Admittedly, it can be tough to sift through the bullshit. And Lord knows there is a lot of it floating around out there. But use some common sense; does waking up and doing an hour of cardio on an empty stomach really seem like a great idea to you?
There are two ways to break this down, scientifically and anecdotally. We will go with science first.
So, here is how it works. When you do steady state cardio (i.e. 45 minutes on a treadmill at one speed), your body will consume a higher number of calories derived from fat tissues. When you perform interval training (i.e. 30 seconds of sprinting, followed by 90 seconds of recovery, repeated multiple times) your body will consume a higher number of total calories.
Lets use some numbers to quantify it. Say you do 45 minutes of exercise, one day it is steady state hamster cardio and the next day is interval training. On hamster day, you burn 350 kcals, and 75 of them are kcals derived from fat sources. Thus, 25% of your total calories were derived from fat sources. The next day you do interval training, and burn a total of 450 kcals, and 75 of them are from fat. This results in only 16% of your calories being derived from fat sources, but you also burned 100 more calories total!
As we know, the name of the game in weight loss is the difference between the calories you can take in vs the calories that you expel (calories in - calories out). If you burn more calories than you consume, you will lose weight. ***
***DISCLAIMER: I am not taking into account changes in body composition. Weight loss is not just about burning calories during exercise, but changing your basal metabolic rate by adding lean muscle mass to your body. I know that weight loss is not comprised of just cardio, but also weight training.
So, if I can burn more calories in the same amount of time by doing interval training, then that sounds like something I would want to take part in, yeah?
So, why don't more people do it? Easy: because intervals suck, they are hard as shit. Average Joe Gymgoer is extremely adverse to doing things that are hard as shit. That is why the squat rack has become the curl rack in many facilities. It is way easier to climb onto the elliptical and grind away for 45 minutes while watching Rachel Ray and reading US Weekly than it is to get onto the rowing erg and crush some 30:30 intervals for 15 minutes, or go to the field and hammer at some 50 yard sprints.
Anecdotally, my favorite comparison is between sprinters and marathoners. Who would you rather look like?
For the most current example, lets see what the winner (and world record setter) of the 2011 Boston Marathon looked like:
This is what Usain Bolt, the gold medal dominator of the Beijing Olympic sprinting events:
Not even a contest, for me.
Ladies? Desiree Davila, the winner of the 2011 Boston Marathon:
LoLo Jones, one of the top sprint athletes for the US Womens Olympic team:
I don't mean to take anything away from the Marathon athletes; they are amazing and do things that I can't even fathom. 26.2 miles in 2:02? I can't even drive that fast. As impressive as they may be, I don't wanna look like them!
Now go sprint somewhere!