Nutrition, Metabolism, and Energy Balance: Study Notes for Human Anatomy & Physiology

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Protein Metabolism

Overview of Protein Metabolism

Proteins in the body are constantly being broken down and replaced. Unlike carbohydrates and fats, proteins are not stored for future use. When dietary proteins are in excess, amino acids are either oxidized for energy or converted to fat for storage.

Deamination: The first step in amino acid breakdown, where the amine group (NH2) is removed.
Fate of Amino Acids: After deamination, amino acids are converted into pyruvic acid or keto acid intermediates of the citric acid cycle.

Three Events of Amino Acid Degradation

Transamination: Transfer of an amine group from an amino acid to α-ketoglutaric acid, forming glutamic acid and a keto acid.
Oxidative Deamination: In the liver, the amine group of glutamic acid is removed as ammonia (NH3), which combines with CO2 to form urea, excreted in urine.
Keto Acid Modification: Keto acids are altered to produce metabolites (e.g., pyruvic acid, acetyl CoA) that enter the citric acid cycle or contribute to gluconeogenesis.

Amino acid degradation pathways in the liver Summary of transamination and oxidative deamination

Example: During fasting, muscle proteins are broken down, and amino acids are used for gluconeogenesis in the liver.

Catabolic-Anabolic Steady State

Dynamic Metabolic State

The body is in a continuous state of breaking down (catabolism) and rebuilding (anabolism) organic molecules. Nutrient pools of amino acids, carbohydrates, and fats are interconvertible, with the liver, adipose tissue, and skeletal muscle directing these conversions.

Amino Acid Pool: Used for protein synthesis, amino acid derivatives, and gluconeogenesis.
Carbohydrate and Fat Pools: Easily interconverted and directly oxidized for energy; excess can be stored, but amino acids cannot be stored as proteins.

Interconversion of carbohydrates, fats, and proteins

Metabolic States of the Body

Absorptive (Fed) State

The absorptive state lasts about four hours after eating, during which anabolism exceeds catabolism and nutrients are stored.

Carbohydrates: Glucose is used for energy or stored as glycogen/fat.
Triglycerides: Hydrolyzed and used for energy or stored in adipose tissue.
Amino Acids: Used for protein synthesis or converted to fat if in excess.

Major events of the absorptive state Principal pathways of the absorptive state

Hormonal Control: Insulin is the primary hormone, promoting glucose uptake, glycogen and fat synthesis, and protein synthesis.

Insulin directs events of the absorptive state

Postabsorptive (Fasting) State

When the GI tract is empty, the body relies on stored reserves. Catabolism exceeds anabolism, and the goal is to maintain blood glucose for organs like the brain.

Sources of Blood Glucose: Glycogenolysis (liver and muscle), lipolysis (adipose tissue), and protein catabolism (during prolonged fasting).
Glucose Sparing: Most tissues use fatty acids, sparing glucose for the brain; ketone bodies are produced during prolonged fasting.

Major events of the postabsorptive state Principal pathways of the postabsorptive state

Hormonal Control: Glucagon is the main hormone, stimulating glycogenolysis, gluconeogenesis, and lipolysis. The sympathetic nervous system and other hormones also play roles.

Glucagon stimulates a rise in blood glucose

Hormonal Influences on Metabolism

Hormone	Effect on Metabolism
Insulin	Stimulates glucose uptake, glycogenesis, lipogenesis, protein synthesis; inhibits gluconeogenesis and glycogenolysis
Glucagon	Stimulates glycogenolysis, gluconeogenesis, lipolysis
Growth Hormone	Stimulates protein synthesis, lipolysis
Thyroxine	Increases metabolic rate, stimulates glucose and fat metabolism
Cortisol	Stimulates gluconeogenesis, protein breakdown
Testosterone	Stimulates protein synthesis

Summary of normal hormonal influences on metabolism

Metabolic Role of the Liver

Functions of the Liver in Metabolism

The liver is central to metabolic regulation, processing nutrients, regulating cholesterol, storing vitamins/minerals, and detoxifying substances.

Metabolic Process	Function
Carbohydrate Metabolism	Glycogen storage, glucose release, gluconeogenesis
Fat Metabolism	Beta-oxidation, lipoprotein synthesis, cholesterol synthesis
Protein Metabolism	Deamination, urea formation, plasma protein synthesis
Vitamin/Mineral Storage	Stores vitamins A, D, B12, iron, copper
Biotransformation	Detoxifies drugs, hormones, bilirubin

Summary of metabolic functions of the liver

Cholesterol Metabolism and Regulation

Cholesterol Transport and Lipoproteins

Cholesterol is essential for cell membranes, bile salts, and steroid hormones. It is transported in the blood by lipoproteins:

HDLs (High-Density Lipoproteins): Remove excess cholesterol from tissues to the liver.
LDLs (Low-Density Lipoproteins): Deliver cholesterol to tissues; high levels are linked to atherosclerosis.
VLDLs (Very Low-Density Lipoproteins): Transport triglycerides from the liver to tissues.
Chylomicrons: Carry dietary lipids from the intestine to tissues.

Composition and function of lipoproteins

Recommended Levels: Total cholesterol < 200 mg/dl, HDL > 60 mg/dl, LDL < 100 mg/dl.

Energy Balance and Regulation of Food Intake

Energy Balance

Energy intake must equal energy output for weight stability. Energy output includes heat, work, and storage. Nearly all food energy is eventually converted to heat, which maintains body temperature and allows metabolic reactions.

Regulation of Food Intake

Food intake is regulated by hypothalamic centers, neural and hormonal signals, and psychological factors. Short-term controls include neural signals from the GI tract and nutrient levels; long-term controls involve hormones like leptin.

Model for hypothalamic command of appetite and food intake

Short-term: Vagal signals, blood glucose, gut hormones (insulin, CCK, ghrelin).
Long-term: Leptin from adipose tissue suppresses appetite.

Metabolic Rate and Body Temperature Regulation

Metabolic Rate

Metabolic rate is the total heat produced by the body. Basal metabolic rate (BMR) reflects energy needed for essential activities and is influenced by age, gender, temperature, stress, and thyroxine.

Body Temperature Regulation

Body temperature is maintained by balancing heat production and loss. The hypothalamus acts as the main thermoregulatory center, receiving input from peripheral and central thermoreceptors.

Heat production and heat loss balance

Heat Production: Basal metabolism, muscular activity, thyroxine, and temperature effects.
Heat Loss: Radiation, conduction/convection, evaporation.

Mechanisms of heat exchange Mechanisms of body temperature regulation

Clinical Correlations and Disorders

Diabetes Mellitus: Inadequate insulin leads to high blood glucose, protein/fat catabolism, and metabolic acidosis.
Obesity: Defined by BMI; increases risk for cardiovascular disease, diabetes, and other conditions.
Metabolic Syndrome: Cluster of risk factors (waist circumference, blood pressure, glucose, triglycerides, HDL) that increase risk for heart disease and diabetes.

Metabolic syndrome illustration

Developmental Aspects and Inborn Errors

Phenylketonuria (PKU): Inability to metabolize phenylalanine, leading to neurotoxicity.
Galactosemia: Inability to convert galactose to glucose, causing mental deficits.
Glycogen Storage Disease: Deficiency in enzymes for glycogen breakdown.

Additional info: Metabolic rate declines with age, and various medications and lifestyle factors can influence nutrition and metabolism.