Full mechanism, in text

The liver's two-step breakdown

1 · ADH cytosol

In the hepatocyte cytosol, ADH turns ethanol into the more toxic acetaldehyde and makes , and this near-saturated step cannot be sped up.Step 1 happens in the hepatocyte cytosol, and the reaction is: ethanol (CH₃CH₂OH) + → acetaldehyde (CH₃CHO) + NADH + H⁺

· Enzyme: ADH (alcohol dehydrogenase), mainly ADH1B / ADH1C
· Location: hepatocyte cytosol (small amount in gastric mucosa)
· Rate: healthy adults ~ 7-10 g/hour, ≈ zero-order (enzyme saturated)
· Genetic polymorphisms:
ADH1B*2 (Arg→His48): much higher activity, so acetaldehyde forms faster (common in East Asians, rare in Northern Europeans)ADH1C polymorphisms: moderate differences in activity
Ethanol itself is toxic, and the acetaldehyde made in between is more toxic still; it also binds DNA to form adducts. The International Agency for Research on Cancer (IARC) lists both the ethanol in alcoholic drinks and the acetaldehyde produced by drinking as Group 1 carcinogens; Group 1 describes how certain the evidence is, not how large the risk is.

The byproduct NADH is also a problem — it accumulates in Step 3 and disrupts hepatic metabolism.

This step cannot be sped up — at ordinary drinking levels ADH is already close to saturated, and no hangover product can make it run faster.

2 · ALDH2 mito

In the mitochondria, ALDH2 turns acetaldehyde into harmless acetate, and the ALDH2*2 mutation common in East Asians slows this step sharply, so acetaldehyde builds up in blood.Step 2 happens inside hepatocyte mitochondria, and the reaction is: acetaldehyde + + H₂O → acetate (CH₃COOH) + NADH + H⁺

· Enzyme: ALDH2 (aldehyde dehydrogenase 2), mitochondrial
· Function: rapidly converts the carcinogenic, DNA-damaging acetaldehyde into harmless acetate.

**ALDH2*2 (rs671, Glu504Lys) mutation — the East Asian key**:

· Heterozygous (E/K): far less than half the enzyme activity (Brooks 2009 records a drop of more than 100-fold)
· Homozygous (K/K): no measurable activity
· According to the Brooks 2009 review, about 36% of East Asians flush when they drink; people with ALDH2 deficiency make up about 8% of the world population, and it is very rare in European and African populations

Blood acetaldehyde after the same one drink:

· Carriers reach a clearly higher acetaldehyde peak than non-carriers
· Homozygous carriers react so strongly that most do not drink at all

Clinical consequences:

· 'Red face' + palpitations + headache — vasodilation + autonomic reaction
· Esophageal squamous-cell carcinoma: ALDH2*2 carriers who drink have a markedly higher risk — case-control studies give between 3.7 and 18.1, mostly above 10 in heavy drinkers (Brooks 2009)
· This is not 'intolerance', it is 'acetaldehyde accumulating in blood' — 'training tolerance' raises the brain's tolerance to ethanol, it does not reduce acetaldehyde damage.

3 · redox crash

ADH and ALDH2 each make one , and the pile-up crashes the NAD⁺/NADH ratio in liver cells, so the liver burns less fat, makes more and stops gluconeogenesis.The two steps ADH + ALDH2 each generate 1 — one drink (14 g ethanol) → ~ 0.6 mol NADH piling up in a short time:

Free NAD⁺/NADH ratio in the liver cell's cytosol:

· At rest: ~ 700 (a cytosolic figure; in the mitochondria it is only ~ 8)
· After drinking: falls sharply.

The cascade consequences of this single ratio collapse:

① β-oxidation pauses:

· β-hydroxyacyl-CoA dehydrogenase needs NAD⁺
· NAD⁺ deficient → fatty acids cannot be burned
· Meanwhile acetyl-CoA floods in → de novo lipogenesis (DNL) ↑
· → accumulation → the chemical root of alcoholic fatty liver.

② cycle slows:

· isocitrate + malate → oxaloacetate all need NAD⁺
· Lactate accumulates → lactate competes with uric acid for excretion in the kidney → blood uric acid rises, one of the ways drinking triggers gout.

③ Gluconeogenesis stops:

· Lactate/pyruvate ratio up + α-ketoglutarate down → gluconeogenesis stops
· Fasting + drinking → severe hypoglycemia (diabetic patients on glucose-lowering drugs + alcohol = ER danger).

④ Ketogenesis ↑ (when fasting):

· Acetyl-CoA abundant + TCA backlogged → ketones pile up → alcoholic ketoacidosis (AKA).

This is the chemistry linking drinking to liver fat: it is not only the calories in the drink; the NADH build-up itself makes the liver burn less fat and make more.

4 · CYP2E1 + Tylenol

Chronic drinking induces a second route, CYP2E1, which generates large amounts of that drive alcoholic liver disease and raises the risk that paracetamol injures the liver.Long-term drinking induces a second metabolic route:

CYP2E1 (cytochrome P450 2E1):

· Chronic drinking induces CYP2E1 markedly
· This is the -dependent oxidation route (not ), in the endoplasmic reticulum
· The product is still acetaldehyde, but with large amounts of (reactive oxygen species) generated
· → lipid peroxidation + protein damage + mitochondrial damage → alcoholic liver disease progression.

The interaction with paracetamol (acetaminophen):

· A small part of paracetamol is turned into NAPQI by CYP2E1 in the liver (a toxic metabolite)
· Normally: little NAPQI is made, and glutathione (GSH) neutralizes it
· In long-term heavy drinkers:
CYP2E1 is induced → more NAPQI is madePoor nutrition → low GSH reservesThe risk of liver injury is higher
In practice:

· Long-term heavy drinkers should ask a doctor or pharmacist before taking paracetamol, and never raise the dose on their own
· (N-acetylcysteine) is the antidote for paracetamol overdose: it supplies the raw material for GSH

Alcoholic liver progression: fatty liver (reversible) → alcoholic hepatitis (partially reversible) → fibrosis → cirrhosis (irreversible) → hepatocellular carcinoma.

At the fatty-liver stage, stopping drinking can usually reverse it; fibrosis is partly reversible, cirrhosis is not — the earlier you stop, the better.

These 4 steps happen with every drink — the only differences are how much and how often. ALDH2*2 carriers who flush and keep drinking for years have a clearly higher risk of esophageal cancer; the most effective step is to drink less or not at all.

Full mechanism, in text

1 · ADH cytosol

2 · ALDH2 mito

3 · redox crash

4 · CYP2E1 + Tylenol

Alcohol · ADH → acetaldehyde → ALDH2 → acetate

Full mechanism, in text

1 · ADH cytosol

2 · ALDH2 mito

3 · redox crash

4 · CYP2E1 + Tylenol

Alcohol · ADH → acetaldehyde → ALDH2 → acetate