Admit it—you too have spent countless hours at the gym doing "low intensity" cardio to burn more body fat. C'mon, it's OK, we've all wasted hours walking on a treadmill...a moving belt with no real destination because, well, the belt doesn't take you anywhere. At least at an airport the moving belt moves you forward.
Isn't it strange that we get in our cars, drive however many minutes to a gym, to walk in place? We're really no different than rats that run around their wheels hoping to get somewhere.
And while I'm picking on treadmills, all the cardio equipment is the same; you pedal on a bike at a low intensity to go nowhere, or whatever other piece of cardio equipment you may use. I digress.
The Misconception Of Low Intensity
Many folks have turned to low intensity cardiovascular work to "burn more body fat than carbohydrates." First, let's get that theory out of the way. Carbohydrates and fat are the body's primary fuel source during activity. Carbohydrates are used first and in terms of carbohydrate sources, it will first be blood glucose that's utilized for energy.
Next, stored carbohydrate (glycogen) will be turned into glucose and utilized. As exercise continues (and I'm talking hours, like a marathon), fat will become the primary fuel source.
Keep in mind that there is never only one fuel source, it's always a mixture of fuels for the body; however, the ratios change (e.g. 90% carbohydrates, 10% fat at the start of exercise, then the carbohydrate percentage decreases and fat percentage increases as exercise continues.).
It is also important to note that fat requires oxygen to be burned as fuel, which is where the misconception about low intensity aerobics burning more fat comes into play. The thought is that if you have more oxygen available (e.g., you're not gasping for air because your intensity is so low), you'll be able to burn more fat.
Here's The Truth
It is true that a greater percentage of calories burned will come from fat if exercising at a low intensity. However, because you're exercising at a lower intensity, you will burn fewer total calories.
On the contrary, if you do a harder, higher intensity activity, like sprinting on a track, or a bike, you'll burn an overall greater amount of calories.
So even though a lower percentage of these calories will come from fat, a lower percentage of a higher number is still greater than a higher percentage of a much lower number. Make sense?
Here's an example with some actual numbers because it can get confusing. These numbers are used solely for ease; they are not exact for what actually happens.
Low Intensity Walking For 60 Minutes At 3 mph:
Since there are approximately 100 calories used for every mile, this 60-minute session would burn approximately 300 calories.
It was low intensity, so let's say 60% of those calories came from fat (and by the way, it's circulating fat, not body fat that you'll first use). 60% of 300 is 180 calories that came from fat.
High Internsity Sprinting For 20 Minutes:
Next, consider a high intensity sprinting session. Since I used the treadmill in the first example, I'll use the running for this (but it's much easier to do these outside vs. a treadmill). If you are able to do sprints for 20 minutes, you may burn 600 calories (again, just an estimate).
Since you were doing these at a higher intensity, you used less fat itself for fuel and more carbohydrates. Let's say that 40% of the fuel source utilized was from fat. That means 40% of 600 is 240 calories.
You still used more fat as fuel with sprinting and it's not body fat directly that you're using; it is circulating fat in the blood. Most importantly, it's the overall calories you use that you should be concerned with vs. what the specific fuel source is anyhow.
A Couple Of Studies
Experimental Biology Study
Don't believe me? Check out this recent study presented at the Experimental Biology meeting that showed fat "burning" may require those high intensity sprints we all love to hate.
This study included 11 men and 11 women in which they were cycling sprints at various intensities. The researchers learned that the maximal fat utilization occurred at an intensity near anaerobic threshold, which is essentially the state you're in when sprinting.
Moral Of The Story:
Stop wasting endless hours doing some low intensity cardio—kick it up a notch to really see the results!
McMaster University Study
Maybe you're less concerned about fat loss, but think that sprinting will negatively affect your endurance capacity. Well, looks like this isn't true either.
A study conducted at McMaster University in Ontario and published in the highly-regarded Journal of Applied Physiology suggested that short sprint interval training increased muscle oxidative potential and doubled endurance capacity during intense aerobic cycling in recreationally active individuals.
Sixteen healthy individuals participated in this study. Out of the 16, 8 subjects were in the experimental (sprint) group and 8 were in the control group (no training intervention).
The sprinting sessions consisted of six sessions of sprint interval training spread over a 14-day period (not quite every other day).
The sessions each consisted of a 30-second "all out" effort on a cycle. Subjects then rested 4 minutes between sets and either completely rested or performed light cycling with little resistance.
In this short, 2-week intervention, it was demonstrated that brief repeated bouts dramatically improves endurance capacity during a fixed workload. Moreover, it was shown that intense exercise can rapidly stimulate improvements in muscle oxidative potential comparable to or greater than that of standard aerobic based training. This basically means the capacity to perform aerobic work over time.
Moral Of The Story:
Sprinting is more effective for fat loss sprinting vs. low intensity walking or cycling for hours on end. It also frees up a lot of time to get home and recover.