Human Metabolism and Energy Balance

Adipocytes and Leptin

• Adipocytes secrete leptin, a hormone.
• Most obese humans are not deficient in leptin.

Brain Neurotransmitters and Food Intake

• Neuropeptide Y is a brain neurotransmitter that stimulates food intake.

Thermodynamics and Energy Balance

• The first law of thermodynamics states that energy cannot be created or destroyed.
• Energy input must equal energy output.
• One kilocalorie (kcal) is the energy needed to raise one liter of water by 1°C.
• A bomb calorimeter measures heat produced from food combustion.

Metabolic Rate and Respiratory Quotient

• Metabolic rate is determined by measuring heat release and oxygen consumption.
• Respiratory quotient is highest for carbohydrates compared to proteins and fats.
• Basal metabolic rate is best estimated when a person is resting after a 12-hour fast.

Metabolism and Anabolism

• Metabolism encompasses all chemical reactions in an organism.
• Anabolism is the synthesis of biomolecules, requiring energy input.
• The brain primarily uses glucose for energy. Without it, brain cells die quickly.

Gluconeogenesis and Enzymes

• Gluconeogenesis converts non-carbohydrate precursors into glucose (glycerol, amino acids, lactate).
• Lipase is an enzyme that digests fats.
• The liver and skeletal muscles store glucose as glycogen for energy.

Nutrient Energy Value

• Fats provide the most energy per gram when metabolized.
• Protein ingestion leads to the highest diet-induced thermogenesis.

Fasting State and Energy Sources

• During fasting, adipose tissue releases glucose to support the body.
• High-density lipoproteins (HDLs) carry cholesterol and phospholipids from tissues to the liver.

Amino Acid Metabolism

• Deamination precedes amino acid metabolism for energy.
• Nitrogen from amino acid metabolism is converted to urea and excreted by the kidneys.
• Lipolysis breaks down fats into glycerol and fatty acids.

Fatty Acid Metabolism

• Beta-oxidation breaks down fatty acids into two-carbon units within mitochondria.
• Ketone bodies are harmful byproducts of fatty acid metabolism.

Pancreatic Hormones

• Alpha cells in the pancreas produce glucagon.
• Beta cells in the pancreas produce insulin.
• Insulin is released when blood glucose levels are high ("fed" state).
• Glucagon is released when blood glucose levels are low ("fasting" state).
• Feeding and satiety centers are in the hypothalamus.

Absorptive and Fasting States

• During the absorptive state (high blood glucose, insulin), the liver synthesizes glycogen.
• During starvation, gluconeogenesis rapidly increases.
• Thyroid hormones and epinephrine increase basal metabolic rate (BMR).

Insulin and its Function

• Insulin stimulates glycolysis and inhibits gluconeogenesis.
• Glucagon stimulates gluconeogenesis, mostly in the liver.
• Insulin secretion is stimulated by parasympathetic neurons.
• GLUT4 transporters are stored in vesicles and found in adipose and muscle tissue.

Diabetes and Hyperglycemic State

• Dehydration can result from heat exhaustion and diabetes.
• In type I diabetes, a hyperglycemic hyperosmotic state can occur. This means plasma glucose and blood osmolarity are elevated above normal.

Thermoregulation

• Homeothermy involves maintaining a constant body temperature.
• Convective heat loss involves warm air rising from the body.
• Radiation is the primary method of heat loss from the body.
• Heat loss is also promoted by sweating and dilated cutaneous blood vessels.
• The hypothalamus governs responses to temperature changes.

Diabetic Responses to Carbohydrate Intake

• Following carbohydrate ingestion in an insulin-deficient diabetic, protein breakdown and ketone production occur.

Treatment and Signs of Diabetes

• Drugs can stimulate beta-cell insulin secretion.
• High fatty acid metabolism increases ketone body levels.
• Symptoms of type I diabetes include high blood glucose (hyperglycemia), glucose in urine (glucosuria), ketoacidosis, thirst, and excessive urination (polydipsia).

Type II Diabetes

• In type II diabetes, insulin levels are often normal but cells are resistant to insulin's effects, suggesting a defect in the signal transduction pathway.

Basal Metabolic Rate

• An individual needing to consume more calories to maintain weight = higher oxygen consumption, i.e. Bill consuming 20 liters of oxygen per hour.

Traveler's Diarrhea and Ketosis

• Ketosis, increased blood urea, decreased blood pH, and increased gluconeogenesis are signs of using alternative energy sources due to reduced carbohydrate intake (as in traveler's diarrhea).

Insulin and Glucagon Actions

• Insulin and glucagon, both peptide hormones that target liver cells, have opposite effects. This indicates that each hormone activates a different secondary messenger system.

Brown Fat

• Brown fat is innervated by the sympathetic nervous system.
• Brown fat is found in infants.
• Brown fat helps with non-shivering thermogenesis.
• Brown fat has a rich vascular supply.