Q: I want to lose fat but with all the different information in magazines, I'm confused when it's time to choose the intensity that I should train at. High Intensity or Low intensity? Can you help me understand?

A: It has been well established that exercise plays an important role in the regulation of body fat accumulation However, there is little agreement as to the how much exercise is needed and how hard the exercise should be. The answer below will demonstrate to you that the exercise intensity for fat loss should not be based on the concept of high or low intensity exercise but on the unique physiology of the individual. Once understood, this concept will help anyone who wants to lose fat, regardless of the amount of fat to lose.

Low Intensity Aerobic Exercise
The amount of fat that is used during an exercise session can be estimated in a lab setting using a measure called the respiratory exchange ratio (RER). A low RER means that more fat is being used. A high RER means more carbohydrate is being used. As the intensity of an exercise session increases the amount of fat that is used as an energy source decreases In exercise of low intensity (50% of VO2max) the oxidation of fat can account for as much as 90% of the energy production . While high intensity exercise may result in a greater number of calories used, the proportion of calories derived from fat will be very small. A study on untrained subjects conducted by Chad and Quigley (1991) found that exercise at 50% of VO2 max resulted in approximately twice the amount of fat being metabolized as exercise at 70% VO2 max. The use of fat as a fuel source during exercise is necessary for efficient and effective fat loss.

High Intensity Aerobic Exercise
High intensity exercise is advocated by some groups as a means of altering body composition because number of calories burned during higher intensity exercise is greater than that burned during low intensity exercise. There has also been some research on post exercise metabolic rate that indicates that higher intensity exercise cause a higher post exercise metabolic rate that remains elevated longer than with lower intensity exercise (Schuenke, Mikat, and McBride, 2002). While high intensity exercise may cause a significant increase in post exercise fat use it is a statistical, not a physiologically significant increase. Binzen, Swan and Manore (2001) found the greatest increase in post exercise metabolism to occur during the first 30 minutes, increasing 18.6%. This may sound like a large increase but if we accept the average resting metabolism to be about 1.5 kcal/min an 18.6% increase results in only an extra 0.28 calories per minute consumed or 16.8 kcal hour beyond resting metabolism, at this rate it would take about 200 hours to lose a pound of fat. High intensity exercise at or above anaerobic threshold, characterized by an increased reliance of carbohydrates as a fuel source, has several drawbacks when exercising for fat loss. First, even though a greater number of calories may be burned during a given time period there is very little use of stored fat as an energy source during the exercise session. According to Flatt (1988) exercise which causes a depletion of carbohydrate stores in the body will stimulate the intake of more food, which is often high fat food, in an attempt to replenish the stores. This process in effect negates any energy imbalances created by the exercise session. It can be very difficult, particularly in untrained people, to maintain high intensity exercise for long periods of time due to carbohydrate depletion (Saltin and Astrand, 1993).

It is important to note that as an individual becomes more fit they also burn fat more efficiently at higher intensities. In fact, some elite endurance athletes burn fat most efficiently at 80% VO2 max or above. This means that exercise intensity for fat loss should not be based on the concept of high or low intensity exercise but on the unique physiology of the individual.

Steve Prud'Homme
Fitness Director, Peak Centre Director


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