International Journal of Molecular Zoology 2024, Vol.14, No.5, 255-264 http://animalscipublisher.com/index.php/ijmz 256 2 Fundamentals of Animal Energy Metabolism 2.1 Basic concepts of energy metabolism Energy metabolism in animals encompasses a series of biochemical reactions that occur within cells to sustain life. These processes are organized into metabolic pathways that either synthesize (anabolism) or degrade (catabolism) complex macromolecules to provide energy for cellular functions (Judge and Dodd, 2020). The primary pathways include glycolysis, the citric acid cycle, and oxidative phosphorylation, which collectively convert nutrients into adenosine triphosphate (ATP), the energy currency of the cell. Understanding these pathways is crucial for optimizing animal productivity and efficiency in agricultural settings (Kerr, 2021). Macronutrients play distinct roles in energy metabolism. Carbohydrates are primarily broken down into glucose, which enters glycolysis and the citric acid cycle to produce ATP. Fats are metabolized into fatty acids and glycerol, with fatty acids undergoing β-oxidation to generate acetyl-CoA, which then enters the citric acid cycle (Judge and Dodd, 2020). Proteins are broken down into amino acids, which can be deaminated to enter various points in the citric acid cycle (Nafikov and Beitz, 2007). The efficiency of these processes and the balance between them are critical for maintaining optimal energy levels and supporting growth and development in farm animals (Drackley et al., 2006; Nafikov and Beitz, 2007). 2.2 Measurement of energy metabolism Several techniques are employed to measure energy metabolism in livestock. Indirect calorimetry is commonly used to estimate heat production and energy expenditure by measuring oxygen consumption and carbon dioxide production (McBride and Kelly, 1990). Multicatheterization techniques allow for the quantification of nutrient fluxes and energy use by specific tissues, such as the liver and digestive tract (Drackley et al., 2006). Additionally, molecular biology approaches, including the measurement of mRNA abundance for key metabolic enzymes, provide insights into the regulation of metabolic pathways. These methods collectively enhance our understanding of how different nutritional strategies impact energy metabolism and animal productivity (McBride and Kelly, 1990; Drackley et al., 2006). Energy metabolism in animals is influenced by various factors, including age, breed, and environmental conditions. Younger animals typically have higher metabolic rates due to growth demands, while older animals may have reduced metabolic efficiency. Breed differences can affect metabolic rates and nutrient utilization, with some breeds being more efficient in converting feed into energy (Hocquette et al., 2007). Environmental factors, such as temperature and housing conditions, also play a significant role in energy metabolism. For instance, extreme temperatures can increase energy expenditure as animals work to maintain homeostasis (McBride and Kelly, 1990). Understanding these factors is essential for developing nutritional strategies that optimize energy use and enhance productivity in agricultural production systems (McBride and Kelly, 1990; Hocquette et al., 2007). 3 Nutritional Strategies to Optimize Energy Metabolism 3.1 Carbohydrate management Carbohydrates are a primary energy source for many animals, including monogastric species like pigs and ruminants like cattle. They play a crucial role in maintaining metabolic functions and supporting growth and production. For instance, in dairy cows, non-fibrous carbohydrates (NFC) are recommended during the prepartum period to facilitate the transition to lactation, although results on their effectiveness are mixed (Mendoza et al., 2019). In weaned piglets, the structure of dietary starch significantly affects serum glucose levels and intestinal health, highlighting the importance of carbohydrate type in energy metabolism (Gao et al., 2020). Optimizing carbohydrate utilization involves selecting appropriate carbohydrate sources and managing their intake. For example, in dairy cows, increasing NFC intake prepartum can marginally affect postpartum intake and metabolic status (Mendoza et al., 2019). In weaned piglets, diets with higher amylose ratios improve blood glucose and insulin concentrations, while high amylopectin diets enhance nutrient digestibility (Gao et al., 2020). Additionally, the use of carbohydrases in grazing beef cattle can improve ruminal fermentation and nutrient absorption, particularly during the dry season (Acosta et al., 2021).
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