Alcohol causes cancer, according to a growing number of groups making increasingly stronger claims linking the two. The U.K. Department of Health made the bold assertion two years ago this month, and just this past November they were joined by the American Society of Clinical Oncology. But the links were mostly statistical observations – the actual mechanisms by which imbibing promoted tumors was not clearly understood.

Now, a new study by British scientists pinpoints a particular byproduct of alcohol in the body that causes DNA damage and chromosome rearrangements, leading to cancer, as explained in the journal Nature this week.

At most risk are certain people of Asian heritage who do not have key enzymes, they add.

“Some cancers develop due to DNA damage in stem cells,” said Ketan Patel, leader of the team, from the MRC Laboratory of Molecular Biology in Cambridge. “While some damage occurs by chance, our findings suggest that drinking alcohol can increase the risk of this damage.”

The culprit is acetaldehyde, an alcohol-derived metabolite. The appearance of this chemical causes DNA damage in the hematopoietic stem cells, which renew the blood, according to the study.

The hypothesis was tested in mice. The rodents were given diluted ethanol, and then the group of animals were assessed for genetic damage at minute level of chromosome analysis through DNA sequencing.

“This damage results in DNA double-stranded breaks that, despite stimulating recombination repair, also cause chromosome rearrangements,” the scientists write. “These findings characterize the mutation of the stem-cell genome by an alcohol-derived and endogenous source of DNA damage.”

A specific group of people also have further problems with the booze byproducts. Those millions of people lack a group of enzymes called aldehyde dehydrogenases, which break down the acetaldehyde into acetates. This is especially true to people who have heritage from Southeast Asia, many of whom have faulty versions of these enzymes – or none of them at all. The mouse models showed that lacking this enzyme response to the alcohol metabolites resulted in four times the DNA damage, they report.

The research was supported by the group Cancer Research UK. Since alcohol contributes to an estimated 12,000 annual cancer cases in the country, they contend, the study is further proof that most should cut back on how much they indulge.

“This thought-provoking research highlights the damage alcohol can do to our cells, costing some people more than just a hangover,” said Linda Bauld, a scientist with the group.

The American Society of Clinical Oncology referenced more than 150 papers in its November meta-analysis of the alcohol-cancer evidence.

“The more that a person drinks, and the longer the period of time, the greater their risk of development of cancer, especially head and neck cancers… Alcohol is a cause of cancers of the oral cavity, pharynx, larynx, esophagus, colorectum, liver and female breast,” they wrote.