Ask almost any trainer and they’ll tell you that to burn the most amount of fat during exercise you need to perform long duration, moderate intensity activities, somewhere between 45 and 60 minutes at approximately 60 - 70% of your maximal heart rate (MHR).
Indeed research shows that the highest rate of fat burning does occur between approximately 60 - 70% of ones MHR and that higher intensities depend to a greater extent on carbohydrate for fuel. Furthermore, as the duration of the exercise increases, not only does the number calories burned increase, so to does the dependency upon fat as a fuel source. In light of these observations, it’s easy to see how the infamous “Fat Burning Zone” was born. While certainly not incorrect, prescribing exercise based upon this information is a bit misleading.
To do so assumes that the number of calories expended during exercise and the source of those calories is vital to improving long term body composition. Current research, however suggests that this may not be the case.
In comparing a 20 week “endurance” training protocol with a 15 week high intensity intermittent training program ( i.e. interval training), studies have found that, even though the endurance trained group expended more than twice as many calories ( 28,800 vs 13,800) over the course of the research, the high intensity interval training group lost significantly more body fat. Yes, you read that correctly.
Cycling uninterrupted for 30 - 40 minutes 4 or 5 times a week resulted in LESS body fat lost than performing 6 - 10 short (20 - 90 seconds), all out sprints. At first glance, this appears to contradict perhaps the most widely accepted principle of exercise prescription, yet subsequent research has supported these original findings.
So the question is, how does brief, high intensity exercise, which burns predominantly carbohydrate for fuel, lead to greater long term fat loss? The answer lies in what happens after you stop exercising.
Quite understandably the earlier studies (upon which the “fat burning zone” is based) examined only the exercise period itself and failed to take into account the number of calories expended in the post exercise period which, depending on the severity of the exercise bout, can be quite substantial. Returning the body to it’s pre-exercise state (i.e. replenishing energy stores, regulating body temperature, etc.) requires energy and as you might now quess, the substrate used to fuel these processes comes from pre- existing fat stores.
Intense activities (>80% MHR) such as interval training and resistance training can result in higher than normal fat burning that lasts long after exercise has ended. In fact, resistance training alone has been shown to raise metabolism by 20% for up to 48 hours! Performed 3 or 4 times a week, this “after burn” can add up to tens of thousands of calories over the course of a year.
Even if we conservatively estimate the post exercise expenditure to be in the neighborhood of 150 calories, exercising in this fashion 3 days per week for one year would result in over 23,000 fat burning calories expended above and beyond those burned while exercising. That is the equivalent of nearly 3.5 kilograms of body fat!
In contrast, it appears that the fat burning benefits of low to moderate intensity activities (<70% MHR) cease almost as soon as you stop working out. If you’re lucky you might expend an additional 10 - 20 calories in the post exercise period which would add up to a whopping 3000 extra calories or so over the course of a year, less than that contained in a single kilogram of body fat. It is for this reason that men and women who engage in low to moderate intensity aerobic exercise without performing resistance training (or make changes to their diet) typically experience extremely slow fat loss. To make matters worse, unless you are extremely unfit, this type of training will do little to enhance your performance in the intermittent, high intensity environment of most sports.
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