Being overweight or obese is known to have its ill consequences but it appears that having excess weight has its benefits when it comes to odds of developing dementia.

A new study suggests that being obese may provide protection against mental decline challenging expert advice that claims individuals who are obese in the middle age have increased risks for dementia.

Earlier studies have found an association between higher body mass index (BMI) and the odds for dementia but the new research, which is believed to be the most comprehensive to date, found that individuals who are underweight in middle age have increased chances of developing dementia and those who are very obese have significantly reduced odds of developing the condition compared with those who have healthy weight.

For the new study, which was published in the Lancet Diabetes & Endocrinology on April 9, the researchers looked at the medical records of almost 2 million people who were 55 years old on average. During the follow up period, almost 50,000 of these individuals developed dementia but those who have the highest BMI appeared to least likely suffer from the disorder.

The study found that people who were underweight had 39 percent increased risk for dementia compared with those who had healthy weights. Those who were overweight had 18 percent reduced odds for the condition and the chances were 24 percent less for the obese.

"Being underweight in middle age and old age carries an increased risk of dementia over two decades," the researchers reported. "Our results contradict the hypothesis that obesity in middle age could increase the risk of dementia in old age."

Study author Stuart Pocock, from the London School of Hygiene & Tropical Medicine, said that older studies that found a link between dementia and being overweight were small in scale and these may have involved a random chance factor.

"People like to think obesity is bad in general so there is a sort of passion to think of new handles on which to say obesity is a bad thing, and there are many health issues for which, obviously, it is. But dementia isn't one of them," Pocock said. "That is new information that people need to take on board."

The researchers said that the result of the study suggests that policy makers, public health scientist and doctors should consider rethinking how to identify individuals who are at high risk of dementia.

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