Sitting less than, more than or equal to an average of 6½ hours daily yielded about the same body-fat mass among 13-year-olds, according to a study in the Journal of Pediatrics. However, researchers reported that boys and girls who spent more time doing moderate-to-vigorous activities had a lower body-fat percentage, indicating that a lack of exercise, rather than sedentary time, is linked to body-fat mass in children.