The role of fat in the body PART 1
Fats or fatty acids from foods meet several physiological roles.
- First, is a source of energy. As combustion molecules, are stored as triacylglycerols, triglycerides or neutral fats. When energy intake is low, as on fasting or during some great physical effort, fatty acids from adipose tissue are mobilize to meet the energy needs. In myocardium and skeletal muscle, free fatty acids are activated in acyl-coenzyme A and transported to the mitochondria – the energy factories of the cell – where they are oxidized at the Acetyl-Coenzyme A. The newly Acetyl-Coenzyme A formed in the Krebs cycle, resulting in adenosine triphosphate for muscle activity.
Body fat of an adult contains 70-75 kg contains 10-15 kg of triacylglycerols. In obese people, the triacylglycerols weight deposit can be several times higher. The 15 kg of triglycerides represents 141,000 kcal, which means it could provide the energy required for the 70 days of fasting, on a normal adult, and for over a year, in an obese person. Theoretically, the starvation period
Might be extended even more if half of proteins in the body would be used.
In the first or second day of fasting, liver glycogen decreases rapidly to 10% of its normal concentration, then remaining constant at this low value, long time during fasting.
Muscular glycogen also decreases, but not so much. Glycaemya, however remains relatively constant at around 80 mg / 100 ml for at least four weeks of fasting.
After the exhaustion of the glycogen reserve, easy to metabolise, which occurs after one or two days, increase the operating speed of triacylglycerols in fat reserve, in the abdominal region and in the skin.
Within a few days after the fasting start, the amount of nitrogen excreted in the urine starts to increase, particularly in the form of urea, the final product of amino acids catabolism, which shows that the proteins in the body began to suffer from degradation process.
Someone might ask: If triacylglycerols are in quantities large enough, why are degraded the proteins in the body during starvation? The answer lies in the large amount of glucose that brain needs. As we know, the human brain has a very active metabolism, using at least 20% of the total intake of energy. In addition, the brain normally use only glucose as fuel – about 140 g of glucose per day.
In mammalian glucose is not formed from fatty acids. In starvation, the main source of blood glucose is the proteins in the body. The yield of glucose production from 100 g protein is about 57 g.
There is a certain sequence of reactions by which body proteins are slaughtered for keeping the normal blood glucose values; first that are lost are the digestive enzymes, secreted by the stomach, pancreas and small bowel; they are no longer necessary, as other enzymes and proteins are involved in the synthesis of digestive enzymes.
Very soon are destroyed various enzymes in the liver, which, normally, processes the nutritive substances coming from the intestines, converting them to plasma proteins, lipids and lipoproteins. Then the muscle protein degradation or use begins. In this stage hungry people become physically inactive, which represents a pathophysiology adaptation to the fasting.
In the first week of fasting, the body proteins are used in a high percentage of approximately 100 mg / day. But a new adjusting of the body occurs, to maintain the protein so that after 4-6 weeks of starvation, proteins are used in a very small proportion, between 12 and 15 g / day. This metabolic adaptations, that protects proteins in the body, is due to brain capacity to use as fuel, in addition to glucose, ketone bodies from the blood, resulted from the oxidation of triglycerides.
- The second role of fats is to be important components of cell membranes. Each of the 130 trillion cell of the human body is enclosed by a membrane, which has an extremely important role. Phospholipids and glycolipids are the most important components of cell membranes.
Physical properties of these membranes are influenced by the way of fatty acids in phospholipids. The presence of double bonds in fatty acids chains bend the fatty acid molecules, causing them to occupy more space than saturated fatty acids. This way unsaturated fatty acids take up more space in the cell membrane, increasing the fluidity of the membrane.