Energy Metabolism and Storage in Human Physiology

Energy Release from Fats

Overview of Fat Metabolism

Fats are a major source of energy in the human body, especially during periods of fasting or prolonged exercise. The breakdown of triglycerides releases significant amounts of energy through a series of metabolic pathways.

• Triglycerides are broken down by lipases into glycerol and three fatty acids.

• Glycerol (a 3-carbon molecule) can be converted to dihydroxyacetone phosphate and enter glycolysis and oxidative phosphorylation.

• Fatty acids undergo beta-oxidation in the mitochondria, where two-carbon units are sequentially removed to form acetyl CoA.

• Acetyl CoA enters the Krebs cycle (citric acid cycle), producing NADH, FADH2, and ATP.

• NADH and FADH2 donate electrons to the electron transport chain (ETC), generating additional ATP.

Example: Complete oxidation of one molecule of palmitic acid (a common fatty acid) yields approximately 106 ATP.

• Each round of beta-oxidation produces NADH and FADH2, which are used in the ETC.

• For palmitic acid (16 carbons):

  • 8 cycles of beta-oxidation produce 8 acetyl CoA.

  • Each acetyl CoA yields 10 ATP via the Krebs cycle and ETC.

  • Additional ATP is produced from NADH and FADH2 generated during beta-oxidation.

Key Equations:

• $\text{Net yield for palmitic acid} = 106 \text{ ATP}$

• $\text{Total ATP from complete oxidation of palmitic acid} \approx 334 \text{ ATP}$ Additional info: This value includes ATP from all steps, including activation and conversion processes.

Energy Storage: Function of Adipocytes

Role of Adipocytes in Energy Homeostasis

Adipocytes, or fat cells, are specialized for the storage and release of fat. They play a crucial role in energy balance, especially during periods of food scarcity or increased energy demand.

• Adipocytes store fat as triglycerides during periods of food uptake.

• During fasting or exercise, stored fat is mobilized and released into the bloodstream for energy production.

• Fat is stored in subcutaneous and visceral depots throughout the body.

Body Energy Stores:

• 15 kg fat = 140,000 kcal

• 9 kg protein = 38,000 kcal

• 0.5 kg carbohydrate (CHO) = 2,000 kcal

Example: The majority of stored energy in the human body is in the form of fat, which provides a dense and efficient energy reserve.

Metabolic Pathways: Carbohydrates, Fats, and Proteins

Overview of Nutrient Metabolism

The body metabolizes carbohydrates, fats, and proteins through distinct but interconnected pathways to meet energy demands.

• Carbohydrates are metabolized via glycolysis and stored as glycogen.

• Fats are metabolized via beta-oxidation and stored as triglycerides.

• Proteins are broken down into amino acids, which can be used for energy during stress, malnutrition, or disease states.

Energy Content of Nutrients:

Nutrient	Calorimeter (kcal/g)	Metabolizable Energy (kcal/g)
Carbohydrate	4.1	4
Fats	9.4	9
Proteins	5.6	4*
Alcohol	7.0	7
Fiber	No calories	-

*Additional info: The difference in protein energy is due to the energy content of urea, which cannot be further metabolized in the body.

Key Terminology in Metabolism

Important Terms and Their Meanings

Understanding metabolic terminology is essential for grasping the biochemical processes involved in energy production and storage.

• -genesis: Formation or creation (e.g., glycogenesis is the formation of glycogen).

• -lysis: Breakdown or destruction (e.g., glycogenolysis is the breakdown of glycogen).

• Phosphatase: An enzyme that removes a phosphate group from a molecule.

• Glycogenesis: The process of synthesizing glycogen from glucose.

• Glycogenolysis: The process of breaking down glycogen into glucose.

Metabolic Pathways: Fed and Fasted States

Energy Utilization and Storage

The body alternates between storing and utilizing nutrients depending on the fed or fasted state.

• In the fed state, nutrients are stored as glycogen, fat, or used for protein synthesis.

• In the fasted state, stored nutrients are mobilized and oxidized for energy.

• Carbohydrates provide rapid energy, especially for the brain and nervous tissue.

• Fats are a slower but more energy-dense source, requiring carbohydrate 'primers' for efficient oxidation.

• Proteins are used for energy primarily during stress, malnutrition, or disease.

Summary Table: Metabolic Pathways of Major Nutrients

Comparison of Carbohydrate, Fat, and Protein Metabolism

Pathway	Carbohydrates	Fats	Proteins
Storage Form	Glycogen	Triglycerides	Body Proteins
Main Metabolic Pathway	Glycolysis	Beta-oxidation	Deamination
Energy Yield (kcal/g)	4	9	4
Special Notes	Rapid energy for brain	Slow, requires CHO primer	Used in stress/malnutrition

Additional Info

• During exercise, the rate of energy supply and the type of substrate used depends on intensity, duration, and availability of nutrients.

• Oxidative phosphorylation is the final step in aerobic metabolism, producing water and carbon dioxide as byproducts.

• Energy balance is regulated by hormones and substrate availability.