Notes on Metabolism and Regulation of Body Temperature

The Liver and Its Functions

  • The liver is the body’s key metabolic organ, critically involved in biochemistry.
  • Main metabolic functions include:
    • Packaging free fatty acids (FFA) for storage and transport.
    • Synthesis of plasma proteins essential for various bodily functions.
    • Formation of nonessential amino acids (a.a.); deaminating a.a. and converting ammonia into urea.
    • Glucose regulation through storage as glycogen and gluconeogenesis, crucial for blood sugar levels.
    • Vitamin storage: stores vitamins A, D, and B12.
    • Iron conservation: salvaged iron from worn-out RBCs as ferritin.
    • Hormonal degradation for excretion.
    • Toxic substance metabolism: including alcohol and various drugs.

Lipoproteins

  • Four major types of lipoproteins transport triglycerides (TGs) and cholesterol in the blood:
    • High-Density Lipoproteins (HDLs): Considered "good" cholesterol; transports cholesterol for degradation.
    • Low-Density Lipoproteins (LDLs): Considered "bad" cholesterol; excessive amounts can lead to arterial cholesterol deposits.
  • Composition:
    • Lipoproteins contain TGs, phospholipids, cholesterol, and protein; higher lipid percentage equates to lower density.
  • Cholesterol intake should aim for below 200 mg/dl of blood.
  • Reducing saturated fats is crucial as they promote synthesis and inhibit excretion of cholesterol.

Thyroid Hormones and Metabolic Regulation

  • Thyroid hormone (TH) is central to regulating metabolic rate, with key structures:
    • The butterfly-shaped thyroid gland located inferior to the larynx; rich blood supply.
    • Main hormones produced:
    • Thyroxine (T4): predominant hormone, converted to T3 in target tissues.
    • Triiodothyronine (T3): more active and co-regulates various bodily functions.
  • Functions:
    • Increases basal metabolic rate (BMR) and heat production (calorigenic effect).
    • Regulates tissue growth and development, critical for fetal and infant development, and reproductive function.
  • Transport & Regulation:
    • T3 is more potent than T4 and binds more strongly to receptors.
    • Peripheral tissues convert T4 to T3; normal regulation involves a negative feedback system with TSH and TRH.

Hypothyroidism

  • Causes may include:
    • Autoimmune issues like Hashimoto’s disease.
    • Inadequate TSH or TRH secretion.
    • Dietary iodine deficiency, leading to goiter due to excess thyroglobulin production without iodine.
  • Symptoms include low BMR, cold intolerance, skin issues, and lethargy.
  • Treatment may involve iodine or hormone replacement (TSH/T4).

Hyperthyroidism

  • Often caused by Grave’s Disease, an autoimmune condition.
  • Symptoms include elevated BMR, weight loss, sweating, and nervousness.
  • Treatments can involve radioactive iodine or thyroid removal.

Calcitonin

  • Secreted by parafollicular cells; it lowers blood calcium levels by inhibiting bone resorption and promoting calcium uptake into bone.

Metabolism Metrics

  • Metabolic Rate: Total energy expenditure rate, including BMR; calculated in kcal/m²/hr.
  • Basal Metabolic Rate (BMR): Energy expenditure at rest under controlled conditions, vital for maintaining essential bodily functions.
    • Influenced by factors such as age, gender, surface area (major factor affecting heat loss), and stress.

Appetite Regulation

  • Balance between energy intake and output is crucial for maintaining weight:
    • Hunger: The physiological need for energy; driven by neural and hormonal signals.
    • Satiety: The satisfaction signifying the end of eating.
  • Influences:
    • Neural signals and hormones (like insulin and leptin) help regulate appetite, with leptin signaling satiety.
    • The influence of macronutrients on appetite and satiety through certain hormones and neuronal pathways.

Summary of Key Hormones Affecting Appetite

  • Insulin: Depresses hunger, acts as a satiety signal post-meal.
  • Cholecystokinin: Released during digestion, inhibits hunger signals.
  • Ghrelin: Stimulation of hunger produced by the stomach, especially in fasting.
  • Neuropeptide Y (NPY) promotes hunger, while POMC/CART suppresses appetite.

Other Influencing Factors

  • Body Temperature: Increased temperatures may decrease hunger levels, while hunger switches in colder climates could see increased caloric intake.
  • Psychological Factors: Emotional states impact hunger levels; the higher set point theory suggests obesity may be linked to body fat storage signals.

Key Takeaway

  • The complex interaction between metabolism, hormones, and regulatory feedback systems shows how body temperature and nutritional states dictate energy intake and overall health.