I am drawn to research on reducing the risk of dementia. A family history of two grandparents, two uncles and a father who all had dementia will do that to you. But this is not just about me. The number of adults with Alzheimer’s disease will rise from 26.6 million today to 106.2 million by 2050. If the onset of Alzheimer’s disease was delayed by a year, the world would have 9.2 million fewer people with the condition.

Likely contenders for preserving cognitive ability include exercise and dietary changes (eg oily fish or other sources of long-chain omega-3 polyunsaturated fatty acids). So how fortunate that last week the Journal of the American Medical Association (Jama) published two randomised controlled trials and an editorial on the effects of fish-oil supplements and exercise. In the US alone, fish oil and other supplements are part of a $1.2bn industry. So should you be helping share prices of supplements rise, take up brisk walking or just accept cognitive decline?

The solution

The jury is still out, and likely to be so for some time. Neither study found a benefit in either the omega-3 fatty acids or an exercise programme (the latter did show some improvement in executive functioning – the ability to organise, apply ideas and problem-solve, but only in people over 80). The fish-oil study was in people with an average age of 73 - so the intervention might have been too late - and looked at supplements rather than foods high in omega-3 fatty acids. Research usually finds that supplements aren’t as effective as proper food.

The exercise study may not have prescribed energetic enough activities and many of the participants were well educated, a factor known to protect against loss of brain power. Dr Sudeep S Gill, the author of the editorial in this week’s Jama, says that these recent research findings shouldn’t make us nihilistic, except about taking supplements, where the evidence is consistently negative.

No single study is going to provide the answers on dementia. Gill has more faith in other research showing the preventative powers of a Mediterranean diet (containing olive oil, nuts, fresh fish and fruit and vegetables). Such a diet may work by reducing blood pressure, keeping the heart healthy and perhaps reducing the risk of dementia through preserving a good blood supply to the brain. Gill says that overall the evidence from many different types of studies suggests we should keep active and eat healthily throughout our lives. But if you think this means it’s too late to change in middle age, Gill would disagree. He told me emphatically: “I’m not sure it’s ever too late to derive some benefit from modifying your lifestyle.”