According to exercise researchers, the percentage of fat calories burned is higher at lower speeds, but the absolute number of fat calories burned is always greater at higher speeds. And for people trying to lose weight, the total number is the only number that matters.

Wayne Wescott, a noted exercise researcher reports that a recent study compared calorie burn rates for a 30-minute walk at 3.5 mph (moderate walking speed) and a 30-minute run at 7 mph (moderate running speed). The walking group burned an average of 240 calories in their half hour, 40 per cent from fat and 60 percent from carbohydrates. The walkers burned an average of 96 calories of fat.

The harder exercising running group split their calorie expenditure differently: 24 per cent from fat, and 76 per cent from carbohydrates. But they burned a whopping 450 calories during the same half hour, which means they burned 108 calories of fat in the same amount of time.

Working easier burns a higher percentage of fat because our "aerobic" energy system provides a greater proportion of the fuel required for exercise. The aerobic system combines the air we breath (oxygen) with the fat stored in our bodies to produce energy.

Working at a higher rate of exertion (running vs. walking, for example) results in a greater proportion of energy being supplied by our anaerobic energy system. The anaerobic system uses as much energy as the aerobic system can provide, and then "tops up" what our body needs through the provision of stored muscle glycogen. Glycogen is a "quick acting" fuel stored primarily in muscle tissue.

One of the best ways to train your anaerobic system to increase the amount of glycogen you can store - thereby increasing the intensity with which you can work out at - is to do interval training. There are many forms of interval training, but it can most simply be described as a form of steady state exercise that includes short bursts of high intensity activity. If you walk for cardio exercise, try picking up the speed or walking on an incline as your high intensity interval. Start out with 30-second intervals, and increase the interval length as you improve your fitness.

If you’re a runner, run at a 5.5 mph pace for 5 minutes. Increase your speed for 30 secs. to 6.0 mph, then increase again for another 30 secs. to 6.5 mph before returning to 5.5 mph for five minutes of steady state recovery. As you get stronger you increase your interval intensity by going faster or longer before returning to your recovery speed.

The morale of this story - if you want fat loss, include some cardio in your training program. If you want more fat loss, increase your cardio intensity by including interval training.