When we do sport, the body has to perform at a much higher level than usual. It is put into a fight-or-flight state. In the “wild” when hunting, fighting or fleeing, energy gels or energy drinks cannot be used to provide the energy required for the higher performance. The body has to draw on its own reserves. Stress hormones, including glucocorticoids and catecholamines, are released during exertion to enable athletic performance. Catecholamines are neurotransmitters, i.e. messenger substances that exert their hormonal effect both in the brain and in the rest of the body. The main players here are dopamine, noradrenaline and adrenaline (epinephrine). The body produces noradrenaline from dopamine and adrenaline from dopamine.
Dopamine is known as the happiness hormone, but this does not adequately describe the effect of dopamine. Instead, dopamine is a “motivational hormone” that is found in all mammals. It “rewards” efforts such as foraging for food by being increasingly produced by the body during and after successful exertion and causing a feeling of happiness. This is why we feel happy and satisfied after training. We are motivated to make another effort. Dopamine is produced from the amino acid phenylalanine (or tyrosine). It has a certain basic level in the body. The higher this level is, the more drive a person feels and the happier they tend to be. Diseases such as Parkinson’s, anxiety disorders or depression are often associated with a lack of dopamine. Sport has therefore been proven to help with depression, as sport helps to permanently increase dopamine levels. Too much dopamine can occur in illnesses such as psychosis, schizophrenia or ADHD.
Noradrenaline, which is formed from dopamine, is released in the brain and sets the course for a fight-or-flight situation, whereby adrenaline is then formed and released in the adrenal medulla. Noradrenaline and adrenaline have similar effects. However, adrenaline is broken down within a few minutes, whereas noradrenaline remains in the body for longer. The hormonal effect of noradrenaline in the body is primarily vasoconstriction and the associated increase in blood pressure. Noradrenaline and adrenaline concentrations increase during exercise.
Adrenaline also causes an increase in blood pressure through vasoconstriction. Immediately before the start of a competition, the adrenaline level rises and with it our pulse, which improves the oxygen supply. Adrenaline causes the so-called smooth muscles to relax, which inhibits gastrointestinal activity. The bronchial tubes are dilated by adrenaline, making breathing easier. Adrenaline also suppresses growth hormones and increases free fatty acids, glycerol and lactate in the blood plasma, as a central task of adrenaline is to improve the supply of energy. This is achieved on the one hand by increasing the breakdown of fat (lipolysis) and on the other by supporting the formation of new glucose (gluconeogenesis) and other energy sources in the liver. During prolonged exercise, adrenaline facilitates the mobilization of substances (metabolites) for gluconeogenesis in the liver. Energy can be provided by exogenous and endogenous energy sources.
If we obtain energy from food, these are exogenous energy sources. The energy sources present in the body that do not come directly from the intake or digestion of food are referred to as endogenous energy sources. Between meals, half of the sugar in our blood comes from stored carbohydrates from the liver, which are broken down there and released into the blood. The breakdown of stored carbohydrates is called glycogenolysis. The other half of the blood sugar comes from newly formed glucose via gluconeogenesis in the liver and a smaller proportion in the kidneys. In contrast to the kidneys, the liver can store glucose as glycogen through certain enzymes and later release it into the bloodstream. During physical exertion, the liver plays the decisive role in gluconeogenesis. The most important substrates from which glucose is produced via gluconeogenesis are: Lactate, glycerol and certain amino acids. The (ketogenic) amino acids leucine and lysine cannot be used for energy production via gluconeogenesis. All other amino acids (glucogenic amino acids) are mainly used for energy production during fasting, sport and very protein-rich meals. When muscle protein is broken down, one of the products is the amino acid alanine. Alanine is released into the blood and absorbed by the liver, where it is converted into the energy source pyruvate and ammonia. While the resulting ammonia is excreted via the kidneys in the form of urea, the pyruvate is converted into glucose via gluconeogenesis and then returns to the muscle via the blood. This circulatory process is called the alanine cycle. In another cycle, the so-called Cori cycle, the working muscle releases lactate into the blood, which is transported to the liver and absorbed there. The liver converts lactate into glucose, which is returned to the muscle via the blood. In a fasted state, this can lead to the liver reducing the lactate concentration in the blood via the Cori cycle. In addition to lactate and amino acids, a breakdown product of fat metabolism is also converted into glucose in the liver. Glucose cannot be synthesized from most fatty acids. However, the glycerol present in the blood, which can be converted into glucose in the liver, comes to a large extent from the breakdown of triglycerides, which make up our body fat. When we burn fat during exercise, glycerine is produced which can be converted into glucose. The better trained a person is, the higher the training-related increase in glycerol during exercise due to fat burning. However, gluconeogenesis in the liver is limited by the presence of enzymes and cannot produce an infinite amount of glucose. The body must therefore fall back on other stored energy sources.
Although fat is an almost inexhaustible source of energy for our body, the supply of energy via the macronutrient fat is slow and also requires oxygen. The increased energy requirement in a short period of time during sport cannot be covered by fat, which is why other energy substrates must be used in addition to fat. Proteins are another macronutrient and are not normally counted as an energy source for the exercising muscle. However, this is a fallacy and the rate of amino acid breakdown increases significantly during training. This is due to the training-related increase in various metabolic processes. By suppressing protein synthesis and thus muscle building (anabolism) during physical exertion, amino acids are available for breakdown (catabolism). There is evidence that the basic concentration of amino acids in the blood and in the muscle is higher in trained people than in untrained people. Animal experiments have shown higher concentrations of free amino acids in slow-twitch muscles than in fast-twitch muscles. The utilization of protein therefore depends on the respective load and the individual metabolism. The utilization of protein therefore depends on the respective load and the individual metabolism. An intake of fat and protein is generally not necessary during training and competition and can sometimes be counterproductive, as it can delay and impair digestive processes. The situation is different with the third macronutrient, carbohydrates. Blood sugar (glucose) and stored carbohydrates (glycogen) in particular are essential as an energy substrate. Our body’s glucose metabolism is not only required during food intake, but also during sport. Two important hormones for regulating blood sugar and the supply of glucose are the two peptide hormones insulin and glucagon. More on this in the next part.
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