BMR alone should never be used to set a diet target — it tells you what your body burns doing nothing, not what it burns living your actual life. Using BMR as a calorie target (rather than TDEE, which factors in activity) puts almost anyone into an unintentionally severe deficit, which is one of the more common mistakes people make when they encounter a BMR number without understanding what it represents.

The "slow metabolism" explanation for weight gain is largely a myth in the way it's commonly used — genuine metabolic differences between individuals of similar size are real but modest, while the actual adaptive thermogenesis that occurs during sustained dieting (a real, measurable 15-20% drop in BMR beyond what weight loss alone explains) is a different and more nuanced phenomenon than a permanently "broken" metabolism.

Formula-based BMR estimates carry a real margin of error (roughly ±10%) even though Mifflin-St Jeor is the most accurate of the commonly used equations. For clinical precision — useful for elite athletes or anyone with a suspected metabolic or thyroid issue — indirect calorimetry, which directly measures oxygen consumption, is the gold-standard alternative worth discussing with a doctor.