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It is well known that fat makes its greatest contribution as an energy source during low to moderate intensity exercise. During exercise at 40 to 60% of VO2Max, fat breakdown supplies about half of the energy needs (1). This has led to recommendations that individuals who want to burn more fat to lose weight should exercise at a lower intensity.
The concept of "fat burning" versus "carbohydrate burning" exercises is a common misconception shared by many aerobics instructors and clients in the weight loss arena. They believe that individuals who want to lose body fat should exercise at a lower intensity since fat contributes more to the metabolic mixture. Unfortunately, this assumption misses the point -- regular exercise is beneficial for weight loss because it creates a prolonged caloric deficit.
The fuel being burned to create this caloric deficit (fat or carbohydrate) is probably secondary. Currently, there is no research suggesting that the selective use of fat as fuel will translate into greater fat loss compared to an equal caloric deficit created by "carbohydrate burning" exercise.
For example, it is doubtful that a runner would lose more body fat by jogging five miles slowly than by running five miles at race pace. Although fat contributes more calories during jogging than when racing, both activities have the same caloric cost and would be expected to have the same effect on body fat loss.
Proof of this came about 10 years ago, when Gaesser and Rich studied two groups of college-age men who exercised on cycle ergometers three days per week for 18 weeks (2). One group exercised for 50 minutes at about 45% of VO2Max and the other group for 25 minutes at about 85% of VO2Max.
The total energy expenditure was 300-350 calories per session, and most importantly, it was nearly identical for both groups. The low intensity group burned more fat during each of the training sessions, but after 18 weeks of training both groups had lost equivalent amounts of body fat (about 3 pounds) and both had reduced their percent body fat by the same amount.
What explains these contradictory results? How can an individual lose body fat via exercise when little fat is burned during the exercise period? The answer lies in what occurs after the exercise is completed. The more carbohydrate burned during exercise, the more fat burned after exercise. This makes sense: when there is not much carbohydrate to burn, fat oxidation will naturally be accelerated.
Bahr and Sejersted observed this process in six men who exercised for 80 minutes at three different intensities -- 29%, 50%, and 75% of V02Max (3). The higher the exercise intensity, the lower the respiratory quotient dropped after exercise, indicating greater fat utilization. The higher exercise intensity also caused the respiratory quotient to stay below normal resting levels for a greater time period -- up to 14 hours longer!
Wilmore points out that although a greater percentage of fat may be burned with low intensity exercise, the total grams of fat burned is actually greater with high intensity exercise because the total energy output is higher during intense activity lasting when the time spent exercising is the same (4). Wilmore also notes that the substrate burned during exercise doesn't matter when the goal is to lose body fat.
A more correct explanation is that low intensity, steady state aerobic exercise uses a greater percentage of fat than high intensity exercise, but these fat calories (and carbohydrate calories) are being burned at a relatively slow rate (e.g. four to five calories per minute.) By comparison, high intensity exercise uses a smaller percentage of fat, but this smaller percentage (along with carbohydrate) is being burned at a much higher rate (e.g. 10 to 13 calories per minute.) Thus, as Wilmore points out, the total amount of fat burned is greater during high intensity exercise (70 to 75% of V02Max) compared to a similar duration of low-moderate intensity exercise (40-60% of VO2Max).
Many aerobic instructors have confused the proportion of fat used as fuel with the more important rate of fuel utilization, which is a key concept in exercise induced body fat loss.
The bottom line is that when the goal of an exercise program is to lose weight, the exercise should create a caloric deficit. To lose a pound, an individual must expend 3,500 calories, whether those calories come from fat or carbohydrate.
It should be noted that low to moderate intensity exercise is recommended for overweight people trying to lose body fat who are new to exercise. High intensity exercise is associated with increased injuries and poor adherence in this population.
The American College of Sports Medicine and the Centers for Disease Control recommend that individuals accumulate 30 minutes or more of moderate intensity activity over the course of most of the days per week (5). This amount of activity should provide enough of a caloric expenditure to promote body fat loss. The goal is to achieve a weekly caloric expenditure of at least 1,000 calories, since this seems to represent a threshold for body fat loss (6).
Individuals with a low level of fitness should exercise at a lower exercise intensity. The only drawback of low intensity exercise is that the person must exercise longer to achieve a significant caloric deficit. Otherwise, a low intensity workout that expends 300 calories in an hour is just as beneficial as high intensity exercise that expends 300 calories in 30 minutes.
Keep in mind that the decreased number of calories burned during low intensity exercise may be detrimental for fit people trying to lose body fat. If the fit person's food intake remains the same and the amount of time spent exercising doesn't increase to compensate for the decreased rate of caloric expenditure, lower intensity exercise may result in slow but steady weight gain.
References
1. Nieman, DC. Nutrition and physical performance. In: Fitness and Sports Medicine: An Introduction. Palo Alto, CA: Bull Publishing Company; 1990:221-269.
2. Gaesser GA, Rich RG. Effects of high- and low-intensity exercise training on aerobic capacity and blood lipids. Med. Sci. Sports Exerc. 1984; 16:269-274.
3. Bahr R, Sejersted OM. Effect of intensity of exercise on excess postexercise post VO2 consumption. Metabolism. 1991; 40:836-841.
4. Gatorade Sports Science Institute. Commonly asked questions regarding nutrition and exercise: what does the scientific literature suggest? Sports Sci. Exch. Roundtable. 1992; 3:2.
5. Centers for Disease Control and Prevention and American College of Sports Medicine. Summary Statement: Workshop on Physical Activity and Public Health. Sports Med. Bull. 1993; 28:7.
6. American College of Sports Medicine. The recommended quality and quantity of exercise for developing and maintaining fitness in healthy adults. Med. Sci. Sports Exerc. 1990; 22:265-274.
It is well known that fat makes its greatest contribution as an energy source during low to moderate intensity exercise. During exercise at 40 to 60% of VO2Max, fat breakdown supplies about half of the energy needs (1). This has led to recommendations that individuals who want to burn more fat to lose weight should exercise at a lower intensity.
The concept of "fat burning" versus "carbohydrate burning" exercises is a common misconception shared by many aerobics instructors and clients in the weight loss arena. They believe that individuals who want to lose body fat should exercise at a lower intensity since fat contributes more to the metabolic mixture. Unfortunately, this assumption misses the point -- regular exercise is beneficial for weight loss because it creates a prolonged caloric deficit.
The fuel being burned to create this caloric deficit (fat or carbohydrate) is probably secondary. Currently, there is no research suggesting that the selective use of fat as fuel will translate into greater fat loss compared to an equal caloric deficit created by "carbohydrate burning" exercise.
For example, it is doubtful that a runner would lose more body fat by jogging five miles slowly than by running five miles at race pace. Although fat contributes more calories during jogging than when racing, both activities have the same caloric cost and would be expected to have the same effect on body fat loss.
Proof of this came about 10 years ago, when Gaesser and Rich studied two groups of college-age men who exercised on cycle ergometers three days per week for 18 weeks (2). One group exercised for 50 minutes at about 45% of VO2Max and the other group for 25 minutes at about 85% of VO2Max.
The total energy expenditure was 300-350 calories per session, and most importantly, it was nearly identical for both groups. The low intensity group burned more fat during each of the training sessions, but after 18 weeks of training both groups had lost equivalent amounts of body fat (about 3 pounds) and both had reduced their percent body fat by the same amount.
What explains these contradictory results? How can an individual lose body fat via exercise when little fat is burned during the exercise period? The answer lies in what occurs after the exercise is completed. The more carbohydrate burned during exercise, the more fat burned after exercise. This makes sense: when there is not much carbohydrate to burn, fat oxidation will naturally be accelerated.
Bahr and Sejersted observed this process in six men who exercised for 80 minutes at three different intensities -- 29%, 50%, and 75% of V02Max (3). The higher the exercise intensity, the lower the respiratory quotient dropped after exercise, indicating greater fat utilization. The higher exercise intensity also caused the respiratory quotient to stay below normal resting levels for a greater time period -- up to 14 hours longer!
Wilmore points out that although a greater percentage of fat may be burned with low intensity exercise, the total grams of fat burned is actually greater with high intensity exercise because the total energy output is higher during intense activity lasting when the time spent exercising is the same (4). Wilmore also notes that the substrate burned during exercise doesn't matter when the goal is to lose body fat.
A more correct explanation is that low intensity, steady state aerobic exercise uses a greater percentage of fat than high intensity exercise, but these fat calories (and carbohydrate calories) are being burned at a relatively slow rate (e.g. four to five calories per minute.) By comparison, high intensity exercise uses a smaller percentage of fat, but this smaller percentage (along with carbohydrate) is being burned at a much higher rate (e.g. 10 to 13 calories per minute.) Thus, as Wilmore points out, the total amount of fat burned is greater during high intensity exercise (70 to 75% of V02Max) compared to a similar duration of low-moderate intensity exercise (40-60% of VO2Max).
Many aerobic instructors have confused the proportion of fat used as fuel with the more important rate of fuel utilization, which is a key concept in exercise induced body fat loss.
The bottom line is that when the goal of an exercise program is to lose weight, the exercise should create a caloric deficit. To lose a pound, an individual must expend 3,500 calories, whether those calories come from fat or carbohydrate.
It should be noted that low to moderate intensity exercise is recommended for overweight people trying to lose body fat who are new to exercise. High intensity exercise is associated with increased injuries and poor adherence in this population.
The American College of Sports Medicine and the Centers for Disease Control recommend that individuals accumulate 30 minutes or more of moderate intensity activity over the course of most of the days per week (5). This amount of activity should provide enough of a caloric expenditure to promote body fat loss. The goal is to achieve a weekly caloric expenditure of at least 1,000 calories, since this seems to represent a threshold for body fat loss (6).
Individuals with a low level of fitness should exercise at a lower exercise intensity. The only drawback of low intensity exercise is that the person must exercise longer to achieve a significant caloric deficit. Otherwise, a low intensity workout that expends 300 calories in an hour is just as beneficial as high intensity exercise that expends 300 calories in 30 minutes.
Keep in mind that the decreased number of calories burned during low intensity exercise may be detrimental for fit people trying to lose body fat. If the fit person's food intake remains the same and the amount of time spent exercising doesn't increase to compensate for the decreased rate of caloric expenditure, lower intensity exercise may result in slow but steady weight gain.
References
1. Nieman, DC. Nutrition and physical performance. In: Fitness and Sports Medicine: An Introduction. Palo Alto, CA: Bull Publishing Company; 1990:221-269.
2. Gaesser GA, Rich RG. Effects of high- and low-intensity exercise training on aerobic capacity and blood lipids. Med. Sci. Sports Exerc. 1984; 16:269-274.
3. Bahr R, Sejersted OM. Effect of intensity of exercise on excess postexercise post VO2 consumption. Metabolism. 1991; 40:836-841.
4. Gatorade Sports Science Institute. Commonly asked questions regarding nutrition and exercise: what does the scientific literature suggest? Sports Sci. Exch. Roundtable. 1992; 3:2.
5. Centers for Disease Control and Prevention and American College of Sports Medicine. Summary Statement: Workshop on Physical Activity and Public Health. Sports Med. Bull. 1993; 28:7.
6. American College of Sports Medicine. The recommended quality and quantity of exercise for developing and maintaining fitness in healthy adults. Med. Sci. Sports Exerc. 1990; 22:265-274.
