CHICAGO (Reuters) - Eating fish at least once a week slows the toll aging takes on the brain, while obesity at midlife doubles the risk of dementia, a pair of studies concluded on Monday. Omega-3 fatty acids contained in fish have been shown to boost brain functioning as well as cutting the risk of stroke, and eating fish regularly appears to protect the brain as people age, the six-year study of Chicago residents said. "The rate of (mental) decline was reduced by 10 percent to 13 percent per year among persons who consumed one or more fish meals per week...