basal metabolic rate

Basal metabolic rate is the energy your body burns just to stay alive while completely at rest — the “idle speed” of the engine. Even if you lay perfectly still all day, your heart would keep beating, your brain would keep thinking, and your cells would keep their machinery running, and all of that costs calories.

BMR is measured under strict conditions: awake but resting, fasted, and in a comfortable temperature. It typically accounts for around 60–70% of total daily energy expenditure, far more than exercise for most people. The biggest determinant is lean body mass (especially organs and muscle), which is why larger and more muscular people have higher BMRs. Thyroid hormones (T3 and T4) are the main endocrine regulators, setting the metabolic tempo of nearly every tissue.

Hormonal disease shifts BMR noticeably: it rises in hyperthyroidism (causing heat intolerance and weight loss) and falls in hypothyroidism (causing cold intolerance and weight gain). BMR also drops during weight loss by more than body-size changes alone would predict — a form of adaptive thermogenesis that helps explain weight regain.