Questions: Energy Metabolism, Caloric Needs, and Basal Metabolic Rate

Question 1

A person's BMR is measured at 1,600 kcal/day. Which statement best describes what this means?

• A. They must eat exactly 1,600 kcal/day to avoid weight gain
• B. They burn 1,600 kcal/day through exercise and daily movement
• C. Their body requires approximately 1,600 kcal/day to sustain vital functions at complete rest
• D. They will lose weight if they consume fewer than 1,600 kcal/day

Answer: (C)

BMR is the energy needed to sustain essential functions — breathing, circulation, thermoregulation, cell repair — at complete rest. It does not include physical activity or the thermic effect of food. TDEE is higher than BMR because it adds these components (typically BMR × 1.2–1.9 depending on activity level). Eating at BMR would create a caloric deficit for anyone who is not completely immobile.

Question 2

Because human metabolism obeys the first law of thermodynamics, reducing caloric intake by exactly 500 kcal/day will reliably produce exactly 1 pound of fat loss per week, indefinitely.

• T. True
• F. False

Answer: False

The first law applies, but biological adaptation complicates simple arithmetic. Sustained caloric restriction triggers adaptive thermogenesis — the body lowers BMR, reducing the effective deficit. Hormonal changes (e.g., decreased leptin, increased ghrelin) increase hunger and decrease energy expenditure. The '500 kcal = 1 lb/week' rule is a useful approximation for the short term but systematically overestimates fat loss over longer periods.

Question 3

Two people have the same total body weight but very different BMRs. What is the most likely explanation?

Model answer: BMR is driven primarily by fat-free mass (muscle, organs, bone), not total weight. Two people at the same weight can differ substantially in body composition — the person with more muscle mass will have a significantly higher BMR, because metabolically active lean tissue demands more energy at rest than adipose tissue.

This is why resistance training raises BMR over time, and why aggressive weight loss that reduces muscle mass can lower BMR — making further weight loss harder. Other contributors to BMR variation include age (declines ~2% per decade), sex, thyroid hormone levels, and genetic factors.