“The very active group, people doing 10-plus hours of activity a week, lost about a third of the mortality benefits,” compared to people exercising for 2.6 to 4.5 hours a week, said Dr. James O’Keefe, a professor of medicine at the University of Missouri-Kansas City and director of preventive cardiology at the St. Luke’s Mid America Hear Institute, who was an author on the study. (subscription may be required)