TweetBurning more calories does not necessarily mean you are burning fat. If you do cardio to intensly you can easily burn up lean muscle tissue which is of course not beneficial. Look at long distance runners...how many of them have any muscle? Now look at sprinters...they are fuckin buff. Also, it's true that your glucose levels are the lowest in the morning, but so are your amino acid levels circulating through your blood stream. I'd like to see some of the studies that indicate that high intensity cardio burns more "FAT", not calories, than cardio done within the target heart range for lipolysis (fat oxidation) ACE certification is very credible, and this is the method they recomment for maximum fat burning. I have also spoke with most of the trainers at my gym and they tell me the same thing. But the most important aspect that a trainer told me is that is you do cardio in your target hear range for 40 minutes, you are far less likely to loose any muscle tissue than if you ran hard for 20-30 minutes. I'm not saying that you burn more calories doing it this way, but I truly believe you burn more "fat" from this method of cardio