People who are obese in middle age run a lower risk of developing dementia later, said a large and long-term study Friday whose findings challenge the prevailing wisdom. 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 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.