Basics of energy metabolism

Those who want to lose weight in the long term should first understand the physiology of the body. Find out here about the most important basics of the metabolism.

The uptake, transformation and excretion of different substances is the most important component of an organism. This is called metabolism, or metabolism. Most athletes will have heard something about anabolism (anabolic chemical metabolism) and catabolism (chemical metabolism). This article is intended to give a simplified overview of the most important metabolisms occurring in the human body.


Humans as well as all animals are, in contrast to plants, heterotrophic organisms. That is, they need organic substances, the nutrients, to build up the cells and preserve their vital function.

These are used for energy, for chemical processes and for maintaining body temperature. So that the so-called energy metabolism can be maintained, a sufficient amount of energy sources (such as fats, carbohydrates and proteins) must be supplied.

The fuel that drives the body and moves the muscles is the primary energy source adenosine triphosphate (ATP for short). If this disintegrates, adenosine diphosphate (ADP), a phosphate residue and energy is created. However, the ATP memory of a muscle is very limited and just enough for 0-3 seconds, so this must be replenished as quickly as possible. This happens on the one hand by the "recycling" of the remaining ADP by means of adenosine monophosphate (AMP) to new ATP, on the other hand by another phosphate binding, the creatine phosphate (KrP). But the storage capacity of KrP is limited to about 9 seconds. Thus, this period represents the phase of the alaktazid anaerobic energy production - without lactate formation and without oxygen consumption.

Energy supplying processes

Logically, in addition to these primary energy sources, the body has additional compounds for reprocessing ATP in order to perform movements with a time span of more than 9 seconds.

Carbohydrates are stored as glycogen in muscle and as glucose in the blood and are the only substrates that are suitable for an oxygen-free treatment of ATP. The heavier the load, the higher its share of energy turnover, while at light to medium burdens their participation shrinks below 50% and another substrate comes to the fore.

Fats are found in almost all cells, but their main storage site is subcutaneous fat. They supply more than twice as much energy as carbohydrates, but they require oxygen (aerobic metabolism) for the fatty acid degradation and the associated energy production. Proteins primarily have an upbuilding function and are therefore not an energy carrier, but can also be converted into such a deficiency situation, ie in the absence of the two substances mentioned above. Their energy value is then comparable to that of a carbohydrate.

The 3 metabolic processes of these substances are:

1) glycolysis

When glucose is broken down into pyruvate, this process is called glycolysis. This process takes place in the sarcoplasm of the muscle cell. Hydrogen is produced, which binds to the coenzyme NAD and is subsequently oxidized in the respiratory chain. The released energy is used in the construction of ATP and KrP. The resulting pyruvate can now be further degraded in two ways.

Lactate is produced without oxygen, which is why it is also known as lactic acid. This possibility already prevails after 5 seconds - thus still in the anaerobic alaktaziden phase - and reaches after approx. 20 seconds the maximum intensity.

If, on the other hand, pyruvate is further degraded with oxygen to form carbon dioxide and water, this is referred to as the aerobic generation of energy. Although the aerobic use is faster, but has the disadvantage that it gives away a lot of energy and can lead to hyperacidity of the muscles through lactate formation, which makes further movement impossible. After 45 seconds, the percentage of glycolysis in the total energy supply is maximized and decreases in the following time. After 3 minutes, the aerobic energy supply predominates.

2) citric acid cycle

The aerobic energy production takes place in the cell's own power plants (mitochondria). From the already mentioned pyruvate, from fatty acids and more rarely from amino acids, the active acetic acid acetyl-coenzyme A (acetyl-CoA) is formed. As this acid continues to degrade, it produces carbon dioxide and hydrogen. In the subsequent respiratory chain the high energy potential of the hydrogen is used and finally water is generated by the oxyhydrogen gas reaction. Worth mentioning is also the different energy balance, because from 1 mol glucose (carbohydrate) 38 mol ATP and from 1 mol triglycerides (fat) 129 mol ATP arise. The stock and the associated performance in this energy production are almost infinite.

3) lipolysis

The lipid metabolism covers the energy requirement at low load and during rest periods. First, in the fat cell by the enzyme lipase, a fat molecule is split into 3 molecules of fatty acid and one molecule of glycerol. The glycerol is further processed in glycolysis. The fatty acids are further broken down and eventually end up as acetic acid. This process is called beta oxidation. Here again energy is released and acetyl-CoA is produced, which is further processed in the above-mentioned citric acid cycle. This requires oxaloacetate, which is produced during glucose breakdown. The synergy of the metabolism of carbohydrates and the burning of fat should be clear here, no glucose can be burned without glucose degradation.

The motto is therefore: fats burn in the fire of carbohydrates!

energy needs

The energy stored in the nutrients is now expressed in joules (J). More common is still the outdated expression calorie (cal). 1 kilocalorie is equivalent to about 4.19 kilojoules. A kilocalorie represents the energy needed to heat one liter of water from 14.5 to 15.5 degrees Celsius. The calorific value of the different substances is the energy released during the combustion of one gram. For carbohydrates it is 17.2 kJ (4.1 kcal), protein 17.2 kJ (4.1 kcal) and fats 38.9 kJ (9.3 kcal).

The daily energy requirement of a person depends on age, gender and individual physical and mental work. It is different

- The basal metabolic rate, which indicates the amount of energy that a person needs in 24 hours of complete rest and bed position alone for the vital metabolic processes,

- the turnover of work or work, by which is meant the additionally applied energy for the performed physical work, and

- the total turnover, the sum of GU and AU.

There are many, different complex and exact formulas for the calculation of the GU and AU. As orientation this simple GU-formula should serve:

Basic sales of the man in Kcal per day900 + 10 x kg body weight
Basic conversion of the woman in Kcal per day
700 + 7 x kg body weight

Keep this in mind when dealing with diet, nutrition, fitness and eating habits.

Read also: Fitness for Beginners III: Fundamentals of Nutrition (Part 1)

Marcel Kremer

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