Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Lipids
Lipid Metabolism During Muscular Activity

The utilization of fats for energy supply during muscular activity depends on its intensity, duration, the athlete's training status, and the degree to which various types of Muscle fibres are recruited during physical work. The following pattern has been established:

✵ fats are utilized to power working Muscles during prolonged physical activity of moderate intensity; they are recruited into METABOLISM/26.html">Energy Metabolism after a significant decrease in Blood glucose levels and muscle Glycogen stores;

✵ as endurance training progresses, The rate of carbohydrate oxidation decreases, while the rate of fat oxidation increases (likely involving muscle triglycerides).

The primary lipid energy substrates in Skeletal Muscle metabolism during endurance exercise are muscle triglycerides (TGs), as well as plasma triglycerides, free Fatty acids (FFAs), and Ketone Bodies. Among non-muscle fat sources, FFAs play The most significant role in muscle energetics. Their delivery to muscles and other Tissues depends on the rate of fat mobilization (lipolysis) in adipose tissues, the concentration of FFAs in plasma, and the rate of their transport into muscles.

Physical exertion enhances fat lipolysis in adipose tissue. After just a 30-minute cycle ergometer workout, the concentration of lipolysis products in fat Cells increases by 35–50%, and after a 4-hour treadmill run, it increases more than 5–6 fold. The rate of lipolysis in fat cells is regulated by hormone-sensitive lipase.

As a result of enhanced fat mobilization in adipocytes, blood levels of FFAs and glycerol increase significantly. High concentrations of FFAs, along with changes in transport mechanisms (carrier Functions), promote the accumulation of FFAs in skeletal muscles and activate their oxidation Enzymes. The penetration of FFAs into skeletal muscle fibres occurs via Active Transport involving carrier Proteins. Once inside the muscle cells, FFAs are either used in aerobic oxidation or partially synthesized into triglycerides.

THE CONTRIBUTION OF fats to muscular energetics increases as the duration of low- and moderate-intensity exercise increases. From the very beginning of such work and until its intensity reaches 60–70% of $\text{VO}_2\text{max}$ and lactic acid begins to accumulate, the rate of FFA mobilization and utilization increases. At high exercise intensities, the rate of FFA utilization by muscles decreases, while the rate of their mobilization remains high, resulting in persistently elevated plasma FFA concentrations.

A substantial contribution to muscular energetics is made by intramuscular TGs. They can provide approximately 65% of the energy generated through lipid oxidation. The rate of intramuscular TG utilization during exercise also depends on the intensity and duration of work, and on the degree of recruitment of various muscle fibre types into contractile activity. The highest utilization of intramuscular TGs occurs in fast-twitch oxidative-glycolytic fibres (type IIa), moderate utilization in slow-twitch oxidative fibres, and it is virtually absent in fast-twitch glycolytic fibres (type IIb). This difference in intramuscular TG utilization correlates with the varying activity of oxidative enzymes in these muscle fibre types, which is discussed in more detail in Chapter 14.

Intramuscular TG utilization depends on the level of training. It has been established that following a 12-week endurance-training program, performing a two-hour cycle ergometer workout at approximately 65% $\text{VO}_2\text{max}$ increases TG utilization twofold (Fig. 78), whereas in untrained individuals, the same exercise induces only a 20% increase in muscle TG utilization. This is because training increases The activity of enzymes involved in the activation, transport, and Catabolism of Fatty acids. Approximately a twofold increase in the activity of skeletal muscle ATP-dependent palmityl-CoA synthetase, carnitine palmityl transferase, and palmityl-CoA dehydrogenase has been observed. Training enhances the capacity of muscles to synthesize triglycerides, leading to an increase in their intramuscular stores.

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Fig. 78 Changes in muscle triglyceride (TG) content in untrained and trained individuals during endurance exercise

The mechanisms of fat mobilization and utilization during muscular activity are complex and not yet fully understood. Blood catecholamines (adrenaline) and Insulin play a crucial role in these processes, as the hormone-sensitive lipase system is highly responsive to them. Adrenaline increases lipase activity and fat mobilization, whereas insulin suppresses lipase activity and fat breakdown. During physical exertion, blood insulin concentration decreases, leading to increased fat mobilization.

Despite the fact that Hormonal Influences are the primary factors regulating lipolysis in adipose tissue, glucose concentration also affects lipolysis independently of changes in plasma hormone levels. Hyperglycemia ($10\text{ mmol} \cdot \text{l}^{-1}$) equally suppresses (by approximately 32%) the rate of formation of both FFAs and glycerol in healthy individuals. Consequently, independently of hormonal shifts, glucose regulates fat mobilization by inhibiting lipolysis. The recruitment of fats into energy metabolism is interrelated with the body's carbohydrate reserves. Fats become the primary energy substrate when glycogen stores are depleted and blood glucose levels drop. This is observed at the 30th–40th minute of submaximal aerobic exercise.

Thus, during bodily adaptation in the course of training, the efficiency of fat utilization increases against the Background of preserved carbohydrate reserves. This occurs due to adaptive Changes in the activity of enzymes responsible for fat oxidation and Oxygen transport.

To accelerate the recruitment of fats into the Energy supply of muscular activity, various lipolysis activators are used: caffeine (in non-doping amounts), Choline, Folic acid, vitamin $\text{B}_{12}$, carnitine, phentolamine, propranolol, etc. They accelerate fat mobilization, improve tissue oxygen utilization, and enhance the Fatty acid oxidation process itself.

Review Questions

1. Name the General Properties inherent to all Lipids.

2. What are the main biological functions performed by lipids in the body?

3. Into which classes and according to what principle are lipids classified?

4. What are the differences between saturated and Unsaturated fatty acids?

5. In what forms does neutral fat exist in the body? What is the Biological Role of each?

6. What are the main physical and Chemical properties of fats? Write the Hydrolysis and saponification reactions of fat.

7. Write the formula of a triglyceride composed of palmitic, stearic, and linoleic fatty acids.

8. What properties will a fat containing predominantly saturated or unsaturated fatty acids exhibit?

9. What is the Structure and biological role of Phospholipids, Lipoproteins, and Glycolipids?

10. What class of lipids does Cholesterol belong to? Write down its chemical formula.

11. What is the daily requirement for lipids, specifically in your sport?

12. What are the Specific features of fat breakdown during Digestion?

13. What is The Role of Bile acids in lipid digestion?

14. What is The Mechanism of fatty acid absorption?

15. How is glycerol utilized in the body?

16. In which cellular structures does ß-Oxidation take place, and what is the energy yield of palmitic acid ß-oxidation?

17. What are ketone bodies? Write down the reactions of their formation.

18. Under what conditions are ketone bodies produced in excess? What is their role in metabolism?

19. What are the primary mechanisms regulating lipid Metabolism in the Body?

20. What are the patterns of changes in fat metabolism during muscular activity?