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By Mike Clark Mike Clark is an Exercise Physiologist and Personal Trainer at the Duke Center for Living at Fearrington. He received his BA in Exercise and Sports Science from UNC.

Getting In The Zone

By Mike Clark Posted June 12, 2013 at 11:49 am

Many treadmills, elliptical trainers, and bikes employ the notion that different exercise training intensities, like exercising at lower heart rates can elicit superior “fat burning,” or exercising at higher heart rates will elicit superior “cardiovascular training.” But, given the vast amount of exercise information, or misinformation available, you probably have some skepticism over claims like this. So…is it true? The answer is undoubtedly yes. Exercising at higher and lower heart rates does in fact prioritize the usage of fats over carbohydrates, or vice versa, depending on how high the heart rate goes. But, please read on to understand why it really isn’t that important to emphasize the infamous fat burning zone.

One of the first lessons in exercise physiology comes from bioenergetics, which describes how the body utilizes fat, carbohydrate, and protein as fuel for movement and organ function. To sum it all up quickly, the body relies mostly on fat at rest. As exercise intensity increases, or the heart rate goes higher and you begin to breathe harder, the body begins to move away from utilizing fat as the main source of fuel, and relies mostly on carbohydrates, or sugar, in the blood and stored in muscle. It is important to mention, however, that some fat, some carbohydrate, and even some protein are always being used for energy, regardless of rest or exercise status.

One question you may ask is “how can they tell whether fat or carbohydrate is being used during rest and exercise?” For one, exercise physiologists have long known that oxygen consumption increases as the heart rate, or exercise intensity increases. Fat, a great source of energy at rest and low intensity exercise, is just unable to yield energy fast enough for strenuous activities, such as jogging. So as oxygen demands increase with exercise intensity, the body is forced to use greater percentages of carbohydrate, which yields energy much quicker than fat. So if there is a way to measure how much oxygen is consumed during exercise, there is a way to measure what fuel sources contribute to the exercise. This can be done by measuring the amount of oxygen inhaled and the amount of carbon dioxide exhaled during rest or exercise. Data like this is collected in a lab with special equipment that participants breathe into while resting and exercising. The measure used to calculate how much fat and carbohydrate is used for energy is called the Respiratory Exchange Ratio (RER).

carbs mike clarkThe normal range for the RER is somewhere between 0.7, which is when the body is consuming 100% fat, and 1, which is when the body is consuming 100% carbohydrate. If the RER is .85, then equal amounts of carbohydrate and fat are being used for energy production. Interestingly, the RER is about 0.7 at rest, meaning the body is consuming mostly stored fat. As exercise intensity increases, the RER climbs, eventually reaching 1 when a maximal intensity is achieved (i.e. as fast can be maintained on a treadmill).

So the theory behind the fat burn and cardiovascular training zones may be true, but if you want to emphasize burning mostly fat and very little carbohydrate, resting may be your best bet. However, the idea that you can burn mostly fat and do a low intensity workout is a great way to motivate individuals to exercise, or a great way to sell exercise equipment. Let’s try to put this in perspective with some numbers. During a very light 30 minute walk you may burn 100 calories with 80% coming from fat, the rest coming from carbohydrates. So 80 calories came from fat, and 20 calories came from carbohydrates. Let’s say that during a more intense 30 minute exercise session you are able to burn 200 calories, with only 40% coming from fat, 120 calories from carbohydrates. If this were the case, you still burn 80 calories from fat, plus 120 extra calories.

The truth is this, if weight loss is your goal, burning more calories than you eat is the most important thing to remember. So it really doesn’t matter if the calories come from fat or from carbohydrates, just that we create a deficit in calories at the end of the day. You can do this with any decent diet and exercise plan, regardless of the exercise intensity. However, working at higher intensities does make the heart muscle stronger, so pushing the pace can help you lose weight, achieve and maintain a high quality of life, and even live longer.

Feature image by US Army Korea via flickr

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