What they found was the 57 per cent of these cases were related to chronic heavy drinking.
Image for representational purposes only. Therefore, the study also suggests that alcohol use disorders may contribute in many ways to the risk of dementia.
According to the World Health Organisation (WHO), "heavy drinking" is defined as consuming more than 60 grams of pure alcohol a day for men and more than 40 grams per day for women.
Researchers said almost 40 per cent of the 57,353 cases of early-onset dementia (below 65 years) were directly alcohol-related, and 17.6 per cent had an additional diagnosis of alcohol use disorders.
Dr. Michael Schwarzinger, the lead researcher, said the increased risk was likely to be explained by the fact alcohol could cause "permanent structural and functional brain damage".
The study tracked more than 31 million patients discharged from hospitals in France between 2008 and 2013, including 1.1 million cases of dementia.
But for early-onset dementia, there was a significant gender split. Study author Michael Schwarzinger says that while the rate of alcohol use disorders is lower in the United States than in France, "it remains substantial enough to be considered [a] major risk factor for dementia onset". But they also looked for evidence of alcohol use disorders-identified as alcohol-related mental, behavioral or physical health conditions (like liver disease or head injury) listed on patients' hospital records.
Dr Rehm added that alcohol use disorders can shorten life expectancy by more than 20 years, with dementia being one of the leading causes of death. But experts said more should be done to tackle drinking levels across the population.
"Given the strength of the association, what is the most surprising to me is that alcohol-use disorders had received so little interest in dementia research and public health policies", Schwarzinger said.
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". They excluded those at risk of developing rare forms of dementia, such as those brought on by infectious diseases like HIV or other neurological disorders.
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. In a recent review by the Lancet Commission on Dementia Prevention, Intervention and Care, reducing alcohol consumption was not included as one of the nine lifestyle changes that may reduce the risk of dementia. In men, the risk was increased by a factor of 4.7, while in women, it increased by a factor of 4.3. "And the direct association of heavy drinking with dementia goes far beyond 65 years old".
Chronic heavy drinking is more than four drinks a day for a man, and more than three for a woman.