The Truth About the “Fat Burning Zone”
By now we are all familiar with the “fat burning zone” theory… that wondrous aerobic training ideal that rapidly took on an almost mythical like status for its alleged capacity to melt fat off the frames of its disciples. According to the adage, if fat loss is our goal it is within this relatively low intensity range of 50-70% maximum heart rate (MHR) that the majority of our exercise minutes should be spent. Hence the origin of the long, slow, steady state aerobic zone workouts that now don the screens of cardio machines the world over and continue to pervade most weight loss programs today.
More recently however the fitness industry has undergone a paradigm shift toward higher intensity exercise modalities like HITT (high intensity interval training) in order to meet those ever-elusive weight loss goals. According to HIIT proponents, training at higher intensities, or more simply – working harder, is the most effective means to this end. So, what’s the deal? Should we be grinding out those long training runs? Endless hours on the elliptical? Is that the most efficient means to the fat/weight loss end? Or is HIIT where its at? Heck, is the “fat burning zone” even real?” Well…
The truth is…. (insert climactic drumroll here)… Yes, the “fat burning zone'“ is 100% real! During lower intensity cardio workouts, caloric expenditure rises, yet fat remains the primary fuel source. You see, humans draw primarily on 2 fuel sources to meet metabolic demands. At rest, when oxygen is readily available, the body preferentially draws on fats, both dietary and stored body-fat. As oxygen demands increase, such as in exercise, the body experiences a gradual metabolic shift toward a less oxygen dependent fuel source. Enter carbohydrates! This sliding metabolic scale effectively allows us to maintain a high level work output by shifting fuel demands to a less oxygen reliant source. In fact, at approximately %85 MHR, carbohydrates become our sole fuel source. Thus, on one end of the spectrum carbohydrates act as the sole supplier for high intensity exercise, while on the other extreme fat is the “snoop dogg” at rest. Of course, this begs the question, if we burn the most fat at rest, why not just sit on our ass all day? Based on “fat burning zone” logic… Hmmmm… but I digress.
Figure 1 demonstrates the inverse relationship between fats and carbohydrates across the exercise spectrum. At rest fats act as the sole contributor. As exercise intensity increases, fat contribution gradually decreases while carbohydrate input steadily rises. At approximately %85 MHR or the anaerobic threshold, carbs take over – fat no longer contributes to energy demands.
You see, what the “fat burning zone” proponents failed to consider was a) the overall caloric cost of intense exercise and b) the metabolic demands following bouts of high intensity training. Put simply, the higher the exercise intensity, the greater the caloric cost. Period. Beyond that however, the body experiences little metabolic stress during low intensity exercise. As such little work is required to regain metabolic balance. As exercise intensity rises, so too does the metabolic stress on the body. Thus, it takes much longer for the body to normalize. Aptly referred to as afterburn, it is during this post exercise recovery period that the fat burning magic of high intensity exercise is experienced. Throughout this period caloric expenditure remains elevated while basal metabolism returns to resting levels. In fact, according to one study, our metabolism actually drops below normal resting levels! Moreover, it remains lower for up to 14 hours following intense exercise.(1) In other words, post workout fat utilization increases beyond our normal resting point and remains elevated for an extended period of time!
So, what does all this mean? Well…, put simply, as in all areas of life, the harder the work, the greater the reward! Fat loss is no exception! Not only do we experience greater caloric expenditure during high intensity training, energy demands remain elevated for hours to follow; a time when increased caloric demands are easily met by the metabolism of fat. So, while on average we burn more fat during low intensity exercise, the sum cost is notably higher following intense bouts of work. Does this mean we should abandon low intensity aerobic work like, running, walking, hiking… not if it brings you joy! If you are doing it under the guise of transforming your body on the other hand, rest assured there is a better, far more efficient way! More on that… Stay tuned!
Bahr R, Sejersted OM. Effect of intensity of exercise on excess postexercise post VO2 consumption. Metabolism. 1991; 40:836-841.
