Carbohydrates are sources of energy needed to sustain our aerobic energy system. It operates at low and medium intensity, when muscle cells can obtain sufficient amount of oxygen needed for "burning". Our muscles "burn" fat then.
Just a small digression, if we take a closer look at the molecule structure of fats and carbohydrates, we can see that the major difference between them is that the former ones contain more carbon atoms. To "burn" more carbon atoms we need more oxygen, and as long as we have plenty of it, muscles "burn" fat. The increase in the intensity of the sports activity after a while leads to the condition when muscle cells get less oxygen than needed to burn the abundant carbon atoms in fat, and relative hypoxia occurs. Simultaneously with the occurrence of this hypoxic condition, muscles turn to carbohydrates for energy which, unlike fat, can only be stored as glycogen in finite amount in our body. When muscle cells cannot obtain oxygen at all, the anaerobic energy system turns on which can provide energy from carbohydrates only with low efficiency and for a fairly short period of time. To put the learning material short and simple, 1 kg of stored fat provides more than enough energy for an ironman distance triathlon race. The fittest and most ascetic athlete's body stores enough fat for several ironman distance races. The real challenge is to sustain the truly efficient aerobic energy sytem, because it is worth sustaining it.
The following example illustrates the efficiency of the particular energy systems: by burning one glycose (one of the simplest carbohydrates) molecule we get energy enough for two steps. From the same molecule our aerobic energy system produces energy for 38 steps, while from fat molecules our it transports energy enough for 460 steps to the muscles. The problem is that not only oxygen is needed for burning fat. "Fat is burnt in a carbohydrate fire." Which is the most significant conclusion of the bible for people engaged in endurance sports. I would skip the detailed desription of the citric acid cycle and the route of fat molecules there now, but believe me that the place for the final destruction (and the creation) of fat molecules is a small ingenious apparatus, the so called Krebs cycle. For the constant"rolling" of the citric acid cycle, only carbohydrates can transport one of the "ingredients" (oxaloacetate). So, in order to sustain our extremely efficient aerobic energy system capable of producing enough energy for 460 steps from one fat molecule, we need carbohydrates in addition to oxygen. Should we run out of the former or the latter one, this magic power plant ceases to operate, and an unusually unpleasant condition, which I call "delirium" occurs: our performance decreases drastically, we start to feel dizzy and sick.
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To be continued!
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