Integrative Metabolism

Respiratory quotient (RQ) = CO2 produced / O2 consumed: ~1.0 indicates carbohydrate metabolism, ~0.7 indicates fat metabolism, intermediate values reflect mixed fuel use.
Basal metabolic rate (BMR) = energy cost of essential functions at rest; influenced by age, sex, genetics, and especially lean body mass (more lean mass = higher BMR).
Body weight reflects energy intake vs. expenditure balance — excess energy is stored mainly as adipose triglycerides.
Hunger/appetite hormones: ghrelin (stomach) increases hunger; leptin (adipose tissue) decreases appetite, signals energy sufficiency; orexin promotes appetite and arousal.
Thyroid hormones raise BMR; insulin, glucagon, cortisol, and catecholamines coordinate fuel mobilization depending on nutritional/stress state — tying hormonal, tissue-level, and whole-body regulation together.

Respiratory quotient (RQ): Ratio of CO2 produced to O2 consumed; estimates which fuel (carb vs. fat) the body is using.
Basal metabolic rate (BMR): Energy required to maintain essential physiological functions at rest.
Ghrelin: Stomach hormone that increases hunger.
Leptin: Adipose-tissue hormone that decreases appetite and signals energy sufficiency.

Hormone	Source / Effect
Ghrelin	Stomach / Increases hunger
Leptin	Adipose tissue / Decreases appetite, signals energy sufficiency
Orexin	— / Promotes appetite and arousal

RQ ≈ 1.0 means carbohydrate use, NOT fat — a common inversion trap given fat's lower RQ (0.7).
Ghrelin increases hunger; leptin decreases appetite — opposite directions, easy to swap under time pressure.
BMR is about resting energy cost, not total daily energy expenditure (which also includes activity and diet-induced thermogenesis).

What RQ value indicates predominant fat metabolism?

What hormone increases hunger, and where is it produced?

What factor most influences BMR besides age and sex?

Biology chapters 1–3 are free. Sign up to unlock every chapter.