Basal metabolic rate reflects the energy cost of maintaining cellular and organ function at rest; it varies with age, sex, body composition, and thyroid status. Total energy expenditure includes basal rate, thermoregulation (including cold and diet-induced thermogenesis), and activity. Hormones (thyroid, catecholamines) modulate metabolic rate to match demands; chronic overfeeding increases metabolic efficiency and resistance to weight loss.
From your study of thyroid hormones and thermoregulation, you understand that T3 increases oxygen consumption and heat production in nearly every tissue. Energy expenditure and metabolic rate are the whole-body manifestation of these cellular processes — the total rate at which your body converts chemical energy from food into heat and work. Understanding the components of energy expenditure explains why two people of similar size can have very different caloric needs.
Basal metabolic rate (BMR) is the energy your body uses at complete rest, in a thermoneutral environment, after an overnight fast. It accounts for 60–75% of total daily energy expenditure in most sedentary people — a surprisingly large share that reflects the continuous cost of maintaining ion gradients (especially the Na-K-ATPase, which alone consumes roughly 20–30% of BMR), synthesizing proteins, and fueling obligate organ metabolism. The brain, liver, heart, and kidneys together account for about 60% of BMR despite comprising only 5–6% of body mass. Skeletal muscle, because of its large total mass, contributes another 20–25%. This is why body composition matters: lean tissue is metabolically expensive, and individuals with more muscle mass have higher BMRs even at the same body weight.
On top of BMR, two additional components make up total energy expenditure. Diet-induced thermogenesis (also called the thermic effect of food) represents the energy cost of digesting, absorbing, and processing nutrients — typically 8–15% of caloric intake. Protein has the highest thermic effect (~20–30% of its caloric content), while fat has the lowest (~2–3%). Activity thermogenesis includes both planned exercise and non-exercise activity thermogenesis (NEAT) — fidgeting, postural maintenance, and spontaneous movement. NEAT varies enormously between individuals and is a major reason why some people seem resistant to weight gain: their NEAT increases unconsciously with overfeeding.
Hormonal regulation ties these components together. Thyroid hormones set the baseline metabolic tempo by upregulating metabolic enzymes and uncoupling proteins. Catecholamines (epinephrine and norepinephrine) acutely increase metabolic rate during stress or cold exposure by activating beta-adrenergic receptors, stimulating lipolysis, and activating brown adipose tissue for non-shivering thermogenesis. Insulin and glucagon coordinate fuel availability to match expenditure. During prolonged caloric restriction, a phenomenon called metabolic adaptation occurs: BMR drops beyond what can be explained by lost tissue mass alone, driven by decreased thyroid hormone conversion, reduced sympathetic tone, and increased metabolic efficiency. This adaptive response — essentially the body defending its energy stores — is a major reason why sustained weight loss is physiologically difficult and why weight regain is common after dieting.