Ryan makes a good point on heart health vs fat loss. Even the top aerobics guru , Dr. Ken Cooper of the Cooper institute came out a few years ago and reversed some of his opinions. He used to recommend high loads of the long slower cardio aimed at heart rate. What he came out with recently, was to not do it aimed specifically at fat loss per se, but to knock off the high impact stuff, as it was detrimental to the joints (almost all of them), and that to maintain good "heart health" you really only needed to do 3-4 very "brisk walks" for 30-40 mins weekly. On to fat loss and metabolism boost, circuits and complexes, such as Ryan just noted, are akin to sprints as far as that goes. I just finished rounds of a complex consising of clean grip hi pullsX 6, power cleans X6, squat push press (aka thrusters) X 6 good mornings X6, bent rows X 6, P bar dipsX10 @40,55,65, 75,75 lbs, in 14 mins. That was 15 mins ago, and I'm still breathing like a rhino.