The 12 lifestyle choices and conditions which fuel dementia have been identified by the World Health Organisation (WHO) in the most definitive list ever of how to avoid mental decline in later life.

New guidelines based on analysis of decades of research found that physical inactivity, smoking, eating an unhealthy diet and drinking excessive alcohol significantly increased the threat of diseases like Alzheimer's.

Medical conditions including diabetes, high blood pressure, high cholesterol and obesity also played a role in the development of cognitive decline and full-blown dementia.

And health experts also warned of a link between hearing loss and depression although said those factors may be symptomatic, and there was no evidence that hearing aids or antidepressants could halt dementia.

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"In the next 30 years, the number of people with dementia is expected to triple," said WHO Director-General Dr Tedros Adhanom Ghebreyesus.

"We need to do everything we can to reduce our risk of dementia. The scientific evidence gathered for these guidelines confirm what we have suspected for some time, that what is good for our heart is also good for our brain."

Around 850,000 people in Britain are currently living with dementia and the number is expected to rise to one million by 2021.

Despite decades of research no drugs have been developed to halt or reverse the condition meaning the only way to slow down or prevent the disease is through living a healthier life.

Yet although dementia is now the UK's leading cause of death, only one third of adults recognise there is anything they can do to reduce their risk of the condition.

Under the new guidance, over 65s are advised to do at least 150 minutes of moderate exercise per week as well as muscle strengthening activity. Along with smoking and high blood pressure physical inactivity in middle age was found to be one of the biggest risk factors for developing dementia.

The guidelines also included adoption of a Mediterranean or balanced diet which includes 400g of fruit and vegetables a day, less than 10 per cent of energy intake from sugar and less than 30 per cent from fat.

However the WHO report said there was no evidence that dietary supplements such as B vitamins, antioxidants, omega-3 or ginkgo biloba helped stave off dementia.

Instead experts encouraged people to partake in activities that exercise their brains, and get as much social interaction as possible.

Charities said it was important that people realised there were steps they could take to lower their dementia risk.

Dr Carol Routledge, Director of Research at Alzheimer's Research UK, said: "With a rapidly ageing population in the UK and across the world, many people are keen to know the steps they can take to enjoy their later years in good health.

"The findings clarify what we already know about dementia risk, including the value of physical activity and not smoking. Sadly, there will always be individuals who address many or all of these risk factors and still develop dementia.

"Genetic predisposition plays an important role in many people's risk of diseases like Alzheimer's, and while we cannot change the genes we inherit, taking the steps outlined in this report can still help to stack the odds in our favour."

Commenting on the research Robert Howard, professor of old age psychiatry, University College London, added: "The guidelines are based on a comprehensive and carefully conducted review of the published literature and are sensible but unsurprising.

"Keep on doing the things that we know benefit overall physical and mental health, smoking cessation, reduce harmful alcohol drinking, treat hypertension, eat a healthy balanced diet and lose weight if you are obese."

12 dementia risk factors

• Smoking
• Excessive alcohol intake
• Unhealthy diet
• Hearing loss
• Social isolation
• Mental inactivity
• Diabetes
• High blood pressure
• High cholesterol
• Obesity
• Depression
• Physical inactivity