The energy to sustain life and for us to move around is derived from food. Our gut with its gut microbiome is directly and indirectly involved with food metabolism and energy harvest and homeostasis.

Fuel from food metabolism is stored in the form of a high-energy compound, adenosine triphosphate (ATP), produced by body metabolism. ATP powers all forms of biological work and body movements.

Energy is used day and night, even when we are sleeping, to sustain physiological functions such as breathing and generating body heat. About 40% of food metabolism is used to produce ATP and the remaining 60% produces body heat (thermogenesis).

Energy use is at the lowest rate when we are at rest. When measured in bed after eight hours of sleep in a fasted state, our body’s rate of energy use is referred to as a basal metabolic rate (BMR). Activities above this basal level result in increased rate of use of energy.

During body movements, muscles or groups of muscles contract. The energy for muscle contractions is released from ATP when it splits off chemically to adenosine diphosphate (ADP, with two phosphates) and phosphate.

Since ATP is the only direct energy source for muscle contraction, it has to be continuously replenished. The body initiates the process to replenish ATP when ADP accumulates.

Only a small amount of ATP is stored in muscle cells, providing just enough energy for 5–8 seconds of strenuous activity. To keep moving, the body employs several methods to replenish ATP for continued energy supply:

• ATP can be immediately replenished by creatine phosphate, another high-energy molecule in the muscle cells. But creatine phosphate is also in limited supply and can only support muscle contraction for an additional 3–4 seconds.

• Glucose, stored as glycogen in the muscle cells and in the liver, can immediately help synthesize ATP without using oxygen. This process is called anaerobic glycolysis (no-oxygen glucose metabolism), and it provides a net yield of 2 molecules of ATP energy. It can support muscle contraction for about 2–3 minutes, or more strenuous exercise for about 45 seconds.

• Even though anaerobic glucose metabolism kicks in quickly to provide energy, the accumulation of an end product of this process, lactic acid (lactate), interferes with muscle function. A high level of lactic acid increases the acidity of muscle cells and inhibits further breaking down of glucose to supply energy. However, once excess lactic acid is moved to blood, it is transported to the liver and nonworking muscles, where it is metabolized back into glucose.

• Both creatine phosphate and anaerobic glycolysis are mainly used to initiate physical activity, and in short bouts of exercise or sports when great strength or high speed is required. In sports such as weight lifting and short-distance sprints, more than 80% of energy used is anaerobic.

• Oxygen is needed for continued muscle contraction. With oxygen, glucose and fatty acids (stored in muscle cells and fat tissues) can be metabolized into water and carbon dioxide, generating ATP in the process (called aerobic glycolysis).

• Our body prefers aerobic glycolysis for strenuous physical activity. Aerobic glucose metabolism provides a net yield of 38 ATP of energy to support muscle contraction for about 1–2 hours. Fatty acid metabolism (converting fat to energy through fatty acid oxidation) provides an average of 129 ATP and can support muscle contraction for many hours. Aerobic glycolysis has a sluggish start, but can be sustained for a long period of time. In endurance sports such as marathon running, almost all of the energy use is aerobic.

• During extended periods of physical activity, glucose metabolism decreases while fatty acid use increases, meaning that burning of glucose fuel reduces as burning of fat fuel increases. This shift maintains and protects the level of glycogen stored in the liver. This is of utmost importance because the brain can only use glucose from glycogen as fuel. We must ensure a continued and consistent supply of glucose fuel to the brain.

• But when glycogen available for activity is exhausted, the shift to burning fat may not be effective enough to support the intensity and level of exertion the body is actively engaged in. Endurance performance can be compromised as a result, for example, “bonking” in marathons.

• More effective burning of glycogen and fat for energy and performance is an area of intense investigation in sports physiology. Various training and nutritional strategies aimed at improving performance have been developed and employed.

• Protein is rarely used as an energy source, except during periods of starvation. Our body uses glycogen fuel first because the biochemical processes involved require less work. Fat fuel requires more steps to convert into energy, and protein is last-resort fuel.

• Besides the operations to generate energy, the site (or factory) where aerobic metabolism takes place in the muscle cells, mitochondria, is important for performance too. Exercise and sports training can increase the number of mitochondria and the concentration and activity of enzymes required (mitochondria biosynthesis), so that muscles can use more energy from glucose and fatty acids to replenish the ATP energy pool more efficiently.

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