When utilizing established formulas like Mifflin-St Jeor for TDEE estimation in individuals with higher body fat percentages, what is the primary critical limitation that necessitates empirical adjustment?

The primary critical limitation of established formulas like Mifflin-St Jeor for Total Daily Energy Expenditure (TDEE) estimation in individuals with higher body fat percentages is their reliance on total body weight as a primary variable, without adequately accounting for body composition. Mifflin-St Jeor is a predictive equation used to estimate Basal Metabolic Rate (BMR), which is the minimum energy required to sustain vital bodily functions at rest, considering an individual's age, sex, weight, and height. TDEE is then calculated by multiplying this BMR by an activity factor representing physical exertion. The critical limitation stems from the fundamental difference in metabolic activity between lean body mass and fat mass. Lean body mass, which includes muscle, bone, and organs, is significantly more metabolically active than fat mass. Formulas like Mifflin-St Jeor utilize total body weight as a proxy for metabolically active tissue but do not directly differentiate between the metabolic contributions of fat mass versus lean body mass. Consequently, for individuals with higher body fat percentages, a larger proportion of their total body weight consists of metabolically less active fat. The formula implicitly attributes a higher metabolic rate to this fat mass than it actually possesses, leading to an overestimation of their true BMR and, subsequently, their TDEE. This inherent inaccuracy in reflecting the actual metabolically active tissue necessitates empirical adjustment, which means practical, observed data often dictates a downward modification of the calculated energy needs to align with the lower actual energy expenditure of individuals with a higher proportion of fat mass.