Most trainers have addressed this misconception among clients hundreds of times—“Why am I not losing fat when I am working in the “fat burning zone”? This machine’s programs includes a “fat burning program,” isn’t that the best one for weight loss?”
In short, no. It is time to elucidate this issue once and for all: the descriptor “fat burning zone” is essentially a successful marketing tactic. And the science behind the use of these terms is relative and misleading when it comes to body change.
Truth or trick?
The terms “fat burning zone” and “cardio zone” simply refer to the intensity of the workout. It is this intensity that dictates which fuel the body preferably uses to power exercise. It is important to understand that we are always using both fat and carbohydrate calories to power activity, whether we are sprinting down the street, walking down the street or sitting in the street. It is the relative ratio of sugar and fat use that changes as intensity shifts. Relatively more fat calories are used compared to carbohydrate calories at lower intensities, while the proportion of recruited carbohydrates builds as intensity increases.
For example, sitting might burn 4 calories per minute total (1 from sugar and 3 from fat). Walking might burn 8 calories per minute (3 from sugar and 5 from fat). Sprinting might burn 20 calories per minute (12 from sugar and 8 from fat). Thus, the ratio of fuel shifts towards a more carb-heavy use, but the total amount of calories burned also increases. This is a key distinction because although it is true that at lower intensities a larger relative ratio of fat is used to power activity, the absolute amounts of both carbohydrate and fat calories are much less.
So, what is more important to body change—total calorie burn or greater ratio of fat calories compared to carbohydrate calories, even if this number remains low? If the ratio were most important, as we are led to believe by the descriptor “fat burning zone”, than simply lying down would prove the most effective activity for body change considering this is when the fat to carbohydrate ratio is the greatest. And of course lying down is not the optimal way to generate body change.
Intensity affects post-workout fat loss
In order to take this argument one step further, let’s examine the science of energy substrates not only during exercise, but also after. Carbohydrates are like high-octane jet fuel for your workouts—they allow you to push to higher intensities whether you are doing sprint intervals or strength training. Fat is a lower-quality fuel that allows you to go for longer durations at lower intensities like when distance jogging or a leisurely bike ride. Whichever fuel the body recruits for exercise, depending on intensity, will be somewhat depleted during exercise, forcing the alternate substrate to be used during post-exercise recovery mechanisms.
For example, performing a high-intensity interval training (HIIT) cardio workout will recruit a larger proportion of burned calories from sugar. However, the workout lasts only 30 minutes and is very intense. After the workout is over, stored glycogen (sugar) levels are somewhat depleted, forcing the body is to pull from fat stores for energy use during post-workout recovery.
Recovery lasts hours and even days after a tough workout. Wouldn’t you rather burn fat for days post-workout than during the 30 minutes of exercise only?
Conversely, a low intensity 60-minute workout like jogging will keep an individual in the “fat burning zone” so that a higher ratio of fat calories are burned during exercise compared to sugar calories. Post-workout, however, glycogen stores still exist and those carbs will be used as energy post-workout preferably instead of fat. Thus, it follows that it can be beneficial to deplete glycogen during exercise assuring the 24-48 hour window of post-exercise recovery pulls from fat stores. This scenario only occurs when performing more intense workouts in the “cardio zone” or even breaching the lactate threshold.
Endurance training expert and author of “Beat the Gym” (William Morrow, April 2011) Tom Holland agrees. “Current research indicates that interval training, specifically doing short, hard bursts of cardio followed by rest periods, not only burns a significant amount of calories during the workout, but you continue to burn calories even after your workout is finished,” which is ultimately the key to body change.
Bottom line for fat loss
It is important to keep in mind the goal of the workout. If you are a ultra-marathoner and need to train your body to run for hours and hours without stopping, it is probably necessary to perform the bulk of your training in the fat burning zone, assuring that you can endure those miles.
However, if the goal is fat loss or improving speed during endurance exercise, it is necessary to train in the cardio zone (and beyond) for increased fat loss post-workout, as well as improved lactate threshold and VO2 max.
Holland concurs, “have you ever seen a fat sprinter? Who is skinnier, the group of people at the front of a marathon or the group at the back?” It holds true that intensity always trumps duration when it comes to improving body composition.
Endurance athletes who want to improve their times can benefit from dropping body fat as well as getting more cardiovascularly fit. For a well-rounded program, Holland recommends doing steady-state cardio as well as interval training to keep the body and mind working optimally.