When the fasting is prolonged and the glucose continues to decrease (67 mg / dl), the alarms are switched on and the hormonal reactions appear to counteract the hypoglycemia (low blood glucose). Glucagon, noradrenaline and cortisol are secreted.
These last two are hormones closely related to stress. The orders that these three messengers transmit to the liver are:
- The use of glucose is prohibited.
- New glucose must be formed from any available molecule.
- We are going to use fatty acids as an energy source.
Let’s see in more detail what these changes in metabolism involve:
When a cell uses glucose as fuel, this molecule breaks down releasing that energy and leaving as residue two pyruvate molecules. This process is called glycolysis and is slowed down during prolonged fasting. Therefore, the first effect of stopping eating would be the saving of the already existing glucose, and the blocking of the organism of the mechanisms that allow it to be used as an energy source. The intention of this strategy is to reserve the glucose that is already available for exclusive use of the nervous system, especially protecting the brain.
The second order is to form new glucose molecules, and this can be achieved through three different sources: pyruvate, a molecule resulting from other biochemical processes, including the use of glucose that was previously described, but with the process of brake glycolysis, the availability of this molecule is diminished. The second source is amino acids, and here we refer to those that may exist in the liver reserve, but also those that are obtained from the degradation of tissue proteins. It is by this mechanism that too restrictive diets induce loss of muscle mass, as well as flaccidity of the skin.
Finally we have the use of fatty acids. If we remember, triglycerides are formed by three fatty acids and one molecule of glycerol, which can be transformed back into glucose. The fatty acids can also be processed to release energy, and some of them (the odd-chain ones) can be used as a raw material for the formation of glucose. When the fatty acids are metabolized, in addition to releasing energy they produce residual molecules called ketone bodies. These can be used as an energy source in the absence of glucose, even by the brain, but they also acidify the blood, and can have effects harmful on the tissues: if they are present in high amount can be toxic to brain cells, cause cardiac arrhythmias and promote dehydration.
The ketone bodies are especially dangerous in patients with diabetes. Before reaching these extremes, they cause symptoms such as nausea, headache, fatigue, bad breath, cold hands and feet, anorexia, etc. Its presence can be determined easily through a urine test. In healthy people, in addition to prolonged fasting and too restrictive hypocaloric diets, ketosis (increased ketones) can occur in high protein diets, especially when carbohydrates are suppressed excessively.
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