Heart Rate Zone: Must Or Bust?

For years there has been a never-ending debate on heart rate zones and which burns the most fat, the most calories, and which you should be doing to reach your specific goals. 

The myth that in order to lose fat, you need to do low intensity cardio because high intensity cardio only burns carbs, is just that, a myth. Your body utilizes both carbs and fats as energy all day, whether you are exercising or not. But if this is the case, then what type of cardio should you be doing? The issue is rather complicated: there is no simple yes or no answer. Everyone has different heart zones, different anaerobic thresholds, and different goals, so how can we say everyone should be doing just low, just moderate, or just high intensity cardio? Let's start by breaking down the heart rate zones and identifying the different things that are happening in each.

There are 5 different zones, zone 1 being the lowest and zone 5 the highest.  Zone 1 has been nicknamed the “fat-burning zone,” where you are working at about 50-60% of your maximum heart rate at a low intensity.  You aren’t burning a large amount of calories per minute, but being at a lower intensity allows your body to work with aerobic energy systems. This means your body has the ability to efficiently utilize the oxygen you are breathing in to break down fat sources to be used as energy. So yes, it is the “fat-burning zone,” but it's not a high calorie burn zone.  Now, as your heart rate increases and it becomes harder for you to breathe in the amount of oxygen needed to break down fat for energy, your body begins to reach into its carbohydrate stores (glycogen) for energy. 

Next is Zone 2, where you're working at 60-70% of you maximum heart rate. Calorie burn per minute has increased a little from zone 1, and your primary energy source remains fat as your body can still utilize oxygen efficiently in those aerobic energy systems to break down fat stores.   

Zone 3 is the cardiovascular training zone, at 70-80% of your max heart rate. With an even higher calorie burn per minute this zone is right before an individual’s anaerobic threshold.  This is a more difficult training zone to maintain and you may feel like it is challenging for you to finish your sentences when speaking. Energy is coming from a more equal mix of fat and carbs because your body can’t utilize oxygen quickly enough to break down the amount of energy you need as fat alone. To compensate, your body is beginning to reach more into its carb stores for a faster-acting energy.  Now, the amount of fat verses carbs your body is burning depends on your fitness level, training program, recovery, diet, stress, and sleep.  I know it sounds like a lot but we'll dig into all that in future posts.

Zone 4 is where you're at 80-90% of your maximum heart rate. You have now crossed your anaerobic threshold and your body is primarily using anaerobic energy systems that don’t require oxygen. Needing energy quickly, your body is breaking down carbs and lactic acid. The oxygen you are breathing is still being used to break down and burn fat, but again it doesn’t happen fast enough, which is why your body is utilizing more carbs and lactic acid.  The key here is to train your body to become a better fat burner by utilizing all the Zones in your training program.

In zone 5, you aren’t dying but you feel like you might be, because you are working at 90-100% of your max heart rate.  Giving it all you have, you are working for short intervals, possibly only a few seconds, depending on your fitness level. This is the same deal as zone 4, but more extreme. Your heart is working hard and your muscles need energy fast so they’re using their carbohydrate stores and lactic acid as a majority of its fuel source, but fat burn can still occur if you are a well-trained individual and your body has become great at utilizing oxygen. Doing interval training such as HIIT in Zones 4 and 5 is what creates the EPOC effect, which allows your body to have an increased metabolic rate, continuing to burn more calories at an accelerated rate after you have finished your workout.

So what does this all mean? At zone 1, the calories burned are primarily fat and you’re using the largest percentage of fat compared to the other zones, but you are burning the least amount of total calories. At Zone 3 you are burning more total calories, which is a more equally divided mix of carbs and fat being burned, but because you are burning a greater total amount of calories, you may be burning more fat overall. Now training in Zone 5 is exhausting so it will not last long, but it does burn the most calories per minute, and keeps your metabolism up, burning more calories afterwards too. 

So which zone should you be working in? The answer is, all of them. They each have a purpose in a well-rounded training program and in general good health and fitness. Even though you burn fewer calories per minute in zone 1 and 2, these are crucial to train your body to become better fat Burner. By training in zones 1 and 2, you teach your body to favor fat burn, and use more fat as an energy daily as well as while exercising.  Zones 3, 4, and 5 are just as important because these produce a higher caloric burn then zones 1 and 2.  Overall, if you want to lose weight, the goal is to increase your total calorie burn throughout the day.  If you burn more calories than you eat, you’ll lose weight, right?  By training in all the zones, though, your body will be able to adapt and recover more easily, and you’ll have time to recover from sessions of high intensity training, and then hit harder the next time. Not to mention if you do long bouts of HIIT multiple times a week, this could potentially cause too much inflammation and stress in your body, making it more difficult to burn fat. At the end of the day it is possible to burn fat in each zone, and each zone serves a purpose and can be effective.






by Alyssa Reyes

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