Duration Of Cardio Effect On Fat Loss

Working out for a longer time frame will burn more calories than a shorter one if you keep a steady, consistent pace. At a certain point, you will start burning fat instead of sugar. If your time budget for cardiovascular exercise is sparse, don't worry -- you can achieve better fat-burning effects by changing the intensity of your workout.

Fat as Fuel

After 45 minutes of steady pace exercise your body starts burning more fat, according to "Getting Back In Shape," by Bob Anderson. Prior to that, the main fuel of choice for your body is sugar. You make your body more efficient at burning fat as fuel when you work out for more than 90 minutes at least once every two weeks, Anderson notes.

Fat Burning Zone

You can work out in your "fat burning zone," defined as between 50 and 70 percent of your maximum heart rate, for shorter time frames, such as 30 minutes. You will still use a higher percentage of fat calories than when you exercise for the same time frame at a higher intensity. However, while the percentage of fat calories burned might be higher, the overall number of fat calories burned is actually lower. For example, if you burn 240 calories during 30 minutes of walking on the treadmill, 40 percent will be fat calories for a total of 96 calories coming from fat. However, if you run for 30 minutes on the treadmill and burn 450 calories, 25 percent come from fat, translating to 112 fat calories. The bottom line is that you'll always utilize more fat calories when you perform cardiovascular exercise at a higher intensity than when you exercise at a lower intensity, even though the percentage of calories from fat is higher during lower-intensity workouts, says Wayne Westcott, fitness research director at South Shore YMCA in Minnesota and author of "Building Strength and Stamina."

Shedding Fat

Utilize high-intensity interval training (HIIT) to stoke your body's fat-burning furnace because it works better than steady-pace cardio exercise when it comes to promoting fat loss. With this exercise you alternate bouts of high-intensity exercise in which you work out at 80 percent of your maximum heart rate or more with bouts of lower-intensity exercise. This type of training does more to increase fat oxidation in your muscle than steady-pace aerobic exercise, says C.G. Perry, lead author for a 2008 study published in "Applied Physiology, Nutrition and Metabolism." You'll see results as quickly as six weeks. A typical HIIT session has a five- to 10-minute warm-up followed by up to 12 work intervals and a cool-down of five to 10 minutes. The typical interval ratio is one minute work to two minutes recovery.

Endurance Exercise

If you are training for an endurance event like a marathon or triathlon, you will have many longer workouts. In workouts that are done at low intensity and for more than three hours, 70 percent of the fuel your body uses comes from fat. In activity that lasts one to three hours performed at moderate intensity, 35 percent of the fuel your body uses is fat. In high-intensity work done for less than an hour, 7 percent of the fuel used by your body is fat, says Brendan Brazier, Ironman triathlete and author of "Thrive: The Vegan Nutrition Guide to Optimal Performance in Sports and Life."