While everyone knows heavy drinking isn't good for the brain, even moderate drinkers who have the occasional wild bender are putting themselves at increased risk of dementia, new research has found.
Scientists in the UK looked at health data from more than 131,000 people in the UK, France, Sweden and Finland, in particular how much they drank and whether they'd ever lost consciousness from over-indulging.
Some of the findings were obvious - heavy drinkers (defined as having 14 or more standard drinks a week) were 20 percent more likely to develop dementia over the next decade as moderate drinkers (under 14 a week), for example.
But moderate drinkers who'd reported losing consciousness at least once were twice as likely to develop dementia as those who'd kept their drinking under control. Even heavy drinkers who didn't black out weren't as likely to develop dementia.
"Moderate drinkers who drink to excess to the point that they black out, they lose consciousness, that's doing the damage to the brain, that's killing the brain cells, that's leading to early on-set dementia," Alcohol Healthwatch director Nicki Jackson told The AM Show on Thursday.
"We've always known that chronic heavy drinking is related to dementia, but this shows that even if you're a moderate drinker that goes out and hits the bottle really hard one night to the point of a blackout, that will start the development of dementia."
Drinkers who blacked out were more likely to drink spirits and beer, and tended to avoid wine, the study found. They were also more likely to suffer substance abuse problems, poisonings, mood disorders, liver disease, heart failure, epilepsy, kidney failure, injuries, diabetes, bleeding on the brain, stroke, hypertension and arrhythmia.
The scientists said the evidence pointed towards blackouts as being the primary cause of alcohol-induced brain cell death, over heavy consumption.
"The association between alcohol-induced loss of consciousness and dementia was noted also among moderate drinkers, supporting the hypothesis that alcohol-induced loss of consciousness may be harmful for brain health independently of overall alcohol consumption."
The latest Ministry of Health data shows 20 percent of Kiwi adults are hazardous drinkers, with 18- to 24-year-olds leading the way (35.4 percent). Also above average are 25- to 54-year-olds, with rates of consumption tailing off in the older population.
But in recent years youth drinking has declined, offset by a rise in older groups.
"Alcohol is still the drug of choice for an ageing Baby Boomer cohort," an article in the New Zealand Medical Journal claimed last year.
"With the current transition of boomers into 'older adulthood' the rate of older drinkers now consuming alcohol at levels hazardous to their health has significantly increased, as has the rate of older adults with alcohol-related disorders and hospitalisations."
Dr Jackson says this is the "perfect storm" when combined with New Zealand's ageing population.
"We'll be starting to see more alcohol-related dementia."
Some studies suggest moderate alcohol consumption can be good for you, particularly because it relieves stress. Dr Jackson said for some diseases there is no safe level of consumption, the risk increasing from the first sip.
The Ministry of Health's current guidelines are no more than 15 standard drinks a week for men and 10 for women. In the UK it's 14 for both, and in Australia, 10.
"Many New Zealanders reap the social benefits of drinking, and I'm one of them," said Dr Jackson. "But what we want to do is encourage New Zealanders to move into those low-risk drinking guidelines. We could have huge cost savings for society if we just drink less."
The study, published in journal JAMA Network Open, says the findings are observational and "cannot prove that alcohol and passing out actually caused the differences in risk of dementia" observed. The authors suggest people who black out often might be falling over and hitting their heads, which could be the true cause of the increased dementia risk.