Studies of gut bacteria are beginning to untangle how diet affects health in old age — but determining cause and effect is tricky. Almost everything about eating gets more difficult with age. Elderly people typically cannot taste or smell as well as they used to, decreasing the appeal of some foods. Dental issues or a dry mouth can impede chewing; loss of muscle tone in the pharynx can make swallowing difficult; constipation and the side effects of medication can make digestion uncomfortable; and decreased mobility makes a chore of grocery shopping or cooking complex meals. Little wonder that older people...