Does midlife obesity protect against dementia?
On the other end of the scale, however, being underweight in the 40-55 age bracket was associated with a higher risk, the researchers found.
While admitting they were "surprised" by the potential protective effect of obesity, the team cautioned against jumping to conclusions.
The reasons for the observed association were not known, they wrote in the journal The Lancet Diabetes & Endocrinology.
"The message that people shouldn't take away is that it's OK to be overweight or obese," study co-author Nawab Qizilbash of the OXON Epidemiology research company told AFP by telephone from Madrid.
"We do know... that if you are overweight or obese you have a high risk of (early) death, so it is not clear that the net benefit on dementia would be positive.
"In other words, even if there were to be protective effects on dementia from being overweight or obese, you may not live long enough to get the benefit of it."
But the widely-held belief that reducing obesity in middle age could help prevent dementia may also be ill-founded, said the team, and required a rethink of how we identify high-risk individuals.
The researchers combed a British database of patient information recorded from 1992 to 2007, representing some nine percent of the UK population.
In what they claimed was the largest-ever study of any link between bodyweight and dementia risk, the team analysed the medical records of nearly two million 40-plussers.
They compared the patients' BMI (body weight index, a ratio of weight to height) to how many developed dementia later on.
A BMI of 25 and higher is classified overweight, and 30 and over obese. Anything less than 18.5 is generally considered underweight, though for this study the researchers set the bar at 20.
Over two decades, the researchers found, "the incidence of dementia continued to fall for every increasing BMI category with very obese people (a BMI over 40) having a 29 percent lower dementia risk than people of a healthy weight."
Just over 45,500 of the total study group developed dementia.
"Compared with people of a healthy weight, underweight people (BMI under 20) had a 34 percent higher risk of dementia," added the authors.
The underweight category is a wide one, ranging from lean to skeletal, said Qizilbash, who described the increased risk as "significant".
'We were surprised'
Numerous other studies, including one carried by The Lancet Neurology in July 2014, have linked obesity to a higher risk for Alzheimer's Disease and other forms of dementia.
But Qizilbash said most were "fairly small and statistically unreliable".
"We were surprised about the findings. Because most of them, not unanimously but the majority of previous studies, have tended to indicate that people who are overweight or obese in middle age have an increased risk of dementia in older age."
Further research was needed to confirm the link and find an explanation for it, the researcher added.
"There are some potential contenders, particularly in terms of the role that nutrients may play," he said.
The team hoped their results would aid in the search for "protective factors" with a view to new treatments.
Alzheimer's Disease International (ADI) projects the number of people with dementia will rise from 35.6 million in 2010 to 65.7 million by 2030 and 115.4 million by 2050.
Obesity, too, is soaring, having more than doubled worldwide since 1980. By 2014, more than 1.9 billion adults were overweight, of whom 600 million were obese, according to the World Health Organization.
In a comment also carried by The Lancet, neurology professor Deborah Gustafson said the findings should be interpreted "with care".
"The report by Qizilbash and colleagues is not the final word on this controversial topic," she wrote.
© 2015 AFP