Many have heard that fat is not burnt most efficiently during high intensity training. True, while you are completing your workout the greatest amount of fat is going to be burned while training at a lower intensity, more precisely at 55-65% of your maximum heart rate. The mechanism behind this begins inside the muscle while you are producing energy. To truly understand this, a quick look at the two aerobic energy processes in the body is necessary.
At 55-65% of your maximum heart rate (http://www.aspirefitnessandhealth.com/measure-heart-rate/) you are primarily using your aerobic, or oxidative (using oxygen) system. At this heart rate, your body is able to utilize fat very well through Fatty Acid metabolism. Here fatty acids (aka lipids, fats) are able to be converted into a compound called acetyl coA. This coenzyme is a very important step in the citric cycle, which is the aerobic process of producing energy, or ATP. ATP is our body’s energy source and everything that we put into our body with a goal of energy ultimately arrives at ATP. Well, fatty acids are excellent producers of energy, creating more than 4 times the amount than that of carbohydrates (thus the reason why overeating fatty foods leads to weight increases by the storing excess energy). Problem is, the process of breaking for a fatty acid into acetyl coA takes a large amount of oxygen. Therefore, ample oxygen needs to be available in the body for this reaction to occur.
As you increase your heart rate, oxygen is transported and utilized in the body at a higher rate, giving less importance to lending oxygen to the energy producing systems. Therefore, at higher intensities, with a lesser amount of oxygen biologically available, the body tends to use carbohydrates to produce energy, called Glycolysis. Carbohydrates in various forms, both from food and stored as glycogen, break down into acetyl coA and enter into the citric acid cycle, just as the fatty acids. Here however, it takes much more energy to convert the byproducts of carbohydrates, therefore giving a lower resulting ATP result.
More fats are burned at lower intensity cardiovascular exercise. But, is this the only way to lose weight properly? No, it is not. High intensity training is also excellent for weight loss. Although while training at high intensities you are primarily burning carbohydrates, you also burn more calories when compared to workouts of low intensity with the same duration. Ultimately, calories are just a unit of energy. More energy is being produced and used during these high intensity workouts. Although not nearly as much fat as been used, high intensity training has been shown to keep the metabolism and body burning more fuel for a greater amount of time when compared with its low intensity rival. Therefore, you are able to be burning more fuel not just during your workout, but for the next 24-48 hours as well.
So if you are looking to lose weight, which type of training should you be doing? The answer is both. Fat needs to be burned and is burned most efficiently at lower intensities. But, calories need to be burned as a whole as well, regardless of what exact substrate is being used. The most important thing to consider is to continually mix up your workouts. Doing one activity all the time will cause the body to adapt and become very efficient at doing that precise action. Not letting your body become adjusted is key, so the more your body is guessing, the better results you will achieve.