I recently wanted to access some extra cardio equipment on my off days, so I dusted off my membership card and started making weekly appearances at the local “commercial gym”. Last week, during my Stairmaster date, I overheard a conversation between two people on some cardio equipment. The conversation went something like this:
Girl: The fat burning zone is best for losing fat, right?
Guy: All the machines say “fat burning zone”, so it must be right.
That moment inspired me write this article. I want us to all be able to move past the fallacy of the fat burning zone, once and for all. The fallacy of the fat burning zone claims that the fat burning zone is optimal for burning/losing fat. This whole matter isn't helped by the fact that those fancy cardio machines come with color-coded zone displays that can be very misleading. Just in case you’re not sure what I’m talking about, the zones displayed on cardio machines look something like this:
Fat Burning Zone: 50-70% of your max heart rate
Aerobic Zone: 70-80% of your max heart rate
Anaerobic Zone: 80-90% of your max heart rate
VO2Max: 90-100% of your max heart rate
Note: Max heart rate can be estimated by subtracting your age from the number 220.
So how did they come up with the fat burning zone and what actually is it? When you look at the breakdown of the zones, it appears that the fat burning zone is an intensity in which you train at the easiest intensity (50-70% max heart rate), but get to burn the most fat. Sounds like a pretty good deal, right? It’s almost seems like a magical zone, where you work out with less effort, and get more jacked!
Well, to better understand this whole fat burning zone, and how effective it is for fat loss, we need to backtrack to the basics of fuel utilization and metabolism.
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Respiratory Exchange Ratio (RER) is a ratio of the amount of carbon dioxide produced to oxygen consumed1. This ratio becomes closer to one with increasing exercise intensity, since your levels of consumed oxygen become closer to the levels of carbon dioxide produced. This ratio can also be used to determine what kind of fuel you are metabolizing or using during exercise. Fancy equations were originally used to figure this out, but in short, an RER of 1.0 is indicative of using carbs as a fuel source, whereas, an RER closer to 0.7 is indicative of using fat for fuel1.
Now, what does this have to do with exercise intensity? As mentioned above, increasing exercise intensity shifts the ratio closer to one, indicating that intensity plays a huge role in determining which type of energy your body will select to predominantly fuel your actions.
When you exercise, you need energy. How quickly you need that energy (intensity of exercise), and how much energy you need (volume, per say), dictates which energy system your body relies on when executing an activity. Thus, the decision your body makes to use fat to fuel your exercise session is greatly dependent on the type and intensity of exercise you do.
Energy Systems During Exercise
To better understand how exercise intensity dictates fuel use, and what this means in respect to the fat burning zone, we need to review the energy systems your body can rely on during exercise.
ATP-CP (fewer than 10 seconds): Your body uses ATP-CP for very short, all-out, intense activities. Carbohydrates, fat, and oxygen are not used in this process to make energy (ATP). Instead, ATP is produced when creatine phosphate donates an ADP to make ATP2. Although it is the fastest way to make energy, the amount of creatine phosphate in your muscles is limited, and it must be resynthesized before you can repeat all out actions. That’s why very intense activities occur for the shortest amount of time, but take the longest amount of time to recover.
Glycolysis/Glycolytic System (30 seconds to 2 minutes): This energy system helps fuel high intensity activities using carbohydrates stored in your muscle as glycogen or running through your blood as glucose. Keep in mind that glycogen gets broken down to glucose. Glucose, after getting broken down to pyruvate (via glycolysis), yields two ATP molecules3. When the oxygen consumption is higher than the demand (anaerobic activities), pyruvate is then converted lactate. This leads to an increase in hydrogen ions within your muscles, which decrease muscle pH, and results in decreased muscle force and increased muscle fatugue4.
Aerobic System (greater than 2 minutes): This oxygen-dependent energy system uses fat, glucose, and glycogen to resynthesize ATP3. It is used when the oxygen demand is sufficient and greater than consumption (aerobic activities), and results in the breakdown of pyruvate into acetyl-CoA. By breaking down pyruvate into acetyl-CoA, instead of lactate, even more energy can be produced. When complete oxidation of glucose occurs, 26 molecules of ATP are produced via oxidative phosphorylation per molecule of glucose broken down. This process takes a while, but the ATP yield is clearly much higher than the aforementioned energy systems. When fat, instead of glucose, is used to make energy, the process takes even longer. However, the energy yield is the highest. In this process of using fat for fuel, triglycerides are first broken down into glycerol and free fatty acids. The carbons that make up the free fatty acids are then used to produce acetyl-CoA through a process known as beta-oxidation. One 16-carbon fatty acid can yield approximately 129 ATP molecules when completely oxidized!
This is why low intensity exercises are in the “fat burning zone”. At that low intensity, your body knows that energy isn’t needed super fast, and thus preferentially oxidizes fat for fuel. And why wouldn't it? Fat is a nearly endless fuel supply that will help keep you going for quite a long time. As intensity increases from low to high, the percent of calories metabolized as fat declines, and the number metabolized as carbs increases. This is a bit misleading, though, as the total number of fat calories burned can also increase because the total number of calories burned counteracts the decline in the percent of fat calories burned5,6. As you can see, the fat burning zone is directly referring to the zone in which your body, during low intensity exercise, relies on fat for fuel.
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To Sum It Up
If you take the concept of the “fat burning zone” at face value, you would think that if you work out at the easiest intensity, you burn the most fat. And, as we just learned, that is “kind of” true. In the supposed fat burning zone, your body relies on fat oxidation to fuel your activities. However, as intensity increases, your body needs energy faster, and you burn more carbs for fuel.
So, that “fat burning zone “is really a zone where you can burn the maximal number of fat calories per minute. This doesn't mean, however, that training at a low intensity is optimal for creating a caloric deficit that is ideal for weight loss. And just because you are training in the fat burning zone does not mean you will shed pounds of fat.
In fact, high intensity exercise might be a better option for fat loss, since it can create a higher caloric deficit during the activity. Additionally, recovery from high intensity exercise requires a greater recovery caloric expenditure due to the additional energy requirements from elevated body temperature, ventilation, and replenishment of glycogen and ATP stores6. In summary, the fat burning zone is just a zone where your body is maximally using fat as a fuel source. To drop fat, you need to not just consider the fuel source you are burning mid-activity, but also the caloric deficit that the activity created. Think of it like this: if training in the fat burning zone was really ideal for dropping fat, then you might think there would be fewer jacked sprinters and a lot more jacked marathoners running around.
References
- Lusk, G. The elements of the science of nutrition, (W.B. Saunders, Philadelphia,, 1923).
- Robergs, R.A. & Roberts, S. Exercise physiology : exercise, performance, and clinical applications, (Mosby, St. Louis, 1997).
- Brooks, G.A. Exercise physiology : human bioenergetics and its applications, (Mayfield Pub., Mountain View, Calif., 2000).
- McLester, J.R., Jr. Muscle contraction and fatigue. The role of adenosine 5'-diphosphate and inorganic phosphate. Sports medicine 23, 287-305 (1997).
- Wolfe, R.R. Fat metabolism in exercise. Advances in experimental medicine and biology 441, 147-156 (1998).
- Carey, D.G. Quantifying differences in the "fat burning" zone and the aerobic zone: implications for training. Journal of strength and conditioning research / National Strength & Conditioning Association 23, 2090-2095 (2009).
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