Adaptations

During strenuous exercise and sports, skeletal muscle metabolism to generate energy for performance can rise quickly up to fifty times its resting rate. The body’s metabolic, cardiovascular, and respiratory responses must adapt rapidly to the dramatic changes in muscle contraction and tissue demands.

Metabolic preference for energy fuel is determined by several factors, such as:

• types of muscles involved in the activity
• intensity and duration of the activity
• level of training
• condition of the individual, such as diet and nutritional status prior to activity

Both glucose from the liver and fatty acids from the fat tissues are delivered to the active muscles via blood circulation. A good supply of blood to the muscle is essential. Also, aerobic metabolism can only continue with a sufficient supply of oxygen delivered to the muscles.

The body achieves these delivery tasks through a concerted adaptation of the respiratory and cardiovascular functions.

Oxygen uptake from the lungs increases, more blood pumps through the heart (cardiac output), and capillaries in the contracting muscles open up to allow increased blood flow, delivering more oxygen and fuel.

For example, total cardiac output of blood in volume per minute at rest is about 5,800 ml. It increases to about 9,500 ml in light exercise, 17,500 ml in strenuous training, and 25,000 ml in maximal exercise.

Increased delivery to the muscle is not only achieved by increased blood pumped through the heart but also through a major shift in blood distribution throughout the body.

During exercise and sports, blood is shifted to the working muscles from other, less metabolically active, organs of the body such as the gut and kidney. This allows more efficient fuel and oxygen delivery.

The majority of the shift is directed to working muscles. Distribution to the heart also increases as exercise intensity increases. But blood distribution to skin only increases as exercise intensity reaches a strenuous level, and then drops back to at-rest level during maximal exercise.

As in energy fuel preference, blood distribution shift is also determined by various factors. These include the intensity and duration of the activity, the corresponding fuel requirement, body condition (such as hydration status), and environmental condition (such as air temperature).

The composition and function of the gut microbiome is known to determine the effectiveness of energy harvest from food, modulating energy metabolism directly and indirectly in many ways and involving all food fuels (carbohydrate, protein, and fat).

It is estimated that gut bacteria generate up to 10% of human calorie intake. Gut microbial-mediated energy harvest is illustrated in both animal and human studies.

Scientists found that when fed the same diet as conventionally raised mice, germ-free mice without any gut microbes had to eat 35% more food to achieve similar body weights. Yet, they had 40% less fat mass and exhibited near-complete loss of SCFAs in their gut.

An individual’s ability to ferment dietary carbohydrates is highly correlated to gut microbiome composition. For example, most of the healthy participants in a study fermented more than 96% of administered resistant starch, but those who harbored very few of the bacteria species Ruminococci had more than 60% of the administered resistant starch pass through their gut unfermented (not harvested as energy).

A significant portion—an estimated 10–20%—of amino acids derived from dietary protein is taken up and metabolized by gut bacteria in the gut, releasing a range of products involved in making structural molecules, ketoacids, and various lipids important for building cells and tissues, including muscles, and for metabolism.

Absorption of dietary fat (lipid molecules) is tightly mediated by bile acids, which are primarily produced in the liver and secreted into the gut where they are modified by gut microbes into numerous secondary bile acids.

These numerous types of bile acids have different abilities to traffic the different lipids. This is important because lipids are insoluble in water and have detergent-like properties that are dangerous to blood vessels and tissues. They must be packaged before they can travel in blood.

The bile acids are then mostly reabsorbed, alone or with attached lipids, in the gut and returned to the liver. This gut-liver cycling regulates dietary fat uptake, transport, and metabolism. Bile acids also help activate gut cell receptors, influencing a wide range of metabolic effects, including insulin sensitivity.

By imposing dramatic effects on the overall bile acid profile, our gut microbiome significantly impacts transport and metabolism of dietary fat.

Our gut microbiome is also involved in altering triglyceride (fatty acid) production in the liver, and controlling the secretion of hormones, including those that modulate appetite, satiety, and body weight.

Short-chain fatty acids (SCFAs) such as acetate, butyrate, and propionate produced from gut microbial fermentation of carbohydrates (preferably fibers) and proteins provide an estimated 80–200 kcal per day of energy to our body. They are alternative energy substrates for hepatic gluconeogenesis (glucose production in the liver). Their signaling to multiple gut cell receptors is involved in the control of energy consumption, fatty acid oxidation (converting fat to energy), mitochondria biosynthesis, and thermogenesis.

SCFAs can prevent dietary-induced obesity. They increase fatty acid oxidation in multiple tissues and decrease fat storage in white adipose (fat) tissues.

Other ways that our gut microbiome can modulate energy metabolism and fuel generating efficiency are via its actions on the immune system as a major modulator of immune response, inflammation and stress responses, and its crosstalk with the brain that influences cognitive function, mood, appetite, eating behavior, and sleep cycle.

Many studies clearly show that regardless of energy fuel source, depletion of ATP in the active skeletal muscles causes fatigue, reducing performance. Exercising with limited glycogen stores increases muscle protein breakdown and can impair performance. Inadequate glucose in the blood increases perception of effort and causes both physical and mental fatigue.

The capacity and efficiency of our gut and gut microbiome to harvest energy from different fuel sources, to influence mitochondria (energy factory) capacity, to modulate body heat production, and to regulate energy metabolism and homeostasis is therefore crucial in initiating, supporting, and sustaining our movements, impacting performance.

The body responds and adapts to varying intensities and durations of physical activities differently, how exercise and gut/gut microbes impact each other, and the impact of gut/gut microbes on strenuous endurance sports performance are explored in separate sections below.

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