Principles of Human Physiology - The Endocrine System
Chapter 21: The Endocrine System: Regulation of Energy Metabolism and Growth
Chapter Outline
- 21.1 An Overview of Whole-Body Metabolism
- 21.2 Energy Intake, Utilization, and Storage
- 21.3 Energy Balance
Learning Outcomes
- Compare the metabolic pathways operating during energy mobilization to those operating during energy utilization.
- Explain the concepts of negative energy balance and positive energy balance.
21.1 An Overview of Whole-Body Metabolism
- Whole-body metabolism encompasses all the biochemical reactions involved in energy production and utilization within the body.
- Metabolism can be divided into two main categories:
- Catabolism: The breakdown of complex molecules to produce energy.
- Anabolism: The synthesis of complex molecules, which requires energy.
- Metabolism can be influenced by various factors including:
- Hormonal regulation
- Nutrient availability
- Physical activity levels
21.2 Energy Intake, Utilization, and Storage
Uptake, Utilization, and Storage of Energy in Lipids
- Absorbed Lipids:
- Includes triglycerides that circulate in the blood as lipoproteins.
- Key Steps in Lipid Utilization:
- Lipoprotein lipase breaks down circulating triglycerides into fatty acids and monoglycerides.
- Fatty acids are then transported to tissues such as the liver.
- Fatty acids undergo further processing to produce CO2, H2O, and energy.
- Summary of Lipid Processing:
- Form absorbed across GI tract: Monoglycerides and fatty acids (in chylomicrons).
- Form circulating in blood: Free fatty acids, lipoproteins.
- Form stored: Triglycerides.
- Storage site: Adipose tissue.
- Percentage of Total Energy Stored: 77%.
Summary Table of Carbohydrates, Protein, and Lipid Processing
Carbohydrates:
- Form absorbed across GI tract: Glucose.
- Form circulating in blood: Glucose.
- Form stored: Glycogen.
- Storage site: Liver, skeletal muscle.
- Percentage of total energy stored: 1%.
Proteins:
- Form absorbed across GI tract: Amino acids, some small peptides.
- Form circulating in blood: Amino acids.
- Form stored: Proteins.
- Storage site: Skeletal muscle.
- Percentage of total energy stored: 22%.
Lipids:
- Form absorbed across GI tract: Monoglycerides and fatty acids (in chylomicrons).
- Form circulating in blood: Free fatty acids, lipoproteins.
- Form stored: Triglycerides.
- Storage site: Adipose tissue.
- Percentage of total energy stored: 77%.
21.3 Energy Balance
- Energy Balance: Refers to the relationship between energy intake and energy expenditure.
- Positive Energy Balance: Occurs when energy intake exceeds energy expenditure, leading to weight gain.
- Negative Energy Balance: Occurs when energy expenditure exceeds energy intake, leading to weight loss.
- The concept of energy balance is critical in understanding body weight regulation and metabolism.
- Metabolic Rate (MR):
- Defined as the rate of energy expenditure by the body per unit time.
- Formula:
MR = BMR + \text{additional energy expenditure}
- Formula:
- BMR (Basal Metabolic Rate): The amount of energy expended while at rest in a neutrally temperate environment, in the post-absorptive state (the body has not eaten for 12 hours).
- Defined as the rate of energy expenditure by the body per unit time.
- Factors Influencing Metabolic Rate Include:
- Muscle activity
- Gender
- Body surface area
- Environmental temperature
Additional Implications and Insights
The understanding of energy metabolism is crucial for several fields including:
- Nutrition: Provides insights for dietary recommendations and management of body weight.
- Medicine: Helps in understanding metabolic disorders and conditions such as obesity and diabetes.
- Exercise Physiology: Aids athletes in optimizing performance and recovery by adjusting energy intake and utilization based on activity levels.
This knowledge and understanding assist in the broader context of health, wellness, and disease prevention, as metabolic dysregulation can lead to severe health consequences.