Answer to: Why athletes can endure more physical exercises and workouts than normal people?

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Human beings are heterotrophic by nutrition which implies that they acquire energy by the ingestion of food materials produced elsewhere in nature. The ingested material is usually high energy organic compounds which require to be metabolized or simplified in the body for synthesizing energy. Among the high energy compounds (Proteins, carbohydrates, fats, nucleic acids), carbohydrates are the chief source of energy and are utilized by the body in both catabolic and anabolic processes.

Carbohydrates are catabolised for yielding high energy nucleotide phosphate like ATP, which is also termed as the energy currency. The complex metabolic process for ATP generation in the body from carbohydrates requires a series of complex biochemical reactions. These reactions are catalyzed by proteinaceous complexes called biocatalysts or enzymes. The enzymatic hydrolysis of carbohydrate-rich food materials usually converts or degrade into glucose, as the end product. Glucose is the major energy compound in the human body and is also among the fastest metabolizing compound in cells beside alcohol (ethanol).

The six-carbon glucose molecules are degraded into three-carbon compounds, i.e. pyruvate by a series of enzymatic reactions called Glycolysis (French; glyco-sweet + lysis-dissolution). The glycolytic process also produces 2 ATPs and two NADH (coenzymes for energy production). Glycolysis serves as the common pathway for the metabolism of glucose under both aerobic and anaerobic conditions.

Since the two ATPs and two NADHs do not fulfil the energy requirements of the cell, the cell chooses either aerobic or anaerobic pathway for the respiratory process and generation of energy from the pyruvate just formed.

In the aerobic respiration, the pyruvate is modified by removal of a carboxyl group followed by oxidation, and then attached to Coenzyme A. This acetyl CoA now enters Tricarboxylic acid(TCA) cycle and undergoes series of reactions to yield two ATP molecules, ten NADH molecules, and two FADH2 molecules (per two molecules of pyruvic acid).

The reduced NADH, FADH molecules generate ATP's by the process of Oxidative Phosphorylation in Electron transport system, occurring in the mitochondrial membrane (thylakoid) and matrix. The Oxidative Phosphorylation is a chemiosmotic process of transfer of electrons via a series of cytochromes and coenzymes in the Mitochondria, which generates ATP in the process and transfers the electrons to the final electron acceptor i.e. Oxygen in the aerobic systems. This is the major site of ATP production in Eukaryotes and that is why mitochondria are referred to as the powerhouse of cells.

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Now turning back to anaerobic conditions, when the oxygen in cells is insufficient and the pyruvate cannot be oxidized to acetyl CoA to further enter the TCA cycle. In this condition, the cell follows a different process where pyruvate is then converted into lactic acid. The Lactate thus formed in cells is transported into the liver and converted back to glucose when required by the body, which is again utilized as an energy source (The Cori cycle).

When does the body undergo anaerobic respiration?

During physical or mechanical workouts or stress like during vigorous exercise e.g. running, swimming, triathlon, kayaking and cycling,  in case of athletes the oxygen consumption suddenly surges. The ATP depletes quickly and it acts as the limiting factor for the hard work. This increased demand of oxygen supply increases the breathing rate or the oxygen inhalation. But when the amount of oxygen inhaled is insufficient for carrying out the oxidative respiration, the cells change the course of metabolism towards anaerobic respiration process.

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The anaerobic respiration process produces lactate or lactic acid as the final product. The enzyme lactate dehydrogenase (LDH) is responsible for reversible conversion of pyruvate into lactate. During rigorous exercise or physical stress, the conversion of pyruvate into lactic acid occurs rapidly which results in the accumulation of lactic acid in the cells. Lactic acid is toxic in nature, once accumulated causes discomfort, soreness and pain in the muscles. This is associated with muscle pain and discomfort during exercise.

Athletes normally can better endure these exercises and their body is adapted to sustain prolonged physical stress. The chemistry behind this endurance and high activity, despite the equal amounts of lactate, is being produced in athletes compared to normal beings is so-called - Training.

Training here refers to the condition of the cells which can utilize or degrade lactate more efficiently than normal cells. The muscle cells of athletes are adapted to degrading lactose faster as compared to normal people. The lactate formed in muscle is removed immediately. This adaptation is brought about by the increase in mitochondria in the muscle cells of athletes. Therefore, training in a sense denotes to grow more mitochondria in muscle cells. Studies have shown that endurance training reduces the blood level of lactose, despite the same amount of lactose is produced. This was possible for the cells to adapt during the training to produce and accumulate less of the toxic product.

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Image: American Diabetes Association (ADA) 2014

Increase in the number of mitochondria enables cells to efficiently convert more and more lactate into pyruvate and generate more energy. Mitochondria facilitate the conversion of lactate back to pyruvate, for entering the Kreb's cycle. Since lactate is formed in the cytoplasm, the rapid removal of lactate or its high metabolism is dependent on its three factors. 

  • The lactate transporter protein (MCT1) which transports lactate inside the mitochondria, 

  • The enzyme lactate dehydrogenase which oxidizes lactate back to pyruvate and the mitochondrial cytochrome oxidase which generates energy from its metabolism. 

These three components are collectively called Mitochondrial Oxidation Complex. Additionally, the conversion of lactate into pyruvate is necessary for the regeneration of NAD+, which helps in balancing of the cytosolic redox. The pyruvate regenerated also acts as the precursor for gluconeogenesis (the pathway for regeneration of glucose molecule from pyruvate).

Some other enzyme system and molecules also control the ATP generation and recycling generation process in cells like the Adenylyl kinase, Creatine Phosphate. Athletes are somehow adapted to the production of these enzymes in higher amounts as compared to normal people.


1. Lindinger, M. I., Brooks, G. A., Henderson, G. C., Hashimoto, T., Mau, T., Fattor, J. A., ... & Zarins, Z. (2006). Lactic acid accumulation is an advantage/disadvantage during muscle activity. Journal of Applied Physiology, 100(6), 2100-2102.

2. Hashimoto, T., Hussien, R., Cho, H. S., Kaufer, D., & Brooks, G. A. (2008). Evidence for the mitochondrial lactate oxidation complex in rat neurons: demonstration of an essential component of brain lactate shuttles. PloS one, 3(8), e2915.