I found this old Q&A from Jillian Michaels and wanted to share :)
Why does my online workout program consist of just one day of pure cardio and four days of weight training? Although I love weight training, I've always thought it's the cardio that really helps a woman like me lose the weight. Why so little pure cardio?
ASK JILLIAN: Okay — I love dispelling this myth. Resistance training when done in circuits with cardio intervals is the best way to lose weight for the following reasons:
1. Resistance training will help burn intramuscular fat and create lean muscle tissue, but you can't build muscle mass unless you are lifting extremely heavy weights and eating more calories than you are burning in a day. Additionally, it is VERY difficult for a woman to gain muscle mass because we simply don't have the testosterone needed to build muscle tissue the way men do.
2. Resistance circuit training burns more calories than straight cardio both during the workout and after. This is because you are getting the benefits of a resistance-training workout and a cardio workout all in one. Additionally, you will burn more calories for up to 24 hours after weight lifting than you would with cardio alone because your body has to work hard to return your muscles to a pre-lactate state (the state before training).
Here is what cardio is good for: weight-loss extra credit. In other words, you can only train your muscles so many times in a week without overtraining them, which is counterproductive. Your muscles need adequate rest and recovery time for best results, but you can do as much cardio as you want. I mean, you can't run a marathon every day, but you can jog, bike, use the elliptical machine, or swim pretty much as often as you want.