Your fat-burning zone refers to a training intensity that causes your body to mainly burn fat as fuel, often measured using your heart rate. Working out at varying intensities has varying effects on your body, so rather than worrying about your fat-burning zone, you are better off understanding different heart-rate zones. Low-intensity exercises do not demand a lot from your body, which lessens the importance of your fat-burning zones overdisproportionate dependence on fat. As a result, fat-burning zone theory encourages long, low-intensity aerobic training sessions that keep the heart rate in the fat-burning zone.
At first glance, you might seem to be burning more fat working with lower intensity (55%-65% of maximum heart rate) compared to working within the cardio zone (75%-85% of maximum heart rate). At lower intensity, your body can burn 50% more calories from fat, whereas it can only burn 35% more calories at higher intensity. While it is true the body does burn fat in lower-intensity training, fat-burning rates are still lower, and you will need to train for longer to burn as many calories as at higher intensities. Often, a single workout can burn a higher amount of proportional fat, but because the caloric burn is lower than in high-intensity exercise, the total amount of fat burned can actually be lower.
According to fat-burning zone theory, you need to perform longer, lower-intensity exercises in order to utilize your fat stores and burn enough calories in order to see fat loss. Yes, your body is indeed using more fat as fuel when doing the lower-intensity exercises, but you are also burning calories more slowly. This means that after your workout, your body does not keep using stored energy and burning calories. When you burn calories while exercising aerobically, your body adapts, slowing down your metabolism and allowing your body to store more fat.
Your body fuels itself mostly by burning a mixture of stored fat and carbohydrates. The most effective thing your body can do in lower- to moderate-intensity forms of exercise is turn to its stored fat for energy. Getting, combined with having a lack of carbohydrates in the body, means that your body starts using fat as an energy source. When this starts happening, you are not taking in enough oxygen to burn the fat, so your body turns to carbohydrates for energy.
If you are eating prior to doing high-intensity training (and especially if you are eating a high-carb diet), chances are good that your body is burning far more carbohydrates than fat. Depletion means high-intensity workouts are much more effective at burning way more total calories — both glycogen and fat calories. In lower-intensity programs, total calories burned in the workout will be lower than a high-intensity workout – whether those calories are fat or carbohydrates.
Whether the most calories are coming from fat or carbohydrates (in the form of glucose) is also affected by exercise intensity. Ultimately, it is the total amount of calories burned that will result in the greatest weight (and fat) loss. An athletic physique is the result not only of the amount of calories burned while exercising, but also the amount of calories your body is forced to burn consistently.
At lower intensities, the body relies more heavily on fat as its fuel source, since fat takes longer to break down and turn into energy (a longer process). The more physically fit you are, the more the body can rely on fat as energy instead of carbohydrates (this is known as anaerobic threshold). At rest (when your heart rate is lower), your body gets nearly all its energy from fat, which is a highly efficient, abundant fuel source.
Once you are exercising, around 60-80% of your max heart rate, your body gets about half its energy from carbohydrates stores, and half from fat stores (a point scientists call your maximal fat oxidation rate, or MFO). One of these zones, Mike Young, PhD, an exercise scientist who is the performance director for the Sports Performance Training Center, Athletic Lab, in Cary, NC, told SELF, is when you are working out at around 55 to 70 percent of your max heart rate (max HR).
The idea is that if you maintain a heart rate within your fat-burning zone, you magically burn more fat than if you were working out at higher levels of intensity. The best way to burn fat and maintain your health is by working smarter, understanding your heart rate zone. As you learned a moment ago, however, you are still burning primarily fat at around 60-80% of maximum heart rate, which is what most people would consider moderate-to-fairly intense training.
Doing intervals within your aerobic zone (like in HIIT or sprint workouts) will allow your body to more easily burn carbs as well as fat. Your warm-up zone gains benefits from burning calories throughout the rest of the workout. Aerobic Training is the concept that your body burns more fat during lower-intensity aerobic exercises compared to higher-intensity ones.
In high-intensity exercise, while the body uses its glycogen stores for quick energy first, it uses up its glycogen stores quickly enough that the body is forced to draw from its bodys fat stores. After completing a high-intensity workout where you burned lots of carbohydrates, your glycogen levels are depleted, and a lot of the calories you eat later in the day will go toward replenishing those stores, with very little, if any, being stored as body fat. As we discussed, whether we burn more fat or more carbohydrates to make ATP depends on the level of activity that we are doing (although other factors, such as diet and hormones, also play a role).
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