SN4 SHORT NOTE 5M ★ High-yield

Define BMR. Describe the conditions for its measurement and the factors affecting it.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Basal metabolic rate (BMR) = the minimum energy required to maintain the vital functions of the body at complete physical and mental rest, measured in a thermoneutral environment, 12–18 hours after the last meal, while the subject is awake.

Normal Values and Measurement

Approximately 24 kcal/kg body weight/day, or 1 kcal/kg/hour
Adult male 1600–1800 kcal/day; adult female 1300–1500 kcal/day
Expressed per unit body surface area: about 40 kcal/m2/hour in men, 37 in women
Accounts for 60–70% of total daily energy expenditure
Measured by indirect calorimetry — oxygen consumption, using the Benedict–Roth apparatus or a modern metabolic cart. Direct calorimetry measures heat output but is impractical

Conditions for Measurement

Complete physical and mental rest, lying down, awake
Post-absorptive — 12 to 18 hours after the last meal, so no SDA
Thermoneutral environment, about 20–25 °C
No exercise for at least 1 hour; no emotional stress
Usually measured in the early morning

Factors Affecting BMR

Factor	Effect	Explanation
Body surface area	Directly proportional	Heat loss is from the surface — the best single correlate
Lean body mass	↑	Muscle is metabolically active; fat is not
Sex	Male > female by 5–10%	Greater muscle mass, less fat
Age	Highest in infancy, falls with age	Growth and higher surface-to-volume ratio in the young
Thyroid hormone	↑↑ in hyperthyroidism (up to +100%); ↓ in hypothyroidism	The most powerful hormonal influence
Fever	+13% per °C rise	Increased enzyme activity
Climate	Higher in cold, lower in tropics	Heat production requirement
Pregnancy and lactation	↑ 5–20%	Fetal and mammary metabolism
Starvation and undernutrition	↓ up to 30%	Adaptive conservation — the reason weight loss plateaus
Sleep	↓ about 10%	Reduced muscle tone
Adrenaline, growth hormone, cortisol	↑	Catabolic effect
Infection, burns, trauma, surgery	↑ markedly	Hypermetabolic response

Applied Aspects

BMR was once the standard test of thyroid function — long superseded by serum TSH and free T4, which are specific and far simpler
The adaptive fall in starvation explains why weight loss slows on a restricted diet, and why regain is so common
Estimating energy requirements in the critically ill uses BMR-based formulas such as Harris–Benedict with stress factors, though indirect calorimetry is more accurate
Fever, sepsis and burns raise requirements substantially, and underfeeding these patients delays recovery
Obesity is not usually due to a low BMR — a heavier person has a higher absolute BMR; the imbalance is in intake and activity

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