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The body has three sources of fuel: carbohydrates, fat and protein. There are many factors that determine which fuel source your body uses and in what ratio. While doing cardio almost always burns fat stores, there a few extreme situations where this is not the case. If your primary motivation for doing that early morning run is to fit into your skinny jeans and burn fat, make sure you do your cardio in a way that helps you achieve your goals most effectively.
Cardio and Fat Burning
If you want to burn fat, engaging in cardio is certainly one way to do so. Cardio almost always burns both carbohydrates -- in the form of glycogen -- and stored fat together, and training in the low to moderately high range results in fat burning. You are only at risk of turning protein, and thus your own muscle tissue, into fuel under extreme circumstances.
When Cardio Burns Muscle
Fat cannot be burned without the presence of carbohydrates. Once carbohydrate stores are depleted, the body's only other source of fuel is protein; your body breaks down protein into amino acids to make sugar. Often, your very own muscle tissue is this source of protein. Muscle tissue takes hard work to acquire and helps keep your metabolism high. The more muscle mass you have, the higher your metabolism. Burning muscle tissue for fuel works against your efforts to lower body fat.
If you start to feel dizzy, light-headed or as though you are running in quicksand, you may have reached the point many athletes refer to as bonking or hitting the wall. If you are planning on doing an intense cardio workout that lasts longer than an hour or two, refueling with a fast digesting carbohydrate source such as half a banana, 4 ounces of apple juice or a few pieces of dried apricots replenishes your carbohydrate stores and keeps you from using muscle as your energy source.
Optimize Fat Burn
Interval training may be the most effective style of cardio exercise to burn fat in the shortest amount of time. A study published in 2007 by the University of New South Wales found that alternating working at high intensity for short bursts followed by steady state burned more fat in a shorter amount of time than continuous cardio exercise. In the study, one group of exercisers cycled at a steady pace for 40 minutes and a second group alternated eight-second sprints with 12 seconds of light pedaling for only 20 minutes. At the end of the study period, the second group had lost more stored body fat even though it spent half the amount of time cycling. The researchers found that this type of interval training produces high levels of chemicals called catecholamines, which seem to be responsible for the increase in thermogenesis, or fat burning.
"Fat Burn" Versus "Cardio" Program
If you want to make the most of your time at the gym, selecting the "fat burn" program on the elliptical, bike or treadmill may not be your best option. Working out using the "cardio" or "interval" program should result in a higher overall calorie and fat burn. Fitness equipment designer Precor lists different programs on its machines that are geared toward varying intensities. Using an elliptical machine as an example, it notes that the fat burn program intensity is 65 to 75 percent of your maximum heart rate, the cardio program intensity is 75 to 85 percent of your maximum heart rate and the interval program intensity is 65 to 90 percent of your predicted max heart rate. The cardio and interval programs provide a more intense workout, and the higher the intensity, the more total calories you burn and the greater your calorie deficit is at the end of the day.