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Exercise Myths and Facts

by Richard Weil, MEd, CDE

Are you confused by all the exercise advice out there? It’s no wonder: With a dozen fitness magazines on the newsstand, a wealth of health and fitness news streaming into your home over the Internet, bogus ads guaranteeing an effortless 40-pound weight loss or bigger muscles in just 10 days, not to mention the free advice from well-intentioned friends, trainers, and the guy on the bench press next to you, there’s a lot of conflicting information to sort through. Unfortunately, many popular fitness tips not only make exercise seem harder and more complicated than it really should be, but they can also lead to injury. To set the record straight and help you exercise safely, here are the facts — nothing but the facts — behind some of the most common exercise myths.

You don’t start burning fat until 20 minutes into your workout.
This is one of the most popular myths of all time. You may have heard it from a friend or even from a fitness trainer at your gym, but the fact is, muscle burns a combination of fat and carbohydrate (glucose) simultaneously almost all of the time. It’s just that you may burn a higher percentage of one or the other depending on the intensity of the exercise. For example, during high-intensity activities like sprinting or strenuous weight lifting, which get you out of breath, your muscles are burning a higher percentage of carbohydrate than fat (perhaps as much as 80% to 90% carbohydrate and 10% fat). At rest and during light-intensity physical activity (such as moderate-paced walking), when breathing is easier, the percentage could change to 70% fat and 30% carbohydrate. Why does this happen?

As you start to exercise, fat and carbohydrate are released from storage sites in the body as well as from the bloodstream and enter the working muscles. (Protein is not a fuel for exercise unless your body is in a starvation crisis.) Oxygen, transported from the lungs to the muscle, burns these fuels to generate energy. Fat contains more than twice the amount of energy per gram as carbohydrate (fat contains 9 calories per gram while carbohydrate has only 4). Your muscles would prefer to burn fat, because it’s so energy-dense, but the catch is, you must provide adequate oxygen to the muscle to burn it. Since fat contains more calories than carbohydrate, it also takes more oxygen to burn. When you’re really out of breath — during a sprint, for example — there’s not enough time for oxygen to travel from the lungs to the muscles, and so your muscles, low on oxygen, have no choice but to burn the less dense fuel, carbohydrate. As a general rule, unless you are performing brief, very intense exercise, there’s always enough oxygen in the muscles to burn some fat as well as carbohydrate. Which leads us to our next myth…

The “fat-burning” option on the exercise machine at the gym is better for weight loss than the “cardio” option.
This isn’t only a myth; it’s really bad advice. Basically, you want to burn as many calories as possible when you exercise, whether you’re trying to lose weight, increase your stamina, control your diabetes, or improve your general health. Caloric expenditure during aerobic activity is directly related to the distance you travel and how hard you work. When you select the fat-burning mode on an aerobic exercise machine, you in effect minimize the number of calories you burn, because the machine — let’s say a treadmill — keeps the speed slow and the elevation low. On the “cardio” option, the speed and elevation are set higher, so that for an equivalent amount of workout time, you end up traveling farther, working harder — and burning more calories.

The reason the lower level is labeled the “fat-burning” option is that, as described above, at lower intensities you breathe more comfortably, so you deliver more oxygen to the muscles and burn a higher percentage of fat than carbohydrate. This part of the equation is accurate, but the truth is that although you may burn a higher percentage of fat at the slower speed and intensity, you end up burning less total fat. For example, if you weigh 150 pounds and walk on the treadmill for 30 minutes at 3.0 miles per hour, you will burn about 150 calories in one session. If you walk for 30 minutes at 4.0 miles per hour, you will burn 200 calories. Now let’s say that at the lower-intensity, 3-mile-per-hour pace you burn 70% of your total calories from fat. That would be .70 x 150 calories, or a total of 105 calories burned from fat. At the faster, 4-mile-per-hour pace, you burn only 60% of your calories from fat (again, because you are working harder, you can’t get as much oxygen to the muscles to burn fat). So, .60 x 200 calories equals 120 calories from fat. As you can see, at the slower speed, not only is the number of total calories burned less (150 versus 200), but the number of fat calories burned is also less than it is at the faster speed (105 versus 120).

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