During the one hour that the runner completes the race

During the one hour that the runner completes the race, different metabolic processes happen in the body in order to sustain the increasing energy requirement of the body, and the main energy source for the body during these processes are carbohydrates and lipids.
Both Carbohydrates and Lipids are formed with carbon, hydrogen and oxygen atoms. However, carbohydrates are non-linear, branched polysaccharides, and the monosaccharides (glucose) in the structure are linked together by glycosidic bonds while lipids are formed by glycerol and three fatty acids which are linked together by ester bonds. In addition, significant amounts of carbohydrates are stored in the skeletal muscles and liver as glycogen (a more compact, highly branched polysaccharide) when they are not needed, while lipids are stored in the adipose tissues as triglycerides when they are in excess.
Our bodies generally use lipids as the main energy source since lipids can carry and store more energy, which means it can provide energy for a longer period of time and produce more Adenosine Triphosphate (ATP) than polysaccharides. Additionally, the body also undergoes aerobic respiration due to sufficient amount of oxygen intake. Hence, before the start of the race (0 min into the race), the resting muscles in the body utilises fats as the preferred energy source and undergo aerobic respiration. The lipids used are broken down to glycerol and three fatty acids by the enzyme triacylglycerol lipase, where the fatty acids combine with serum albumin and get transported to muscles and tissues, cytosol and mitochondria (further description below). Beta-oxidation of the fatty acids will then occur to form Acetyl-CoA, which goes through the citric acid cycle and the electron transport chains to produce ATP. The leftover glycerol enters the glycolytic pathway, where it forms glyceraldehyde-3-phosphate through phosphorylation, oxidation and isomerisation and then undergoes the glycolysis process (payoff phase onwards).
Once the race has started (5 min into the race), muscles in the body are utilised which means the runner will require more energy. However, the rapid breathing that happens when the runner starts running cause insufficient oxygen intake into the body system to undertake aerobic respiration. Hence at this stage of the race, the immediate needs of the muscles are met by anaerobic respiration and carbohydrates, which can produce ATP more quickly than lipids due to its quicker delivery in energy, are used as the preferred energy source. This is advantageous in this stage of the race where it requires a lot of energy to be released rapidly in order to “pump” the muscles to support the runner during this first stage of the race.
Energy is attained from glucose during anaerobic respiration. As mentioned above, glucose is stored in skeletal muscle and liver when they are not needed. Hence, when immediate energy is required at the first 5 minutes of the race, glucose is readily mobilised from glycogen, which means glycogenolysis occurs. Glycogenolysis is the breakdown of glycogen to glucose and it mainly occurs in the liver. It is stimulated by the hormones glucagon and epinephrine and the main enzymes involved in this reaction are glycogen phosphorylase, phosphoglucomutase and glucose-6-phosphotase. Although glycogenolysis normally breaks down glycogen to glucose, when the reaction occurs in the muscles, the glucose-6-phosphatate formed, through catalysation of Glucose-1-Phosphate, which was broken down from glycogen, will directly enter glycolysis and convert to pyruvate due to the lack of glucose-6-phosphotase which removes the phosphate group from the G-6-P in the muscle and this would be the source of energy for muscle contraction.
After glucose is formed through glycogenolysis in liver, it will undergo the ten enzyme catalysed reactions, glycolysis. Glycolysis is the breakdown of glucose to pyruvate and it occurs in the cytoplasm, it is neither anaerobic nor aerobic. The reaction has 2 phases, the preparatory phase and the payoff phase, where the earlier one consumes energy and the latter one generates energy. One conversion of glucose can produce 2 pyruvates and 2 NAD+ are reduced to 2 NADH + H+. The net ATP yield for glycolysis is 2 ATP. The pyruvate produced from glucose in the muscle cell will undergo the lactic acid fermentation process, which is anaerobic since it doesn’t require oxygen to undertake. In this process, the 2 pyruvates are converted to 2 lactates using the enzyme lactate dehydrogenase. These 2 lactates can be recycled to form pyruvate, which enables the lactic acid fermentation process to occur again. The 2 NADH + H+ are converted back to 2 NAD+ so that glycolysis can continue and energy can be produced out of that process. It is this indefinite loop of reaction (until there is sufficient oxygen supply to undergo aerobic respiration) that allows carbohydrates to provide immediate energy rapidly, hence becoming the preferred energy source for the first 5 minutes into the race.
However, unlike lipids, carbohydrates are mainly for short term energy needs (cannot store much energy), hence approximately after 5 minutes, when the runner starts to get accustomed to the rapid breathing of air, aerobic respiration will take over (it is now possible as there is sufficient oxygen intake since the runner gets accustomed to the new breathing rhythm). Now that the body can undergo aerobic respiration, the runner can attain energy from both carbohydrates and lipids. For carbohydrates, after glycolysis, conversion of 2 pyruvate to 2 acetyl-CoA will occur in the mitochondria. This conversion is an oxidative decarboxylation of pyruvate, it is catalysed by pyruvate dehydrogenase complex and requires 5 other coenzymes, Coenzyme A, NAD+, TPP, Lipoyllysine and FAD to complete. This step will produce 2 NADH, and as 1 NADH is equivalent to 2.5 ATP, hence On the other hand, for lipids, they are broken down to glycerol and three fatty acids by the enzyme triacylglycerol lipase as mentioned above. The fatty acids combine with serum albumin in the bloodstream and get transported to skeletal muscles, heart and renal cortex. After that, they dissociate from the serum albumin and get transported to the cytosol, where the fatty acids are activated to Fatty Acyl-CoA with the addition of 1 ATP and Coenzyme A. The fatty acids are transported to the mitochondria via the transporter carnitine in order to undergo beta-oxidation. Beta-oxidation of the fatty acids then occurs to form Acetyl-CoA. For each cycle of beta-oxidation, 2 carbons are removed from fatty acids and 1 Acetyl-CoA is formed. At least 14 ATP equivalents are produced in a cycle of beta-oxidation as the cycle produces 1 FADH2, 1 NADH and if 1 acetyl-CoA is formed, then 3 NADH, 1 FADH2, and 1 GTP would be produced, where 1 NADH is equivalent to 2.5 ATP, 1 FADH2 is equivalent to 1.5 ATP, and 1 GTP is equivalent to 1 ATP.
The process continues as the Acetyl-CoA formed from carbohydrates and lipids enters the Citric Acid Cycle (CAC).