The more you move your body, the more energy you burn. But how does the body burn energy? And what type of exercise produces the highest energy expenditure?
WRITTEN BY: JAN HELGERUD, PROFESSOR OF MEDICINE
The explanation of the mechanisms that occur in the body can be a little more complicated to understand. Here we give you the complete answer to how the body converts the energy we take in.
Oxygen uptake is energy metabolism
Oxygen uptake or fitness is measured in L/min. For every L of oxygen, you can release 5 kcal. When we measure oxygen uptake, we are actually measuring energy metabolism. Once we have measured your maximum oxygen uptake, we know how big an "engine" you have. But it's not enough to have a big engine if you drive a truck. Then you use up a lot of the engine capacity to transport yourself.
You need fuel for the engine
When you eat food, you get the fuel for your engine. The most important nutrients are carbohydrates (sugar) and fat. For every gram of fat you can release 8 kcal, while for every gram of carbohydrate you only get 4 kcal. In addition, 1 gram of carbohydrate binds 3 grams of water, a total of 4 grams. This means that you only get one kcal per gram. When the carbohydrates are therefore converted to fat in the body, your weight will be reduced to 1/8!
Why you need carbohydrates
The "heavy" carbohydrate stores in the body are therefore limited to a minimum. When the stores are full (1-1.5 kg or 4-6,000 kcal), they only last for 1.5 hours of hard work, and you "burst" or "hit the wall". Further work is only possible with carbohydrate replenishment and/or greatly reduced intensity. But what does 1.5 kg of carbohydrates weigh in the body? Well, together with 1.5 kg of carbohydrate, 3 times 1.5 kg of water is bound, a total of 6 kg. With such an extra weight to carry, you might be glad of the relatively limited carbohydrate stores in the body.
Carbohydrates release energy faster than fat
The human engine typically uses a mixture of carbohydrates and fat as fuel. The intensity and duration of work will determine the distribution. From rest and up to 80-90% of maximum oxygen uptake, there will always be a mixture, at higher intensities only carbohydrates are used. With increased work intensity, the need for energy per minute increases. Since carbohydrates can release energy 2-3 times faster than fat, they are preferred at high intensity.
Intervals vs. long runs
An interval training session will therefore almost exclusively use carbohydrates as fuel. If you work at 50% of the engine capacity (long run or "talking speed"), the work can last for hours. You then use about ½ fat and ½ carbohydrate. A load at 40-60% of maximum oxygen uptake gives the highest fat turnover per minute ("fat burning"). But you should note that you burn the same amount of carbohydrates at the same time.
Duration is crucial for energy consumption
The duration of the work also affects the distribution between the fuels. During moderate work at e.g. 60% of maximum oxygen uptake in 1.5 hours, the distribution at the start of the ride may result in a 60/40 carbohydrate/fat distribution. As the carbohydrate stores are reduced, it becomes necessary to switch to more fat metabolism. At the end of the ride, the split could then be 40/60 for carbohydrate/fat.
Moderate exercise is not our recipe
However, moderate exercise over a long period of time is not our recipe for effective exercise. You burn just as much energy in about 30 minutes during 4x4 interval training as you do in 50 minutes during moderate training. This is despite the fact that your working hours are reduced by 40%! So when it comes to burning energy, you have a choice: lower intensity for longer. But we remind you that this is a choice you don't have when it comes to the effect of the training on maximum oxygen uptake. There, duration cannot replace intensity in the same way!
Recent research shows that the most important factor for total fat metabolism in the body is maximum oxygen uptake. The paradox is therefore that the best way to increase maximum oxygen uptake is high-intensity interval training, i.e. only carbohydrate metabolism. The result of such training, however, is that your increased work capacity makes you a better fat burner in all your other activities.