Energy Processes during the Exercise for Weight Control

作者是: Prof. Vojko Strojnik, Ph.D.

Muscular work requires energy. The muscles get this energy from the food we consume. Among all, only carbo-hydrates (CH), fat (F) and proteins provide the energy for movement. Among all foods a lot of energy comes from alcohol, but it is not possible to be used for muscular work.

Energy sources for muscular work don't contribute to energy processes equally. The most important are CH and F, because proteins cannot be used as an energy source in normal circumstances. Despite that fact, proteins are important for endurance sports activities, because they are necessary to repair muscular microdamages that occur during the activity.

Differences between Energy Sources

There are important differences between CH and F in provided amount of energy and power. Normally fed person has about 8.3MJ (2000kCal) of energy stored in CH and 400MJ in F. Situation is reverse when we talk about power. F can provide only about 50% of power that can be provided by CH.

This is why movement speed drastically drops when we run out of CH.

Big consumption of energy is associated with accessibility of oxygen. F can only be decomposed with sufficient presence of oxygen, while CH can also be decomposed without it (anaerobic metabolism, glycolysis).

CH do not decompose completely during the glycolysis (to water and CO2) and only 8% of energy gets discharged before the process is interrupted. Even though this is very little energy, more power is available than in aerobic decomposition. The toll is raised fatigue in muscles, which decreases the time of its work. Thus we don't want muscular fatigue when we workout to lose weight.

Source of Energy Is Related to Strain

Relative part (%) of both sources during the movement can be estimated by respiratory coefficient (based on analysis of inhaled and exhaled gas). CH and F contribute approximately similar shares to energy consumption when resting. Increasing the strain, the ratio doesn't change fundamentally, up to approximately 40% of maximum oxygen consumption, which is approximately 40% of heart reserve (Karvonen Formula for determination of intensity based on heart rate). Then the share of CH starts to rise proportionally and it becomes almost the only source at the highest activity intensity. Estimated shares of CH and F for different moving intensity are presented in table 1.

Table 1: Share of CH and F for different moving intensity (max heart rate is 200)

Share of CH and F for different moving intensity
Intensity (maximum HR %) Pulse (bpm) % CH % FAT
65-70 130-140 40 60
70-75 140-150 50 50
75-80 150-160 65 35
80-85 160-170 80 20
85-90 170-180 90 10
90-95 180-190 95 5
100 190-200 100 -

The best method for successful control of body weight is the intensities where the body entirely suffices its energy needs with decomposition of CH and F with oxygen. Usually this line is between 70 and 80% of the maximum oxygen consumption. The 24alife app can help you monitor your heart rate during the workout with a sensor, connected to your smartphone. 

Influence of Physical Readiness

Duration of activity in the presence of enough oxygen increases the share of burned F as an energy source. Higher aerobic power increases F part at the same intensity. Production of milk acid reduces the F consumption, so it is important to prevent it to use F as an energy source. This often occurs to physically unfit people than to fit ones.

Influence of Fat Consumption

Type of food strongly affects shares of CH and F as energy sources. With nutrition, rich in CH, the share of F decreases and the CH share increases. The opposite happens with nutrition, poor with CH.

It is important to know that nutrition, poor with CH, the exhaustion occurs earlier. Thus the CH rich foods (that provides the longest exhaustion time) ensures bigger absolute share of consumed fats.


From "exercise for weight loss" point of view, it is the most important to move in the lines of aerobic decomposition of energetic sources and to keep moving long enough to importantly contribute to energy consumption.