Energy and Metabolism

Food as Energy

There are three macro nutrients required by the body: carbohydrates, protein, and fat.

Carbohydrates are one of the main sources of energy for the body and are broken down into glucose or stored as glycogen in the liver and muscles for future use. What ever is not used or stored as glycogen gets converted to triglycerides and stored as fat. 

Glucose is a simple sugar the body uses for energy production on the cellular level. 

Triglycerides are a chemical compound formed when three fatty acids combine with glycerol. It is also the most abundant fat in the body.

When the body is at rest, it is estimated that approximately 70 percent of the body’s energy needs are met by fat sources and approximately 30 percent of the energy needs are met with carbohydrate sources. 

When energy needs are sufficient for immediate demands, excess carbohydrates are stored in adipose tissue (body fat) as triglycerides. 

Protein is not a primary substrate for energy metabolism unless the body is in a state of severe starvation or when the intake of other macronutrients is insufficient to support energy demands. 

The body will use each macronutrient differently, and, depending on the activity level and energy demand, the mix of which macronutrient is providing most of the energy will vary. 

Adenosine Triphosphate (ATP)

Macronutrients are not directly used as energy, nor are the resulting substrates from digestion. Instead, these substrates (glucose and fatty acids) are converted into adenosine triphosphate (ATP), the energy currency of the cells. 

During physical exercise, ATP is used in muscle cells to generate muscle contraction. Muscle contraction requires two molecules of ATP to complete the contract/relax sequence. For the cell to continue to work, more ATP must be created. This is done in several ways depending on the intensity and duration of the activity. 

The body metabolizes the food we eat through three distinct energy pathways: the ATP/creatine phosphate system (ATP/CP), anaerobic glycolysis, and the oxidative pathway. Each pathway is effective at producing energy for various intensities and durations of activity. 

Anaerobic Energy Production

ATP/Creatine Phosphate (CP) Energy Pathway

The body stores a limited amount of ATP in the muscles cells and this ATP is available for immediate energy needs. Stored ATP can only supply energy for up to ten seconds of work. After stored ATP is used, the cell creates energy using the immediate energy of there ATP/creatine phosphate (ATP/CP) pathway. This pathway is anaerobic, meaning it does not require the presence of oxygen.

After immediate energy stores of ATP are used, creatine phosphate (CP), a compound stored in muscle cells, is broken down to create more ATP. 

ATP becomes ADP (adenosine diphosphate) when a phosphate bond is broken and the resulting energy is used for work. CP lends a phosphate group to ADP to create another molecule of ATP.  

During maximum-intensity activity, CP stores can be depleted in less than 10 seconds. 

Anaerobic Glycolysis

For activities lasting from 10 to 120 seconds, (two minutes) and when the immediate demand for oxygen is greater than the supply, the body must tap into a second energy pathway: anaerobic glycolysis. 

Anaerobic glycolysis uses one molecule of ATP to convert glucose to glucose phosphate. Glycogen can also be used in this process. 

Anaerobic glycolysis produces a metabolic by-product called lactic acid and is sometimes referred to as the lactic acid system. Lactic acid, also called lactate, is used in the body in three ways: to make ATP, to make glucose in the liver, and as a signaling molecule. 

Recent findings suggest that lactic acid is a major source of energy used to repair and refuel the energy systems when those systems are taxed to the point that metabolic by-products are generated (metabolic stress).

During intense activity, mitochondria in the cell prefer lactate for energy. Lactate also signals the body to stop the metabolism of fat for energy and switch to the faster metabolism of glucose and glycogen. 

The heart and the brain prefer lactic acid for energy, however, when there is an excess of lactic acid, and hydrogen ions build up in tissues as a by-product of metabolism, they lead to muscular fatigue and muscular soreness. This buildup in the muscle cells, causes the burning sensation many people describer during intense activity. 

The point at which the body switches from metabolism requiring oxygen to primarily anaerobic metabolism is called the anaerobic threshold. 

The point where muscle tissue begins to make large amounts of lactate is referred to as the lactate threshold and can lead to lactic acidosis. Physical training increases the efficiency of the cells to use lactate for energy production.

Aerobic Energy Production

When cells exhaust the immediate ATP energy stores and glucose has been depleted, the aerobic energy pathways will begin to dominate energy production. The aerobic energy pathways are dominant in sustained activities more than 120 seconds (2 minutes) and include the process of aerobic glycolysis, fatty acid oxidation, and in extreme circumstances, gluconeogenesis. 

The oxidative energy pathway is a primary source of energy when the body is at rest or during low-intensity activities. Carbohydrates and fats are the primary fuel for this system, with fat providing most of the energy when energy demands are low and the glucose from carbohydrates increasing in comparison as the intensity of activity and immediate energy needs increase. 

Oxidative Energy Pathway

Aerobic metabolism produces a large amount of ATP, but it does so through a series of steps including: glycolysis, the Krebs cycle, and the electron transport chain. 

The aerobic production of ATP is more efficient but it also takes more time to occur. 

Glycolysis means the breakdown of glucose. This metabolic process occurs both anaerobically and aerobically. In the absence of oxygen, the process is anaerobic glycolysis and the by-product is lactate. In the presence of oxygen, the process is called aerobic glycolysis and the by-product of this energy pathway is pyruvate. 

Pyruvate serves as a transitional molecule in the many stages of aerobic metabolism. 

Pyruvate is broken down into acetyl coenzyme (also known as acetyl-CoA), which then enters the Krebs cycle in the mitochondria during metabolism. When acetyl-CoA is oxidized, it creates two molecules of ATP, carbon dioxide, and hydrogen ions. 

Hydrogen ions released during the Krebs cycle move onto the electron transport chain (also known as oxidative phosphorylation). These electrons contain a large amount of energy and are passed down a series of proteins located in the membrane of the mitochondria. A series of reactions happen as the hydrogen ions are transported across the membrane of the mitochondria, and the process produces 35-38 molecules of ATP. 

Gluconeogenesis

Amino acids are a last resort energy substrate. Gluconeogenesis is the process by which muscle protein is broken down or catabolized. The amino acid alanine is the most prominently used amino acid for this process. 

Gloconeogenesis is limited by the availability of the enzymes required to drive protein breakdown. During long-duration activities or starvation, hypoglycemia - or excessively low blood glucose levels - can occur. Low blood sugar stimulates the production of the hormone glucagon, which in turn stimulates the production of the enzymes required for gluconeogenesis and will stimulate protein breakdown. 

The Energy System Overlap

All these energy systems are interconnected and all three are operating at all times. It is the intensity and duration of activity which dictates which energy system dominates at any moment in time. 

During low-intensity, long duration exerciser, aerobic metabolism supplies the body with energy and fatty acids are the primary substrate used. 

During high intensity exercise, the body relies on both anaerobic and aerobic energy systems and carbohydrates are the preferred energy substrate. 

Amino acids are oxidized when muscle glycogen is used up and quickly used carbohydrates are a limited fuel source. Glocuneogenesis is the backup fuel generation system for low to moderate intensity activity.  

Metabolism and Energy Balance

Metabolism is the detailed and complicated chemical process of aerobic and anaerobic metabolism occurring within the cells of the body. 

The breakdown of nutrient in food yields calories. A calorie is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree celsius at a pressure of 1 atmosphere. 

 Resting Metabolic Rate (RMR)

The RMR is the rate of energy expenditure when the body is at rest. The RMR is directly correlated to body size and sex. 

The Bland-Altman analysis has been widely used to predict an individuals RMR

Men = 66.4730 + (13.7516 x weight in kg) + (5.0033 x height in cm) – (6.7550 x age in years)

Women = 655.0955 + (9.5634 x weight in kg) + (1.8496 x height in cm) – (4.6756 x age in years)

Caloric Expenditure

The Harris-Benedict equation is used to estimate total calorie expenditure (DCE). 

Thermic Effect of Food (TEF)

The TEF is the energy associated with the breakdown of food by the body. 

Physical Activity

Physical activity is second only to the RMR in terms of its contribution to daily energy expenditure.  

TIPS!

For a fat loss goal, a deficit of 200 to 500 calories per day from the DCE is recommended to create a Calorie deficit but still support body functions. This is a negative energy balance; Calories out are greater than Calories in.

Conversely, for a muscle gain goal, a surplus of 200 to 500 Calories (or more) is recommended to support muscle repair and muscle building. The actual Calorie surplus will be individualized based on the client’s training frequency, intensity, and recovery needs. This is a positive energy balance; Calories in are greater than Calories out.

Source: Foundations and Applications for a Certified Personal Trainer, Tenth Edition

Copyright 2021 ISSA LLC