Sprinting in intervals is better if your only goal is to lose fat, however, if you are interested in actually becoming a fitter person, it is important to mix in steady state work as well. The reason the last poster said to hit the 60 minute mark is that around 40 minutes into an aerobic workout your body starts using fat almost exclusively as fuel. This doesn't actually make a difference as far as losing body fat though because your liver can turn any substrate into fat to replace it. Think of losing fat as creating a caloric deficit and really it won't make a difference what types of calories you are burning as long as you're burning more than you're putting in your body. The reason that sprints are preferable to steady state for burning more calories is that you can keep your heart rate higher for longer by doing quick bursts, then a short recovery time repeatedly. Also, you are putting your muscles under more stress, so you will burn more calories due to your metabolism being higher during recovery. Like I said though, you really want to mix things up if you're going for good overall fitness. Hope that helped. Feel free to email me with any other questions.