Alcohol directly increases blood pressure and the chances of having a stroke, so refuting the claim that moderate drinking can stave off the risk, reveals a genetic study,* published today in The Lancet.
Heavy drinking is known to increase stroke risk, but until now it wasn’t clear whether moderate drinking boosted or curbed that risk.
People who have one or two alcoholic drinks a day had previously been observed to have a slightly lower risk of stroke and heart attack than non-drinkers.
In East Asian populations, there are common genetic variants that greatly reduce alcohol tolerability and therefore the amount people can drink. Because they are unrelated to other lifestyle factors such as smoking, they can be used to study the causal effects of alcohol intake.
The researchers used a statistical technique called Mendelian randomisation, which allows researchers to rule out the influence of various factors that might otherwise make it harder to determine a possible cause.
Lead author Dr Iona Millwood, from the Medical Research Council Population Health Research Unit at the University of Oxford, UK, says: “Using genetics is a novel way to assess the health effects of alcohol, and to sort out whether moderate drinking really is protective, or whether it’s slightly harmful. Our genetic analyses have helped us understand the cause-and-effect relationships.”
The research team quizzed 500,000 men and women in China about their alcohol intake and then tracked their health for 10 years. In over 160,000 of these adults the researchers measured two genetic variants (rs671 and rs1229984) that substantially reduce alcohol intake.
Among men, these genetic variants were associated with a 50-fold difference in average alcohol intake, from near zero to about four units (drinks) per day.
The genetic variants that cubed alcohol intake also lowered blood pressure and stroke risk.
From this evidence, the authors concluded that alcohol increases the risk of having a stroke by about one third (35%) for every four additional drinks a day (280 g of alcohol a week), with no protective effects seen for light or moderate drinking.
Study co-author, Professor Zhengming Chen, from the Nuffield Department of Population Health, University of Oxford, says: “There are no protective effects of moderate alcohol intake against stroke. Even moderate alcohol consumption increases the chances of having a stroke. The findings for heart attack were less clear-cut, so we plan to collect more evidence.”
About 10,000 of the men whose genetic variants were measured had a stroke and 2,000 had a heart attack during the monitoring period.
Few women in China drink alcohol: less than 2% of women in the study drank in most weeks, and when they did drink they consumed less than men.
And the genetic variants that cause alcohol intolerance had little effect on blood pressure or stroke risk. Women in this study therefore provide a useful control group, which helps confirm that the effects of these genetic variants on stroke risk in men were caused by drinking alcohol, and not by something else.
The authors say their findings should be applicable worldwide. Although the alcohol consumed was mainly spirits, they expect the findings to apply to other drinks.
In China, more years of life are lost to stroke than to any other disease. This study estimates that, alcohol causes 8% of all ischaemic strokes and 16% of all haemorrhagic strokes.
Writing in a linked comment,** Professor Tai-Hing Lam and Dr Au Yeung, from the University of Hong Kong, call for a WHO Framework Convention for Alcohol Control (FCAC), similar to the Framework Convention on Tobacco Control (FCTC).
“Alcohol control is complex and stronger policies are required. The alcohol industry is thriving and should be regulated in a similar way to the tobacco industry,” they write.
* Millwood IY, et al. Conventional and genetic evidence on alcohol and vascular disease aetiology: a prospective study of 500000 men and women in China. The Lancet, 2019. DOI: 10.1016/S0140-6736(18)31772-0
** Tai Hing Lam, Shiu Lun Au Yeung. Unite for a Framework Convention for Alcohol Control. The Lancet, April 2019. DOI: 10.1016/S0140-6736(18)32214-1