Most of the people that come to me for personal training on Cape Cod typically already know at least a workable amount about exercises and functional exercise form. What they are lacking and what prevents most of them from reaching their fitness goals, is a lack of basic knowledge about what energy systems they are using for their current exercise routine. Like all the folks that pile into gyms every January to attack weight loss goals, jumping on a treadmill for 20 minutes and " sweating off the fat" never really works the way they expect. Whether you are training for short burst, explosive strength, building muscle or looking to burn fat, you must be training at intensity levels with the appropriate set/ rep schemes that will use the correct energy systems to get the results you want. When you train with me, the issue of bioenergetics will be addressed in nearly every training session.
The human body needs a constant supply of energy to function properly. Those energy requirements can change, and exercise places unique demands on the body’s ability to supply energy. In addition to increasing energy demand, exercise also requires the body to handle additional metabolic by-products.
The food we eat contains carbohydrates, proteins, and fats, which are needed by our cells to produce energy and function properly. The energy stored in these food sources, through a series of chemical reactions, is converted to a high-energy compound called adenosine triphosphate (ATP), which serves as the main form of energy in the human body .The role of energy metabolism during exercise involves understanding how energy is supplied, which energy systems are used during exercise, how quickly energy can be supplied, and how cells generate ATP.
Energy metabolism, or bioenergetics, is the study of how energy is transformed through various biochemical reactions. Energy is required to sustain life, support exercise, and promote recovery from physical activity or structured exercise. The term metabolism refers to all the chemical reactions that occur in the body to maintain itself. As mentioned earlier, the main sources of chemical energy for humans are carbohydrates, fats, and protein. Exercise metabolism refers to the examination of bioenergetics as it relates to the unique physiological changes and demands placed on the body during exercise.
Dietary food provides energy to sustain life and support physical activity, but not directly; it first must be broken down by the digestive system into smaller by-products called substrates. Proteins (more specifically, chains of amino acids), carbohydrates, and fats constitute the main substrates used to transfer metabolic energy to be used for all types of cellular activity and life (Becker & Smith, 2006; Gleeson, 2005; Kalish et al., 2012; Maughan, 2005). Since all energy substrates are forms of organic matter, many can be converted from one to the other within the body depending on what is needed. For example, the body can convert carbohydrate-based foods to fat molecules in order to store energy for later use. However, a small selection of substrates cannot be created internally in this manner and must be consumed in the diet. Those nutrients we must eat to live healthily are termed essential.
Glucose is one of the main sources of energy, particularly for brain function and higher-intensity activity. Glucose can be made in the body from other substrates (fats and amino acids), but a large majority of our daily glucose needs come from consuming carbohydrate-based foods. Carbohydrates are consumed and broken down into glucose through digestion. Glucose is then absorbed and transported in the blood, where it circulates until it enters cells and is either used to make ATP or is stored for later. When it is stored, it is stored as string molecules in a branched structure called glycogen. Glycogen is stored in the liver and muscle cells and can be broken down rapidly to provide energy when there is not enough free glucose in the blood.
Glucose makes a relatively small contribution to overall energy production during rest or low-intensity exercise. The brain always requires glucose to function, but fats are what primarily fuel the body when it is not active.
As the intensity of an activity increases, the body transitions from using mostly fat as fuel to using mostly glucose to provide energy. This is because glucose can be used much faster than fat and can also be metabolized without oxygen, whereas using fat for fuel always requires oxygen.
As activity intensity increases, the usage of carbohydrate as an energy source becomes 50%, and the usage of fat becomes 50%. This metabolic marker is referred to as ventilatory threshold 1 (VT1). This will be an important concept to keep in mind during cardiorespiratory assessment and programming, especially as it pertains to maximizing both fat loss goals and performance goals for clients. As exercise intensity increases further to maximal levels, ventilatory threshold 2 (VT2) is reached (Ballweg et al., 2013; Foster et al., 2008). VT2 represents the point where activity is so intense that glucose is providing virtually all of the energy for the activity, as fats metabolize too slowly to keep up with maximal demands. If the supplies of glucose and glycogen run out, a person would not be able to continue exercising at maximal intensity and he or she will have to reduce effort to a point where fat usage is once again possible, commonly referred to as hitting a wall. This is part of why some athletes use “energy gels” and other carbohydrate supplements during prolonged strenuous training and in competition.
FREE FATTY ACIDS
An equally important source of energy are fats, also known as lipids. This energy source is particularly important during rest and lower-intensity activity (i.e., below VT1). The chemical (or substrate) form in which most fats exist in food (as well as in the body) is called triglyceride . Triglycerides, more commonly referred to as free fatty acids when they are in the blood stream, are derived directly from fats contained in foods or are made by the body to store excess energy when more food is consumed than is needed to support activity.
Before cells can use consumed fat or stored body fat as a fuel source, it first needs to be broken down into free fatty acids. Free fatty acids are then used exclusively in the aerobic metabolic pathway to produce ATP. One of the benefits of having fat as a fuel source is that even relatively lean people still have a large supply stored on their body, which can be broken down into triglycerides and used for energy during prolonged, lower-intensity physical activity and exercise. Any time an individual is exercising at an intensity below VT1, free fatty acids are the primary fuel source.
AMINO ACIDS
The third fuel source is protein, which is made up of long chains of “building block” substances called amino acids. Humans use 20 different amino acids to assemble bodily proteins. Of these 20 amino acids, nine are called essential amino acids, which means that the body cannot synthesize them on its own and they must be consumed in the diet.
The other 11 amino acids are called nonessential amino acids, which means that they can be synthesized by the body (from consumed carbohydrate or fat substrates) as long as overall nutrition intake is adequate.
When a person consumes protein, it is broken down into its component amino acids. Those amino acid building blocks will then, ideally, be used to synthesize human bodily proteins that build up muscle and repair cellular machinery. However, the amino acids from dietary protein can also supply energy for ATP production if carbohydrate and fat sources are low. This situation should happen rarely to “spare protein,” which is why adequate carbohydrate intake is important, especially after intense exercise; therefore, glycogen stores get replenished from the carbohydrate source, and amino acids can fulfill their main postexercise role: build and repair muscle.
Protein rarely supplies much energy during exercise and, in many descriptions, is ignored as a significant fuel source for energy metabolism (Mitchell et al., 2016; Phillips, 2017). During a negative energy balance, amino acids are used to assist in energy production and can come from protein that was eaten or from the breakdown of muscle tissue itself in extreme cases, like starvation or when exercising at extremely high intensities for long periods of time (for example, with Olympic marathon runners). Before amino acids can be used to make ATP, they are further broken down and then recombined into either glucose through a process called gluconeogenesis or ketone bodies through a process called ketogenesis (Maughan, 2005; McArdle et al., 2010).
KETONE BODIES
Ketone bodies is the name collectively used to refer to three molecules—acetone, acetoacetic acid, and beta-hydroxybutyric acid—that can be anaerobically metabolized similar to glucose. These molecules are produced by the liver as a by-product of the breakdown of fatty acids or through the conversion of ketogenic amino acids. The human body does not have the ability to store these molecules, so they are only used acutely to produce energy and are not stored for later like glycogen (Miller et al., 2018). Even though the body does primarily run on free fatty acids during low-intensity activity and rest, it still needs carbohydrate substrates to properly function. So, when carbohydrate stores run low, ketone bodies are produced and used alongside gluconeogenic glucose to help make up for the deficiency. During this metabolic state, the body is said to be in ketosis. Ketone levels can increase in the human body in several ways:
By restricting overall calories to very low levels
By following very low-carbohydrate diets (e.g., ketogenic diet)
By consuming exogenous ketones
When there is a lack of insulin produced (type 1 diabetes) or substantial insulin resistance (type 2 diabetes)
In most cases, when humans engage in the previous dietary habits 1–3, their ketone levels can increase to approximately 0.5–1.5 millimoles per liter (mmol/L) of blood, which is known as nutritional ketosis. This is a different physiological state than what is known as ketoacidosis, which mostly occurs in diabetic individuals. For most people, ketones make up a small portion of the energy-producing substrates in the human body, even in nutritional ketosis. However, there is some research studying the effect of ketosis and ketone-producing diets, such as ketogenic diets, for exercise performance (LaFountain et al., 2019; Volek et al., 2016).
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