And who said exercising couldn't be fun? Khloe Kardashian once said that she chooses to have sex in order to burn more calories. And while we wouldn't usually take her advice, she may be on to something... According to Glamour, you can burn quite a few calories during sex. Professor Antony Karelis, from the University of Quebec, recently conducted a study to find out just how many you actually burn. On average sex lasts 5.4 minutes, which would burn around 17 calories, but that doesn't include foreplay. Karelis decided to get a more accurate figure by using Fitbit-like devices. He got couples to wear the devices while they had sex, including foreplay, and found that the entire session lasted 25 minutes on average. During this time women burned 69 calories, while men burned 100 calories. That's almost as much as you would burn if you walked a mile. Karelis also said, "the use of caffeine or alcohol may increase the number of calories that are burned during sex," but he hasn't studied this directly. Either way, it's another reason to have sex!