Men and women who are too skinny in their middle age could face a heightened risk of developing dementia in later life, while obese people might have a better chance at staving it off, a new study suggests.

Researchers analysed the medical records of nearly 2 million people from the UK, with an average age of 55 and an average body mass index of 26, which is considered just on the heavy side of healthy. None of the people evaluated had dementia when the study began, but over the course of the two-decade follow-up, around 45,000 of the people examined developed the condition.

They found that people who were underweight - with a BMI score under 20 - during their 40s, 50s, and 60s, were 34 percent more likely to develop dementia than similarly aged people with healthy body weights.

But the study also yielded an unexpected result, revealing that people who are overweight and obese in middle age might be even less susceptible to developing dementia than people with healthy body weight - a finding that contradicts previous studies that have linked obesity to higher risks of dementia.

Of the medical records examined in the latest study, British researchers found that the people who were the heaviest at their middle age, with a BMI of 40 or higher - which is considered obese - were 29 percent less likely to develop dementia than people with a healthy BMI between 19 and 25. People who were overweight were 18 percent less likely to develope dementia than healthy individuals. 

"The controversial side is the observation that overweight and obese people have a lower risk of dementia than people with a normal, healthy body mass index," lead author Nawab Qizilbash from OXON Epidemiology in the UK told James Gallagher from the BBC. 

"That's contrary to most if not all studies that have been done, but if you collect them all together our study overwhelms them in terms of size and precision," Qizilbash added.

The results, which were published in The Lancet Diabetes & Endocrinology journal, suggest that body weight might be an important factor in the development and prevention of dementia, and could lead to a rethink of who is most at risk of developing the disease, which is expected to affect some 135 million people by 2050. 

The researchers say it's unclear what, if anything, is responsible for the reduced risk of dementia they observed in overweight and obese people. They suspect factors like exercise, frailty, genetic factors and diet might play a role, but say more research is needed.

Writing in a linked comment piece, neurologist Deborah Gustafson from SUNY Downstate Medical Centre in the US said: "Some studies report a positive association between high mid-life BMI and dementia, whereas others do not... To understand the association between BMI and late-onset dementia should sober us as to the complexity of identifying risk and protective factors for dementia. The report by Qizilbash and colleagues is not the final word on this controversial topic."