Research based on those hospitalised for alcohol abuse showed 39 per cent of early-onset dementia cases were attributable to drinking-related brain damage.
Last night experts said even hitting the bottle occasionally can have adverse affects on brain health, warning people to stick to recommended guidelines.
The latest analysis reinforces how simple lifestyle changes can help beat a condition that in 2016 killed almost 63,000 people in England and Wales, 12 per cent of all deaths.
Study author Dr Michael Schwarzinger, of the French Translational Health Economics Network, said: “Our findings suggest the burden of dementia attributable to alcohol use disorders is much larger than previously thought, suggesting that heavy drinking should be recognised as a major risk factor for all types of dementia.”
The findings come from an observational study of more than a million adults who were diagnosed with dementia between 2008-2013.
Of the 57,000 cases of early-onset dementia, defined as before 65, most were alcohol-related.
Researchers studied a French national database to examine the effect of alcohol disorders, including those diagnosed with mental and behavioural conditions or chronic disease attributable to drink abuse.
It included all patients over 20 discharged with alcohol-related brain damage, vascular dementia or other forms of brain-wasting diseases like Alzheimer’s.
During the five-year period 31.6 million people were discharged from hospital with more than 1.1 million diagnosed with dementia.
One in 20 of those had early-onset. Around three per cent of dementia cases were attributable to alcohol-related brain damage while other alcohol use disorders were recorded in almost five per cent.
Prevalence increased in early-onset dementia where 39 per cent of cases were attributable to alcohol-related brain damage while 18 per cent had other drink disorders.
Overall, alcohol abuse was associated with a three times greater risk of all types of dementia and was the “strongest modifiable risk factor” for its onset.
The World Health Organisation defines chronic heavy drinking as six or more standard drinks a day for men and four for women.
Medical chiefs in the UK recommend the consumption of no more than 14 units a week, equivalent to seven pints of average-strength lager or nine 125ml glasses of average-strength wine.
Dr Schwarzinger said: “The link between dementia and alcohol use disorders needs further research, but is likely a result of alcohol leading to permanent structural and functional brain damage.
“Alcohol use disorders also increase the risk of high blood pressure, diabetes, stroke and heart failure, which may increase the risk of vascular dementia. Heavy drinking is associated with smoking, depression and low educational attainment – also dementia risk factors.”
The research, published in The Lancet, comes after a study last year claimed one in three cases of dementia could be prevented if the risks of becoming a sufferer were tackled.
Scientists said the disease was not an inevitable part of ageing but avoidable if nine key risk factors were eliminated.
Professor Clive Ballard, of the University of Exeter Medical School, called the study “immensely important”, adding: “We should move forward with clear public health messages on the relationship between both alcohol use disorders and alcohol consumption and dementia.”
Dr Sara Imarisio, of Alzheimer’s Research UK, said: “This is not the first time research has revealed a link between alcohol misuse and dementia and the findings lend even more weight to calls for people to drink within guidelines.”
Dr Doug Brown, of Alzheimer’s Society, said: “This study suggests alcohol abuse disorders may be responsible for more cases of early-onset dementia than thought.”