Common Fitness Misconception #2: The myth of the "fat-burning zone"

Happy Saturday to you all! Any fun plans in the works?!

I'm just gonna dive right in here and assume that most of you readers have heard a thing or two about something called the "fat burning zone" in the context of traditional cardio exercise. According to the lore, during low-intensity cardiovascular activity our bodies switch into fat-burning machines. The idea, as it has been promoted, is that we should spend the majority of our workout in this zone, bookended by less intense "warmup" and "cool down" periods.

What do YOU think?!

Let me get one thing out of the way before we delve any further into this. If you're a man reading this post, 1) kudos to you, first of all; and 2) you've likely never even heard of the term that's spawned the myth I am attempting to debunk! Why do you think that is?

I have a pretty darn good theory: it's because the term is associated with traditional cardio, aka the singular type of exercise that has been marketed over the past 40-50 years as "acceptable" for women to do. Things are certainly in the process of changing, thanks to paradigm-shifters like CrossFit, but much remains the same: women are relegated to the treadmills, ellipticals, recumbent bikes, etc. while men happily sidle from stationary machine to stationary machine and beat their chests like King Kong in the free weight room.

#Lunks

This might be an over-exaggeration, but I think a lot of women can relate. For many women, the weight room remains elusive and intimidating. The media doesn't help much, as women are constantly being smacked over the head with messages about how cardio is the best fat-loss tool because it burns the most calories, while helping you achieve that "long, lean, and toned" look. The best exercise for achieving such a look (which, let's face it, is rather amorphous) is a subject for another post, so for now, let's unpack where this notion of the "fat-burning zone" came from.

Don't listen to this machine!

To really understand the genesis of this myth, we must consult science--bioenergetics, specifically. Also called exercise metabolism, bioenergetics "is the study of energy in the human body" (Clark, Sutton, & Lucett, 2014, pg. 70). Our bodies capture energy from different pathways, and utilize that energy from different sources of fuel depending on the type of exercise we are engaging in. Think about the physical difference between stepping onto a treadmill belt set at 9.0 mph for an all-out sprint, and stepping on at 3.5 mph and gradually increasing the speed over time. Depending on the demand placed on our bodies by our exercise of choice, the number of calories we burn from carbohydrate and fat changes. In other words--at the risk of oversimplifying a complex metabolic process--our exercise choices can either make us more dependent on carbohydrates for calories, or fat-burning machines. 

Pretty wild, eh?!

Much of what our bodies ultimately burn as fuel depends on our diets, but exercise has a large impact. Ironically enough, though, that "fat-burning zone" is not the best way to burn fat. There is a formula in bioenergetics called the respiratory quotient, or RQ. This is "the amount of carbon dioxide expired by the amount of oxygen consumed, measured during rest or at a steady state of exercise." A common use of the RQ has been misinterpreted countless times by fitness equipment marketing departments in order to sell people (read: women) more cardiovascular exercise equipment. The long-held belief states that people burn more calories from fat at lower-intensity exercise because such easy work does not require getting energy quickly from carbohydrates (Clark, Sutton, & Lucett, 2014, pgs. 79-80).

To illustrate why this logic is flawed, let's compare the percentage of calories burned from fat by someone engaging in 20 minutes of treadmill walking at 3.0 mph, and the percentage when that same individual doubles the intensity, within the same time frame, to 6.0 mph. Using the RQ formula, the individual burns 67% of energy from fats and 33% of energy from carbohydrates walking leisurely. At this pace (dependent on age and weight, of course), the individual burns 4.8 calories/minute, so that's 3.2 calories from fat and 1.6 calories from carbohydrate. The total calories burned is roughly 96, with 64 calories coming from the metabolism of fat and only 32 from he metabolism of carbohydrates (Clark, Sutton, & Lucett, 2014, pg. 80).

By increasing the treadmill speed to 6.0 mph, the demand for energy from carbohydrates increases. After 20 minutes of intense activity, 54% of the energy to fuel the exercise will have come from carbohydrates and 46% from fat. Regardless of the increased dependence on carbohydrates for fuel, the individual burned approximately 9.75 calories per minute, more than 50% more per minute than during low-intensity exercise. Thus, at an increased intensity, the same individual burned approximately 5.2 and 4.48 calories per minute from carbohydrates and fat respectively, resulting in a total caloric expenditure of calories from fat to 90, roughly 50% more than during low-intensity exercise. This demonstrates the fallacy of the "fat-burning zone;" despite the increased reliance on carbohydrates for fuel, at a higher intensity, the caloric expenditure from fat increases (Clark, Sutton, & Lucett, 2014, pg. 80).

It both baffles and saddens me that the myth of the "fat-burning zone" has persisted for as long as it has. It's amazing how exercise equipment marketers have manipulated scientific data to help them sell their products and convince consumers that low-intensity cardio exercise is the fat-loss solution. As a former cardio queen, I can testify to the fact that it wasn't until I cut back on cardio and upped the resistance training that I finally saw results. And by results, I don't mean fewer pounds on the scale: I mean I gained lean muscle, dropped fat, and went down at least three dress sizes! So while I don't think that there's a "one-size-fits-all" solution to achieving the most desirable physique, I believe that women and men alike can certainly benefit from additional resistance training and higher-intensity, lower volume cardio exercise. More importantly, we can all benefit equally from not blindly following what we read, hear, or see from fitness marketers on late-night infomercials! When it comes to exercise, do what you love but make sure it's serving you to help you achieve your goals.

Personally, I'd rather look like this...and this physique was not built with
cardio!