2006A13 Describe the factors that influence metabolic rate.

 

List:

·       Definitions

·       Factors affecting metabolic rate

 

Definitions:

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

 

Factors affecting metabolic rate:

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