Two people have identical total body weight (80 kg), but Person A has 65 kg of lean mass and 15 kg of fat, while Person B has 50 kg of lean mass and 30 kg of fat. Who has a higher basal metabolic rate, and why?
APerson B, because more fat tissue stores more energy and therefore burns more at rest
BThey are identical, because total body weight determines BMR
CPerson A, because lean tissue (muscle and organs) is metabolically more expensive to maintain at rest than fat tissue
DPerson B, because fat tissue requires more oxygen for lipid storage reactions
Lean tissue (skeletal muscle, organs) has much higher resting metabolic activity than adipose tissue. The brain, liver, heart, and kidneys together account for about 60% of BMR despite being only 5–6% of body mass; skeletal muscle contributes another 20–25%. Fat tissue is metabolically inexpensive — it primarily stores lipids passively. At the same total body weight, more lean mass means a significantly higher BMR because more metabolically active tissue must be maintained. This is why body composition, not just weight, determines caloric needs — and why muscle-preserving interventions during weight loss help maintain metabolic rate.
Question 2 Multiple Choice
A person successfully loses 15 kg over six months through caloric restriction. After the weight loss, their measured BMR is significantly lower than predicted by their new body composition alone. What best explains this discrepancy?
ATheir organs have shrunk proportionally to their weight loss, reducing their metabolic demand
BMetabolic adaptation — BMR drops beyond what tissue loss explains, driven by reduced thyroid hormone conversion, decreased sympathetic tone, and increased metabolic efficiency
CTheir NEAT has increased, which suppresses BMR to maintain energy balance
DThe caloric restriction has permanently damaged their mitochondria, reducing ATP production capacity
Metabolic adaptation (adaptive thermogenesis) is a real phenomenon: during sustained caloric restriction, BMR decreases beyond what can be explained by the loss of metabolically active tissue alone. The mechanisms include reduced T3 (active thyroid hormone) from decreased T4-to-T3 conversion, decreased sympathetic nervous system activity, and increased metabolic efficiency per unit of tissue. This response represents the body actively defending its energy stores — an evolutionary adaptation that made sense in environments with unpredictable food availability but frustrates modern weight management. It explains why weight loss becomes progressively harder and why weight regain is common.
Question 3 True / False
In most sedentary people, physical exercise accounts for the majority of daily energy expenditure.
TTrue
FFalse
Answer: False
This is one of the most common misconceptions about energy balance. Basal metabolic rate (BMR) accounts for 60–75% of total daily energy expenditure in sedentary individuals — a surprisingly large fraction that reflects the continuous, unavoidable cost of maintaining organ function, ion gradients, protein synthesis, and cellular homeostasis. Even with moderate exercise, most people's total activity thermogenesis (exercise + NEAT) accounts for only 15–30% of total expenditure. This has practical implications: exercise alone has a modest effect on total energy expenditure, while interventions that raise BMR (increasing lean mass, optimizing thyroid function) have a larger sustained impact.
Question 4 True / False
Among the three macronutrients, protein has the highest thermic effect — meaning eating protein costs more metabolic energy to digest and process than the same caloric amount of fat or carbohydrate.
TTrue
FFalse
Answer: True
Diet-induced thermogenesis (the thermic effect of food) represents the energy cost of digesting, absorbing, and metabolically processing nutrients. Protein requires ~20–30% of its caloric content just to process (deamination, urea cycle, gluconeogenesis from amino acids are all energetically expensive). Carbohydrates cost ~5–10% to process, while fat is cheapest at ~2–3%. This means 100 kcal of protein yields only ~70–80 kcal of net metabolic energy, while 100 kcal of fat yields ~97–98 kcal net. This is part of why high-protein diets modestly increase total energy expenditure even without changes in activity.
Question 5 Short Answer
Why does metabolic adaptation during caloric restriction represent the body 'defending its energy stores,' and what physiological mechanisms drive this defense?
Think about your answer, then reveal below.
Model answer: Metabolic adaptation is an evolved response to famine conditions. When caloric intake drops below expenditure, the body responds by reducing its energy needs through multiple hormonal mechanisms: (1) decreased conversion of T4 to the more active T3, reducing the metabolic rate per unit tissue; (2) reduced sympathetic nervous system tone, decreasing catecholamine-driven thermogenesis; and (3) increased metabolic efficiency, meaning less energy is wasted as heat per unit of cellular work. The net effect is that BMR falls beyond what lost tissue mass alone would predict — the body is doing more with less to preserve its energy reserves. This is 'defending energy stores' because the adaptations collectively slow the rate at which stored fat is consumed, extending survival in conditions of food scarcity.
Metabolic adaptation is why weight loss is not linear: the same caloric deficit that produced 1 kg/week of loss initially may produce only 0.3 kg/week months later, even if behavior is unchanged. It also explains the clinical observation that former dieters regain weight rapidly when returning to prior intake — their now-lower BMR means that what was previously a maintenance intake becomes a caloric surplus. This biology is not a failure of willpower but a physiological response the body executes automatically.