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You've probably seen lots of estimates like this: Spin class blasts 600 calories an hour! Swim 20 laps to burn off that candy bar! But those numbers are just that—estimates—and depending on your metabolism, you may burn considerably more or less. Even two women of similar age and body-mass index may notice significant differences in calorie burn: A 2013 University of Pittsburgh study, for example, found that when African-American and Caucasian women were placed on the same diet and exercise routine for six months, the white women lost an average of seven pounds more. The black women had lower resting metabolic rates , the researchers found, and expended less daily energy. In order to lose the same amount of weight, they'd need to work out more or eat less.