-up part travels around the body in the blood and widens the small vessels in the face. That is what "turning red" is.
The "weak type" is common among Japanese people
The second enzyme comes in a strong version and a weak version. You inherit one copy from each parent, so there are three possible combinations.
With two strong copies, you rarely flush. With just one weak copy, you are the "weak type", who flushes after a small amount. With two weak copies, you are the "non-drinker type", who can hardly drink at all.
Among Japanese people, the weak type is said to be about 4 in 10, and the non-drinker type a few percent. This weak version is almost never seen in people of European or African descent, and is known to be especially common in East Asia. That is why so many people you know turn red at drinking parties.
If you keep drinking, another enzyme that helps with the first step is said to increase, and the brain also gets used to being drunk. So it feels as though you've "got stronger". But the type of your second enzyme is fixed from birth and doesn't change. Even if you stop turning red, acetaldehyde is thought to be still building up in your body.
Why does alcohol last until the next morning?
Another important point is the speed of breakdown. The amount of alcohol the liver can break down in an hour is said to be about 0.1 grams per kilogram of body weight. For a person weighing 60 kilograms, that is about 6 grams per hour.
Here is the key point. Even if you drink a lot, the liver's enzymes are already working flat out, so the speed of breakdown barely rises. It's like an assembly line in a factory: even if the materials pile up, the amount it can process in an hour is fixed.
Try moving the slider in Figure 2. One 500-millilitre can of beer contains about 20 grams of alcohol. If you finish drinking at 10 pm, three cans won't be fully broken down until around 8 am. At 7 am, when you commute, it's still in your body.
Breakdown speed varies a lot between individuals, and is said to be slower in women, older people and smaller people. "I slept, so it's gone" isn't always true, because the liver can only work at the same speed while you sleep.
So what should we do?
- A red face is a sign that "clearing can't keep up"If you're the type who flushes, drinking less is the quickest way to protect your body.
- You're allowed to say no when offered a drinkYour constitution can't be changed by effort. Forcing people to drink, or drinking in one go, is dangerous.
- Don't drive the morning after heavy drinkingBy calculation, three cans of beer take about 10 hours to clear.
Acetaldehyde has been classified by an international research agency as a substance that causes cancer in humans. People who flush and drink a lot regularly are reported to have a risk of oesophageal cancer that is many times higher. Also, if someone has drunk a large amount in a short time and does not respond when called, has abnormal breathing, or seems about to choke on vomit, call 119 (the emergency number in Japan) without hesitation. Stay with them, and wait with their body turned on its side so they don't choke on vomit.
Summary
In the liver, alcohol is broken down in two steps: "alcohol → acetaldehyde → acetic acid". What reddens the face is the acetaldehyde made along the way. The strength of the enzyme that clears it is fixed from birth, and about 4 in 10 Japanese people are said to be the weak type. And because breakdown speed doesn't rise with more alcohol, it takes as long as the amount you drank requires.
A red face is a sign not of "being drunk" but of "clearing can't keep up".
The liver moves along only one third of a can of beer per hour.
For how the stomach lives with strong acid, see "Why doesn't the stomach dissolve itself, even though it produces strong acid?", and for how much alcohol used in cooking really burns off with heat, see our article on cooking alcohol.
- Soak a few drops of disinfectant ethanol, sold at pharmacies, into the gauze pad of an adhesive plaster.
- Stick it on the soft inner side of your arm, wait about 7 minutes, then peel it off.
- About 10 minutes after peeling, if the patch of skin is red, you may be the type with the weak second enzyme. If it isn't red, you are likely the strong type.
The skin shows the same difference in enzyme activity, so this is said to give a rough idea. But it is only a simple guide, not a definite answer. People with sensitive skin may get itching, so don't push yourself. In Japan you can drink from age 20, but this test itself can be done without drinking any alcohol.
For those who want to know more ― terms, formulas and links to textbooksWe show which level each topic belongs to, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school chemistry and biology
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity specialist courses (biochemistry, pharmacokinetics) not taught in high school
- ResearchNot taught even at university as settled fact; being studied by researchers right now
Middle schoolTerms: this phenomenon has names
- Acetaldehyde: a substance made while alcohol is being broken down. It widens blood vessels and causes facial redness, a pounding heart, headache and nausea.
- Aldehyde dehydrogenase 2 (ALDH2): the second enzyme, which turns acetaldehyde into acetic acid. Weak versions are common in East Asia.
- Flushing response: the reddening of the face or body after a small amount of alcohol.
Middle schoolHigh schoolChecking with a formula: how many hours do three cans of beer take to clear?
Let's check the main text's "finish at 10 pm, gone around 8 the next morning" with a formula.
| In symbols | t ＝ ( V × a × ρ ) ÷ ( k × W ) |
| In words | Time to break down ＝ weight of alcohol drunk ÷ weight that can be broken down per hour |
| Where the formula comes from | The liver's enzymes are already full even with a little alcohol, so the amount falls by a roughly constant weight each hour regardless of quantity (a zero-order reaction). Divide the total amount by that constant speed and you get the time. |
| Symbol | Meaning and unit |
| V | Amount of drink: one can of beer, 500 mL |
| a | Alcohol content: 5% ＝ 0.05 |
| ρ | Density of alcohol: 0.8 g per mL |
| k | Breakdown speed: said to be about 0.1 g per kg of body weight per hour |
| W | Body weight: 60 kg |
| Volume of alcohol in one can (mL) | 500 × 0.05 ＝ 25 |
| Its weight (g) | 25 × 0.8 ＝ 20 |
| Weight for three cans (g) | 20 × 3 ＝ 60 |
| Weight broken down per hour (g) | 0.1 × 60 ＝ 6 |
| Time to break down t (hours) | 60 ÷ 6 ＝ 10 |
| For just one can (hours) | 20 ÷ 6 ≒ 3.3 |
③ In everyday terms: someone who finishes three cans at 10 pm will still have alcohol in their body 10 hours later, around 8 the next morning. In one hour it gets through 6 ÷ 20, about one third of a can. These values are averages, and the actual breakdown speed is said to vary from person to person by anything from half to double.
High schoolHigh school+The textbook's "oxidation of alcohol" is happening inside your body
High schoolIn high-school chemistry you learn that oxidising ethanol gives acetaldehyde, and oxidising that further gives acetic acid. Written out: ethanol (C₂H₅OH) → acetaldehyde (CH₃CHO) → acetic acid (CH₃COOH). In a test tube this reaction is done with substances such as potassium dichromate; the body does it with enzymes at body temperature. The enzyme for step 1 is alcohol dehydrogenase, and the one for step 2 is aldehyde dehydrogenase.
High school+The second enzyme is built from four identical parts joined together. If even one weak part is mixed in, the whole assembly works poorly. So even someone who inherited a weak version from only one parent is said to have activity drop to a tenth or less of the strong type. It doesn't simply "halve", which is one reason a small amount can turn them red.
UniversityEnzyme reaction speed and how blood concentration falls
The speed of an enzyme reaction is described by the Michaelis–Menten equation, which you meet in biochemistry. When the concentration of the material (the substrate) is high enough, the speed sticks at its maximum and no longer depends on concentration. This is a zero-order reaction, and it is why blood alcohol concentration falls in a straight line over time. The Widmark formula, well known in forensic medicine, uses this idea to estimate how much someone drank. In the final stage, when the concentration has dropped low enough, the speed approaches a first-order reaction, proportional to concentration. The weak version of the second enzyme comes from a single-letter change in the gene (a single nucleotide polymorphism), generally called ALDH2*2.
📖 Derivations and further reading: Michaelis–Menten equation (Wikipedia, in Japanese) ／ Aldehyde dehydrogenase (Wikipedia, in Japanese)
ResearchWhat is still not clearly understood
- Why did the weak version spread only in East Asia? It is estimated to have increased in East Asia several thousand years ago, but what made it an advantage is unknown. Hypotheses include resistance to infectious disease and protection against heavy drinking.
- The detailed pathway of flushing. How far substances such as histamine are involved as acetaldehyde widens blood vessels is still debated.
- Combinations with the type of the first enzyme. The first enzyme also has fast and slow versions, and researchers are studying how combinations of the two types change how people feel drunk and their disease risk.
In other words, this article too is an "explanation of what is known so far". The link between constitution and health is still being updated by large follow-up studies.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, year 2: "Digestion and absorption", "Functions of the liver" | The liver breaks down harmful substances |
| High school | Chemistry: "Oxidation of alcohols"; Biology: "Enzymes", "Genes and traits" | Two-step breakdown, and how types are inherited |
| High school+ | Advanced biology: "Quaternary structure of proteins" | A four-part enzyme, where one weak part stops the whole |
| University | Biochemistry: "Enzyme kinetics"; pharmacokinetics; forensic medicine | The Michaelis–Menten equation and zero-order reactions |
| Research | Population genetics, epidemiology | Why the weak version spread, and disease risk |
| ― | Links to daily life | How much to drink, driving the next morning, not pressuring others to drink |
- Brooks PJ, Enoch MA, Goldman D, Li TK, Yokoyama A (2009) The Alcohol Flushing Response: An Unrecognized Risk Factor for Esophageal Cancer from Alcohol Consumption. PLoS Medicine 6(3): e1000050
- Ministry of Health, Labour and Welfare (厚生労働省) e-Health Net (e-ヘルスネット), "Absorption and breakdown of alcohol" (アルコールの吸収と分解) and "Alcohol metabolism and health" (アルコールの代謝と健康)
- IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 100E: Personal Habits and Indoor Combustions (2012)
- Acetaldehyde (アセトアルデヒド) (Wikipedia, in Japanese)
- National Hospital Organization Kurihama Medical and Addiction Center (独立行政法人国立病院機構 久里浜医療センター), explanation of "Alcohol constitution test (ethanol patch test)" (アルコール体質判定（エタノールパッチテスト）)
*This article is a general-audience science explanation. The numbers given are rough guides and estimates to help you understand the mechanism. If you are worried about your constitution or about drinking, consult a doctor. If your condition changes suddenly, do not hesitate to call 119 (Japan's emergency number) or follow the instructions of a medical institution.