nbc's bianca castro reports on a study that says too much of it may not be a good thing for some peopleter: can too much exercise really be bad for your heart? >> more is not always better. >> reporter: dr. danny lee at medical city arlington reviewed a recent study out of chicago that found white men who workout at least 7 1/2 twice as likely to suffer from heart disease than people who workout a moderate amount. vigorous exercise can stress the arteries and lead to plak buildup. but the correlation did not exist for white women or black men, and the researchers don't know why. >> there might be some biological differences among the sex and the race that we have yet to determine. >> reporter: more research is needed to find out why very active white men are 86% more likely to experience a plaque buildup by middle age. however, dr. lee said the findings don't mean stop working out. american heart association recommends 2 1/2 hours of moderate exercise a week for better heart health. >> multiple studies have determined that people are more active and exercise live longer and they decreas