For middle-aged people, being overweight may decrease the risk of dementia, a new study shows -- contradicting previous findings that suggested overweight people were at an increased risk.

Researchers from the London School of Hygiene & Tropical Medicine, and Oxon Epidemiology concluded that middle-aged people with body mass indexes (BMI) of 40 or higher were 29 percent less likely to develop dementia than people of the same age with a lower BMI.

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Middle-aged adults with a BMIs less than 20 were 34 percent more likely to develop dementia than those who were overweight or even normal weight, the study also showed.

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“Our results suggest that doctors, public health scientists, and policymakers need to rethink how to best identify who is at high risk of dementia," said Stuart Pocock, study author and professor of medical statistics from the London School of Hygiene & Tropical Medicine.

Pocock also said that while researchers do not yet understand why a higher BMI would decrease the risk of developing dementia, further down the line, "researchers might be able to use these insights to develop new treatments for dementia."