Fats along with carbohydrates are oxidized in the muscles to supply energy to the working muscles. The limit to which they can compensate for energy costs depends on the duration and intensity of the load. Hardy (> 90 min) athletes usually train at 65-75% V02max and are limited by the reserves of carbohydrates in the body. After 15-20 minutes of endurance loading, the oxidation of fat stores (lipolysis) is stimulated and glycerol and free fatty acids are released. In muscle at rest, oxidation of fatty acids provides a large amount of energy, but this contribution decreases with light aerobic exercise. During intensive physical activity, switching of energy sources from fat to carbohydrates is observed, especially with intensities of 70-80% V02max. It is assumed that there may be limitations in the use of oxidation of fatty acids as an energy source for working muscles. Abernethy et al. Offer the following mechanisms.
- Increasing the production of lactate will reduce the lipolysis caused by catecholamines, and thus reduce the concentration of fatty acids in the plasma and supply muscles with fatty acids. A manifestation of the antilipolytic effect of lactate in adipose tissue is suggested. An increase in lactate can lead to a decrease in blood pH, which reduces the activity of various enzymes involved in the energy production process and leads to muscle fatigue.
- A lower level of ATP production per unit time for fat oxidation compared to carbohydrates and a higher oxygen demand during fatty acid oxidation compared with carbohydrate oxidation.
For example, the oxidation of one glucose molecule (6 carbon atoms) leads to the formation of 38 ATP molecules, while the oxidation of fatty acid molecules with 18 carbon atoms (stearic acid) yields 147 ATP molecules (the yield of ATP from one fatty acid molecule is higher in 3, 9 times). In addition, six molecules of oxygen are required for complete oxidation of one glucose molecule, and 26 molecules of oxygen for complete oxidation of palmitic acid, which is 77% greater than in the case of glucose, therefore, with prolonged loading, the increased demand for oxygen for oxidation of fatty acids may increase the stress of the cardiovascular system, which is a limiting factor in relation to the duration of the load.
Transport of fatty acids with a long chain in the mitochondria depends on the ability of the carnitine transport system. This transport mechanism can inhibit other metabolic processes. The increase in glycogenolysis during the load can increase the concentration of acetyl, which as a result will increase the content of malonyl-CoA, an important mediator in the synthesis of fatty acids. This can inhibit the mechanism of transport. Likewise, enhanced lactate formation can cause an increase in the concentration of acetylated carnitine and a decrease in the concentration of free carnitine, and then weaken the transport of fatty acids and their oxidation.
Although the oxidation of fatty acids during endurance training provides more energy than carbohydrates, the oxidation of fatty acids requires more oxygen than carbohydrates (77% more O2), thus increasing the cardiovascular tension. However, due to the limited capacity of carbohydrate accumulation, the load intensity indicators deteriorate with the depletion of the glycogen reserve. Therefore, several ways of saving muscle carbohydrates and enhancing the oxidation of fatty acids during exercise for endurance are considered. They are as follows:
- feeding triacylglycerides with a chain of medium length;
- oral fatty emulsion and fatty infusion;
- a diet with a high fat content;
- additives in the form of L-carnitine and caffeine.
Observations showed that in trained muscles high lipoprotein lipase activity, muscle lipase, acyl-CoA synthetase and fatty acid reductase, carnitine acetyltransferase. These enzymes increase the oxidation of fatty acids in the mitochondria . In addition, the trained muscles accumulate more intracellular fat, which also increases the intake and oxidation of fatty acids during exercise, thus saving carbohydrate stores during exercise.
Consumption of triacylglycerides with a carbohydrate chain of medium length
Triacylglycerides with a carbohydrate chain of medium length contain fatty acids with 6-10 carbon atoms. It is believed that these triacylglycerides quickly pass from the stomach into the intestine, are transported with blood to the liver and can increase the level of fatty acids with a medium-length carbohydrate chain and a triacylglyceride in plasma. In muscles, these fatty acids are quickly absorbed by the mitochondria, since they do not require a carnitine transport system, and they oxidize faster and more than triacylglycerides with a long carbohydrate chain. However, the results of the influence of the consumption of triacylglycerides with carbohydrate chain of medium length on the performance indicators of exercises are rather doubtful. Data on the preservation of glycogen and / or increased endurance when consuming these triacylglycerides are unreliable.
Oral intake of fats and their infusion
Reducing the oxidation of endogenous carbohydrates during physical exertion can be achieved by increasing the concentration of fatty acids in the plasma by means of fatty acid infusions. However, the infusion of fatty acids during exercise is impractical, and during the competition it is impossible, since it can be considered as an artificial doping mechanism. In addition, oral consumption of fatty emulsions can inhibit gastric emptying and lead to its disorders.
Diets high in fat
Diets with a high fat content can enhance the oxidation of fatty acids and improve the endurance performance of athletes. However, the available data make it possible only hypothetically to assert that such diets improve performance by regulating the metabolism of carbohydrates and maintaining glycogen stores in muscles and liver. It has been established that long-term consumption of high-fat food adversely affects the cardiovascular system, so athletes should use this diet to improve results.
Additives of L-carnitine
The main function of L-carnitine is the transport of fatty acids with a long hydrocarbon chain through the mitochondrial membrane to include them in the oxidation process. It is believed that the oral intake of L-carnitine supplements enhances the oxidation of fatty acids. However, there is no scientific evidence supporting this provision.