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Lipid Oxidation
- Lipids provide a greater energy yield compared to the catabolism of glucose or proteins.
- Only triglycerides are routinely oxidized for energy.
- The two building blocks of triglycerides: glycerol and fatty acids are oxidized separately.

Lipase enzymes separate fatty acids and glycerol backbones.
Glycerol is converted into glyceraldehyde-3-phosphate (G3P) which enters glycolysis to be converted into pyruvate.
- Yields about ½ the ATP compared to glucose since G3P is half of a glucose molecule.
- ATP yield = 15 ATP per glycerol.

Beta oxidation occurs in the mitochondria, where fatty acid chains are broken down into 2C acetic acid fragments.
Both FAD and NAD⁺ are reduced during this process and enter the electron transport chain (ETC) to generate ATP.
Each acetyl group binds to Coenzyme A forming acetyl-CoA, which then enters the citric acid cycle.

Triglycerides are synthesized from glycerol (derived from G3P) and fatty acids (synthesized from acetyl-CoA).
- This process occurs when cellular ATP and glucose levels are high.
- Glucose can be easily converted to fat as acetyl-CoA is an intermediate in its catabolism and a starting point for fatty acid synthesis.
- Dietary glycerol and fatty acids not needed for energy are stored as triglycerides.

Lipolysis is the breakdown of stored fats into glycerol and fatty acids.
- This process is accelerated when carbohydrate intake is inadequate.
- Fatty acids are preferred as fuel by the liver, cardiac muscle, and resting skeletal muscle.
- Beta oxidation of the released fatty acids produces large amounts of acetic acid acetyl-CoA.
Acetyl-CoA can enter the citric acid cycle only if sufficient intermediates are available; otherwise, it can accumulate.
Accumulated acetyl-CoA can be converted by ketogenesis in the liver to ketone bodies.
- Ketone bodies include acetoacetic acid, beta-hydroxybutyric acid, and acetone.
- The formation of ketone bodies is a primary metabolic result of rapid lipid breakdown in the body.

Ketosis is a metabolic state characterized by elevated ketone bodies in the blood, often occurring during prolonged fasting, carbohydrate restriction, or untreated diabetes.
The accumulation of ketone bodies in the blood can lead to ketosis which is observable through the excretion of ketone bodies in urine.
Ketones are acidic; their buildup can result in metabolic acidosis, which may cause dangerously low pH levels in the body.
- Patient's breath can smell fruity due to vaporizing acetone.
- Breathing rate increases to expel CO2 and increase blood pH. Severe cases can lead to shock or death.
- Metabolic acidosis may develop in patients with uncontrolled diabetes (diabetic acidosis).

Proteins degrade over time and need to be continually replaced.
Amino acids are recycled into new proteins or converted into different compounds.
Storage: Amino acids are not stored in the body; if dietary proteins are excessive, amino acids undergo deamination, which may convert them to energy, store them as fat, or excrete them as urea.

Transamination is the formation of nonessential amino acids via the transfer of an amine group (NH3) from an amino acid to a keto acid.
The resulting keto acid can then be converted into different nonessential amino acids.

Amino acids can be converted into intermediates of carbohydrate metabolism, starting typically with transamination or oxidative deamination.
The produced keto acid can enter the citric acid cycle or be converted into pyruvate or acetyl-CoA.

Refers to the time during which nutrients are being absorbed, lasting about 4 hours after each meal.
Cells utilize most of the glucose that enters the blood for energy needs, storing excess nutrients.

Nutrients absorbed from the digestive tract are carried by the blood to the liver.
- Amino acids are converted into energy-storage molecules such as glycogen, fatty acids, and triglycerides.
- The liver synthesizes proteins including plasma proteins from amino acids.
- Fatty acids and triglycerides produced by the liver are released into blood.

Occurs when the gastrointestinal tract is empty (post absorptive state), drawing energy from body fat and carbohydrate reserves.
Promotes the use of fats for energy (glucose sparing) to save glucose for organs such as the brain.

Nutrients Used and Stored
- Nutrients are stored in adipose tissue as triglycerides and in muscle as glycogen.
- Amino acids synthesized into proteins provide energy for tissues.

Using stored energy molecules as sources of energy: glycogen is converted to glucose, and triglycerides are converted to fatty acids.
Molecules from tissues are processed in the liver to create additional energy sources, leading to the release of glucose and ketones into the blood for distribution to tissues.

Key hormones include Insulin, Glucagon, Epinephrine, Growth Hormone, Thyroxine, Cortisol, and Testosterone.
- Insulin stimulates glucose uptake by cells, amino acid uptake, glycogenesis, and fat storage.
- Glucagon stimulates the processes that elevate blood glucose levels including glycogenolysis and gluconeogenesis.
- Epinephrine promotes lipolysis and mobilization of fats.

Bond energy from food must equal the total energy output; energy intake equals the energy released from food oxidation.
- Approximately 60% of energy is immediately lost as heat.
Energy output includes warmth due to metabolic reactions and energy stored as fat or glycogen based on body’s needs and hormonal signals.

Nearly all energy from food is converted to heat, which cannot be used for work but is vital for maintaining homeostatic body temperature and metabolic efficiency.
Body Temperature Regulation: reflects balance between heat production and heat loss, mainly generated by the liver, heart, brain, kidneys, and endocrine organs.
- During exercise, heat production from skeletal muscles increases significantly.
The hypothalamic thermoregulatory center integrates sensory input to regulate heat through mechanisms like vasodilation, vasoconstriction, sweating, and shivering.

Heat Production Mechanisms: Basal metabolism, muscular activity (shivering), action of thyroxine and epinephrine which increase BMR, and the temperature effect where warmer cells have a higher metabolism.
Heat Loss Mechanisms: Include radiation, conduction, convection, and evaporation.

Updated for 4-credit course and verified for accuracy and clarity by Stephen Taylor (stephen.taylor@dtcc.edu), with contributions by Julie Underwood, Laura Bianco, and John Kaminski.