Reserves of Carbohydrates in the Human Body
Carbohydrates, in addition to being simple and complex, are classified according to their availability. All those nutrients that can be used by the body are defined as available; on the contrary, the use of those not available is prevented by difficulties during digestion, absorption or their metabolization.
The body's lipid deposits are much more abundant. An individual in good physical shape possesses in fact about 12 kg of fat mass, which at the time of need can release up to about 100, 000 Kcal.
The body "prefers" to accumulate energy reserves in the form of lipids instead of carbohydrates for two reasons: first of all because lipids are more energetic (9 Kcal / g against 4 Kcal / g of carbohydrates); moreover, very little water is present in the fat deposits.
Carbohydrate functions
The primary role of carbohydrates, also called carbohydrates or carbon hydrates, is to supply energy to the body. These nutrients are an ideal energy substrate for several reasons.
- First of all because their metabolism does not form intermediate products or potentially harmful by-products, unlike what happens during the catabolism of other nutrients. In fact, ketone bodies (acidosis) form from lipids, ammonia (toxic) from proteins, and acetaldehyde (also harmful to the body) from alcohol.
- Some tissues (primarily the nervous one) use glucose as an elective source of energy. Only in conditions of prolonged fasting does the brain resort to ketone bodies to satisfy its energy needs.
- The availability of carbohydrates as an energy substrate saves body proteins. The body, in fact, is devoid of protein reserves to be used for energy purposes.
Despite all these valuable functions, carbohydrates are not essential nutrients, at least in the strict sense. In fact, the body is able to synthesize glucose from different amino acids, lactic acid and glycerol, according to a metabolic pathway called gluconeogenesis. However, a diet that is too low in carbohydrates causes excessive catabolism of fats and tissue proteins.
The consequence of excessive lipid oxidation is the accumulation in the circulation of ketone bodies. These substances are synthesized by hepatocytes starting from the acetyl-CoA produced by the oxidation of fatty acids. Under normal conditions the synthesis of ketone bodies is minimal and the small quantities produced are used for energy purposes by various tissues, such as the muscles and the heart. On the other hand, when a strong catabolism of fats is taking place, for example in the case of hypoglucid diet or prolonged fasting, ketone bodies, produced in excess, accumulate in the blood (ketonemia) causing acidosis. This excess is eliminated in the urine (ketonuria); however, passing into the urine, the ketone bodies attract a lot of water by osmosis, with consequent dehydration of the organism.
Excessive protein catabolism leads to different but equally serious consequences:
- structural and functional deficit;
- ammonia formation (which must be converted into urea by the liver and eliminated in the urine, with consequent hepatic and renal overwork);
- dehydration of the organism.
Carbohydrates should provide 55-60% of the total daily energy intake. However, quantitative is not the only important parameter, in fact it is recommended to favor slow-absorbing complex carbohydrates, while the consumption of simple sugars (no more than 10% of the total caloric intake) should be limited.
The negative consequences of excessive carbohydrate consumption are manifold. First of all, when we introduce too many carbohydrates, our body increases its ability to oxidize them. Therefore, an excessive carbohydrate intake promotes an increase in energy expenditure, because the body catabolizes carbohydrates with greater speed. This argument, however, is valid only if the intake of lipids is at the same time very modest.
When excessive amounts of carbohydrates are consumed, especially if they are simple (sugars), the level of glucose in the blood increases (hyperglycemia). This condition rapidly promotes insulin secretion by the pancreas. This hormone acts just as quickly, causing a rapid accumulation of meal lipids in the deposits. This explains why the simultaneous intake of carbohydrates and excess lipids promotes the storage of these last in adipose reserves.
Insulin also inhibits lipid mobilization from fat deposits and promotes rapid entry of blood glucose into adipocytes and muscle fibers.
Hypoglycemia, which follows the entry of glucose into fat and muscle cells, triggers hunger and leads to the consumption of other food. This is the reason why it is good to favor low glycemic index foods.
CONTINUE: the glycemic index »
