Why Katch-McArdle Over Mifflin-St Jeor?
Basal Metabolic Rate (BMR) represents the energy your body expends performing the most basic life-sustaining functions: breathing, circulating blood, regulating body temperature, growing and repairing cells, and maintaining brain and nerve activity. According to the National Institutes of Health (NIH), BMR accounts for 60–75% of total daily energy expenditure (TDEE) in sedentary adults — making it the foundational variable for any evidence-based nutrition protocol.
The Katch-McArdle formula is considered the most precise predictive equation for individuals who know their body fat percentage. The Mifflin-St Jeor equation (1990) and the older Harris-Benedict equation (1919) both rely on total body weight, sex, and age — which means two individuals weighing 80 kg with vastly different body compositions (15% vs 35% body fat) will be assigned the same BMR. This is biologically incorrect.
Lean tissue is metabolically expensive. Skeletal muscle burns approximately 13 kcal/kg/day at rest, brain tissue burns 240 kcal/kg/day, and the liver burns 200 kcal/kg/day — whereas adipose (fat) tissue burns only 4.5 kcal/kg/day. By using lean body mass as the primary input, Katch-McArdle captures the true metabolic cost of your body composition. This precision matters: a 10% error in BMR estimation, sustained over a year, translates to roughly 73,000 kcal — the equivalent of 21 lbs (9.5 kg) of theoretical body weight drift.
From a longevity perspective, preserving lean mass is non-negotiable. Sarcopenia — the age-related loss of skeletal muscle — begins in the third decade of life and accelerates after age 60, with adults losing 3–8% of muscle mass per decade if untrained. The 2010 European Working Group on Sarcopenia in Older People (EWGSOP) consensus, endorsed by the WHO, identifies low muscle mass as an independent risk factor for falls, fractures, insulin resistance, and all-cause mortality. Higher lean mass is correlated with improved insulin sensitivity, lower visceral adiposity, and reduced systemic inflammation as measured by CRP and TNF-α.
Once your BMR is established, multiplying by an activity factor (1.2 sedentary, 1.55 moderate, 1.725 active) yields your Total Daily Energy Expenditure (TDEE) — the maintenance caloric target. For body recomposition or controlled fat loss, the Academy of Nutrition and Dietetics recommends a deficit of no more than 500 kcal/day below TDEE to preserve lean tissue and avoid the metabolic adaptation (adaptive thermogenesis) seen in aggressive cuts. Pairing this caloric framework with adequate protein intake (1.6–2.2 g/kg) and resistance training is the evidence-backed protocol for sustainable, longevity-focused body composition.