The study identifies five healthy behaviours as being integral to having the best chance of leading a disease-free lifestyle: taking regular exercise, non-smoking, a low bodyweight, a healthy diet and a low alcohol intake.

The people who consistently followed four or five of these behaviours experienced a 60% decline in dementia and cognitive decline - with exercise being the strongest mitigating factor – as well as 70% fewer instances of diabetes, heart disease and stroke, compared with people who followed none.

"The size of reduction in the instance of disease owing to these simple healthy steps has really amazed us and is of enormous importance in an aging population," said Principle Investigator Professor Peter Elwood from Cardiff University's School of Medicine. "What the research shows is that following a healthy lifestyle confers surprisingly large benefits to health – healthy behaviours have a far more beneficial effect than any medical treatment or preventative procedure.

"Taking up and following a healthy lifestyle is however the responsibility of the individual him or herself. Sadly, the evidence from this study shows that very few people follow a fully healthy lifestyle. Furthermore, our findings reveal that while the number of people who smoke has gone down since the study started, the number of people leading a fully healthy lifestyle has not changed," he added.

Recent surveys indicate that less than one per cent of people in Wales follow a completely healthy lifestyle, based on the five recommended behaviours, and that five per cent of the population follow none of the healthy behaviours; roughly equating to a city with a population the size of Swansea (240,000).

Professor Elwood continued: "If the men had been urged to adopt just one additional healthy behaviour at the start of the study 35 years ago, and if only half of them complied, then during the ensuing 35 years there would have been a 13% reduction in dementia, a 12% drop in diabetes, six per cent less vascular disease and a five per cent reduction in deaths."

The Caerphilly Cohort Study recorded the healthy behaviours of 2,235 men aged 45-59 in Caerphilly, South Wales. The study had multiple aims and has been the basis for over 400 research papers in the medical press. One of the most important aims was to examine the relationship between healthy lifestyles, chronic disease and cognitive decline over a 35-year period; and to monitor changes in the take-up of healthy behaviours.

Dr Doug Brown, Director of Research and Development at the Alzheimer's Society, said: "We have known for some time that what is good for your heart is also good for your head, and this study provides more evidence to show that healthy living could significantly reduce the chances of developing dementia. These large, longitudinal studies are expensive and complicated to run, but are essential to understand how dementia can be prevented. We are calling on the G8 Summit next week to commit to greater funding of important studies such as this one which give us hope for reducing the impact of dementia in the future."

Christopher Allen, Senior Cardiac Nurse at the British Heart Foundation, which part-funded the study, said: "The results of this study overwhelmingly support the notion that adopting a healthy lifestyle reduces your risk of cardiovascular disease and dementia. These findings will hopefully go a long way in encouraging people to carefully consider their lifestyle and how it will impact on their health in later years."

Unhealthy living has accounted for around 10% of the costs of the NHS in Wales since the study first started, while the annual expenditure on prevention and public health services in Wales is estimated to have been £280M.

The study was funded by the Medical Research Council, Alzheimer's Society and the British Heart Foundation.