2006A13 Describe the factors that influence metabolic rate.

Metabolic rate

·   Amount of energy consumed by the body per unit time

Basal metabolic rate

·   Amount of energy consumed by the body per unit time when at mental and physical rest, at a thermoneutral temperature, after a 12 hour fast

·   3-4mL/kg/min oxygen

·   Approx 8MJ/day

Physical factors

·   Body surface area ∝ heat loss rate ∝ MR

·   Lean mass ∝ MR (accounts for most of it)

o ↑with athletic training

o ↓with malnutrition

Activity

·   Physical exercise: ↑MR up to 20x (↑ATP consumption; note large and variable)

·   Mental stress, anxiety: ↑catecholamine release

Eating

·   Specific dynamic action of food – especially deamination of protein

·   Protein 20-30%, alcohol 20%, carbohydrate 5-10%, fat 0-5%

·   Lasts ~6 hours

Core temperature

·   Core temp ∝ rate of enzymatic reactions ∝ MR (↑1°C -> ↑MR 7%)

·   ↑T: sepsis, malignant hyperthermia

·   ↓T: cold exposure, hypothermic circulatory arrest

Ambient temperature

·   MR minimized in thermoneutral zone

o 18-22°C clothed adult

o 26-30°C naked adult

o 32-34°C neonate

Active heat loss

(Cause: ↑core temp or ↑ambient temp)

·   Sweating (active glandular secretion)

Active heat gain

(Cause: ↓core temp or ↓ambient temp)

·   Shivering: ↑MR 2-3x

·   Brown fat oxidation in neonates ↑MR 2-3x

o β₃ adrenoceptor activation

o Insertion of uncoupling protein into inner membrane

o Production of heat without ATP synthesis

·   Calorigenic effects of catecholamines on liver and skeletal muscle

Hormones

·   ↑Thyroid hormones -> ↑Na⁺K⁺ATPase activity -> ↑MR

·   ↑Catecholamines -> ↑cardiac work, calorigenic effects on liver and skeletal muscle

Special physiology

·   Neonate: ↑MR 2x (↑ratio surface area : mass)

·   Elderly: ↓MR (mainly ∝ lean mass)

·   Pregnant: ↑MR 1.2x (foetal growth/maintenance, ↑cardiac and resp work)

·   Lactate: ↑MR 1.2x

Anaesthesia

·   Immobility, inhibition of shivering -> ↓muscle activity -> ↓MR

·   ↓CMRO₂ to minimum 40% of normal

Disease

·   Sepsis -> ↑rate enzymatic reactions

·   Malignant hyperthermia -> ↑↑Skeletal muscle Ca²⁺ -> ↑MR

·   Fracture, wound healing, burns -> immune response, ↑protein requirement