The researchers had previously identified a weak spot in the brain which is a specific area that develops later during adolescence, and also shows earlier degeneration in old age.

In the new study some 161 risk factors for dementia were examined, and ranked according to their impact on this brain network, over and above the natural effects of age.

The University of Oxford researchers classified these so-called modifiable risk factors - as they can potentially be changed throughout life to reduce the risk of dementia - into 15 broad categories.

These were blood pressure, cholesterol, diabetes, weight, alcohol consumption, smoking, depressive mood, inflammation, pollution, hearing, sleep, socialisation, diet, physical activity and education.

The findings suggest that these specific parts of the brain are most vulnerable to alcohol, diabetes and traffic-related air pollution.

The researchers investigated the genetic and modifiable influences on these fragile brain regions by looking at the brain scans of 40,000 people in the UK Biobank database aged over 45.

Professor Gwenaelle Douaud, who led the study, said: "We know that a constellation of brain regions degenerates earlier in ageing, and in this new study we have shown that these specific parts of the brain are most vulnerable to diabetes, traffic-related air pollution - increasingly a major player in dementia - and alcohol, of all the common risk factors for dementia.

"We have found that several variations in the genome influence this brain network, and they are implicated in cardiovascular deaths, schizophrenia, Alzheimer's and Parkinson's diseases, as well as with the two antigens of a little-known blood group, the elusive XG antigen system, which was an entirely new and unexpected finding."

Professor Lloyd Elliott, a co-author from Simon Fraser University in Canada, said: "In fact, two of our seven genetic findings are located in this particular region containing the genes of the XG blood group, and that region is highly atypical because it is shared by both X and Y sex chromosomes.

"This is really quite intriguing as we do not know much about these parts of the genome; our work shows there is benefit in exploring further this genetic terra incognita."

The study analysed the unique contribution of each controllable risk factor by looking at all of them together to assess the resulting degeneration of this particular brain weak spot.

Professor Anderson Winkler, a co-author from the National Institutes of Health and the University of Texas Rio Grande Valley, said: "It is with this kind of comprehensive, holistic approach - and once we had taken into account the effects of age and sex - that three emerged as the most harmful: diabetes, air pollution, and alcohol."

The study is published in Nature Communications.

Australian Associated Press