Glucose, amino acids, and fatty acids are used in the body for energy. Depending on the availability of each, they may be stored for later use or broken down to be used immediately. Metabolism is known as the combination of chemical reactions needed to occur for the body to convert the carbohydrate, protein, and fat ingested into energy or ATP for fuel. Glucose is most readily available when carbohydrate sources are ingested in the form of fruits, vegetables, and grains. Protein is made up of amino acids which consist of carbon, hydrogen, nitrogen, oxygen, and a side-chain group. Fatty acids are formed when fat from triglycerides are broken down. The chemical reactions of these substrates to break down or create molecules to use for energy may be catabolic or anabolic. Catabolic pathways release energy when larger molecules are broken down into smaller molecules and anabolic pathways use energy to create larger molecules from smaller molecules (McGuire & Beerman, 2018). Like we discussed last week, depending on the fed state partly determines which pathway is taken - whether catabolic or anabolic. Finally, depending on how caloric intake of any of these substrates compares to energy expenditure, weight gain or weight loss may result. However, despite which substrates are consumed, if there is an excess amount of any substrate, it is mainly converted to fat for later use.  

When energy or ATP is readily available as in a fed state, anabolic pathways are favored and work to create or store the substrates consumed into smaller molecules for later use. However, when energy or ATP is not readily available as in a fasted or starved state, catabolic pathways are favored to break down the energy that had been previously stored for immediate use (McGuire et al., 2018). While the body favors glucose in the form of carbohydrates, one’s nutritional status determines which substrate and pathway is used as “energy availability may be the single most influential factor that governs energy metabolism” (McGuire et al., 2018).   

Carbohydrate 

There are four ways the body is able to break down or create glucose or glycogen when necessary. Glycogenolysis, glycolysis, glycogenesis, and gluconeogenesis are these pathways. Carbohydrate sources are mainly found in fruits, vegetables, and grains. When these foods are ingested, they may be catabolized or broken down into glucose if the cells need immediate energy, or the extra glucose may be stored in the liver and muscle tissue for later use. When the cells are in need of additional glucose, the hormone glucagon stimulates glycogenolysis. This is one catabolic pathway used to break down the glycogen stores into glucose in order to supply energy to the cells (McGuire et al., 2018). 

Another catabolic pathway is glycolysis. This pathway does not require oxygen and anaerobically breaks down glucose to use during high intensities. The result of this is the formation of two molecules of pyruvate to be converted into lactate then to finally be converted into glucose. Although this process is catabolic, and occurs in the mitochondria of the cell, it may also occur as an anabolic pathway in the cytoplasm of the cell. Since these processes may occur in both locations simultaneously, glycolysis is considered an amphibolic pathway (McGuire et al., 2018). 

The hormone insulin stimulates glycogen formation from glucose stored in the liver and muscle tissues. This anabolic pathway uses energy or ATP to create glycogen from glucose and is known as glycogenesis (McGuire et al., 2018). 

Finally, the average person has a supply of glucose stored in the liver and muscles to provide enough energy to last for 8 to 12 hours. Since these stores may deplete quickly depending on the activity levels, when these stores begin to deplete, the body (primarily the liver) is able to create glucose from alternative, noncarbohydrate sources such as amino acids from muscle protein or from fatty acids (McGuire et al., 2018). This process is an anabolic pathway known as gluconeogenesis.    

Protein

Protein is not the body’s first choice to utilize as a food source. However, when energy or ATP in the form of glucose is low and glycogen stores have been depleted, protein becomes an alternate energy source (McGuire et al., 2018). To be used as energy, muscle tissue must first be broken down or catabolized into smaller molecules. Enzymes called proteases speed up this chemical process and allow the muscle tissue to quickly convert into amino acids to finally be converted into glucose for the cells to use when necessary. The pathway of breaking down protein into amino acids is known as proteolysis.  

The pathway of proteolysis is also important when it comes to muscle tissue turnover. Protein sources in the diet are essential in order to maintain the proper balance of quality and quantity of skeletal muscle tissue throughout life (Vliet et al., 2018). As muscles are continuously broken down and repaired, eventually the tissue will “retire” and be used as amino acids to help maintain an adequate supply of usable energy (McGuire et al., 2018). Proteolysis is important in facilitating this breakdown process, but research has shown that this is also important when it comes to producing skeletal muscle fibers and how those muscle fibers are able to adapt to future stress - whether through exercise or daily living activities. Muscle fibers rely upon “appropriate protein degradation to rid the cell of damaged proteins from the mechanical and oxidative stress that accompanies the force-bearing/force-generating function of skeletal muscle” (Bell et al., 2016).   

Fat

There are four ways the body is able to break down or create fatty acids to be used as energy when necessary. Lipolysis, β-Oxidation, ketogenesis, lipogenesis are these pathways. Although the body primarily relies on glucose to fuel cellular activities, when these supplies run low triglycerides may be broken into glycerol and fatty acids. No matter the original source of triglycerides, they must be broken down into glycerol and fatty acids to be utilized. Lipolysis is the first stage of lipid catabolism that breaks down triglycerides into fatty acids and glycerol. When glucose levels are low, this causes glucagon levels to rise and then signals the enzyme lipase to speed up the catabolism of triglycerides into glycerol. During lipolysis, the glycerol can then be converted to glucose and used for energy (McGuire et al., 2018). 

β-Oxidation is the second stage and involves the catabolic breakdown of fatty acids into acetyl-CoA. Once acetyl-CoA enters the citric acid cycle, it is oxidized and turned into energy (McGuire et al., 2018). 

Lipogenesis is the anabolic pathway used to convert stores of adipose tissue consisting of fatty acids and triglycerides into energy (McGuire et al., 2018). 

Finally, to protect the muscle tissue from being used first when glucose is limited, ketogenesis is the anabolic pathway which uses an alternative energy source of the nearly limitless fatty acids to create ketones from acetyl-CoA. Once these ketones are formed, they are released into the bloodstream to be used for energy (McGuire et al., 2018).

Energy Balance

When it comes to foods that are consumed, essentially, there needs to be a neutral energy balance to maintain weight. This means that the caloric intake must be equal to the energy expended - no matter the substrate consumed. Energy expenditure may come in the form of basal metabolism, exercise or physical activity, and the thermic effect of food. The combination of these make up the total energy expenditure (TEE). Basal metabolism is where a majority (50-75%) of energy is expended just to sustain living functions such as breathing and maintaining a heart rhythm. Physical activity makes up 15-30% of energy expended. This may come from a conscious effort to go out for a run or walking to the mailbox after a day of work. Finally, the thermic effect of food makes up the smallest portion (10%) of total energy expenditure and it is the amount of energy required to digest, metabolize, or store food as it is consumed (McGuire et al., 2018). Therefore, to maintain weight, one would need a neutral energy balance where energy expended is equal to energy or calories consumed.   

When one begins to gain or lose weight, there is a shift in energy balance. This shift may be positive or negative. A positive energy balance happens when one consumes more calories than are required to maintain weight. When these extra calories consumed are not met with additional energy expended (oftentimes in the form of increased physical activity), this results in weight gain. Conversely, a negative energy balance occurs when fewer calories are consumed than is necessary for the body to maintain weight. When fewer calories are consumed, yet activity levels remain stable, this results in weight loss. Research has found that a shift in energy balance leads to either weight gain or weight loss in the form of fat. This additional fat may end up being stored under the skin in the form of subcutaneous adipose tissue or may be stored as a layer surrounding the organs in the form of visceral adipose tissue. No matter which form of fat it takes, researchers state, “A positive energy balance, in which energy intake exceeds expenditure causes weight gain, with 60–80% of the resulting weight gain being attributable to body fat. In negative energy balance, when energy expenditure exceeds intake, the resulting loss in body mass is also accounted for by 60–80% body fat” (Hill et al., 2013). 

References

Bell, R. A., Al-Khalaf, M., & Megeney, L. A. (2016). The beneficial role of proteolysis in skeletal muscle growth and stress adaptation. Skeletal muscle, 6, 16. https://doi.org/10.1186/s13395-016-0086-6 

Hill, J. O., Wyatt, H. R., & Peters, J. C. (2013). The Importance of Energy Balance. European endocrinology, 9(2), 111–115. https://doi.org/10.17925/EE.2013.09.02.111 

McGuire, M., & Beerman, K., (2018). Nutritional Sciences: From Fundamentals to Food (3rd ed.). Boston, MA: Cengage Learning.     

Vliet, S. V., Beals, J. W., Martinez, I. G., Skinner, S. K., & Burd, N. A. (2018). Achieving Optimal Post-Exercise Muscle Protein Remodeling in Physically Active Adults through Whole Food Consumption. Nutrients, 10(2), 224. https://doi.org/10.3390/nu10020224