Alcohol, ethanol, is a direct hepatotoxin and a class 1 carcinogen as classified by the International Agency for Research on Cancer. The fact that it is also a widely accepted social lubricant and a legal substance in most of the world does not change its biochemistry. Understanding what actually happens in the liver during and after alcohol consumption provides a more specific and honest risk framework than the conventional moderate drinking discourse.
Metabolism and Acetaldehyde
Ethanol is metabolised in the liver primarily by alcohol dehydrogenase to acetaldehyde, a reactive and toxic compound that binds to DNA, proteins, and lipids, forming adducts that impair cellular function. Acetaldehyde is then metabolised by aldehyde dehydrogenase to acetate. The rate-limiting enzyme is aldehyde dehydrogenase: individuals with genetic variants producing reduced ALDH2 activity, common in populations of East Asian descent, accumulate acetaldehyde rapidly and experience flushing, nausea, and significantly elevated cancer risk even with modest alcohol consumption.
Glutathione Depletion
Alcohol metabolism generates significant oxidative stress through NADH accumulation and reactive oxygen species production. The liver's primary antioxidant defence is glutathione. Chronic alcohol consumption depletes hepatic glutathione, reducing the liver's capacity to neutralise phase 1 detoxification intermediates and reducing protection against oxidative damage from other sources including pollutants, drugs, and metabolic byproducts. N-acetyl cysteine, the glutathione precursor, is used medically in paracetamol overdose for this reason.
Intestinal Permeability and Hepatic Inflammation
Alcohol directly damages intestinal tight junctions, increasing gut permeability and allowing bacterial lipopolysaccharide from the gut microbiome to enter systemic circulation via the portal vein. LPS reaching the liver activates Kupffer cells, the resident liver macrophages, via Toll-like receptor 4, driving hepatic inflammation. This LPS-mediated hepatic inflammation is now understood to be the primary mechanism of alcoholic liver disease progression, explaining why the gut microbiome is a significant determinant of individual susceptibility to alcohol-induced liver injury.
- No level of alcohol consumption is free of hepatic impact: the current evidence does not support a clearly safe minimum dose
- Alcohol-free days are not a substitute for reducing total weekly units: cumulative weekly exposure drives cumulative risk
- N-acetyl cysteine at 600mg daily supports glutathione status and may reduce the oxidative burden of alcohol metabolism in those who drink regularly
- Adequate dietary protein supports the amino acid substrates for glutathione synthesis
- Consider liver function tests annually if you consume alcohol regularly: ALT and GGT elevation is an early indicator of hepatic stress
Sources
- Zakhari, S. (2006). Overview: how is alcohol metabolized by the body? Alcohol Research and Health, 29(4), 245-254.
- Thurman, R.G. (1998). Alcoholic liver injury involves activation of Kupffer cells by endotoxin. American Journal of Physiology, 275(4), G605-G611.