Total daily energy expenditure (TDEE) and caloric intake dictate the overall trajectory of body mass change based on the energy balance equation: $\Delta \text{Energy Stores} = \text{Energy Intake} - \text{Energy Expenditure}$. However, while caloric surplus or deficit governs net weight flux, macronutrient partitioning determines tissue composition—determining whether tissue mobilized during a deficit is predominantly adipose triglyceride or skeletal muscle contractile protein.

The thermic effect of food reflects the metabolic cost of enzymatic nutrient breakdown, peptide absorption, and substrate processing. Dietary protein exhibits a thermic coefficient of 20% to 30%, meaning that for every 100 calories of protein consumed, 20 to 30 calories are dissipated as heat during peptide hydrolysis and urea synthesis. In contrast, carbohydrates require 5% to 10% and dietary fats require merely 0% to 3%.

Dietary fats should rarely fall below 15% to 20% of total caloric intake. Lipids are essential for the endogenous biosynthesis of steroid hormones (testosterone, progesterone, estradiol), systemic membrane fluidity, and the physiological transport of fat-soluble vitamins (A, D, E, K). Chronic severe dietary fat restriction impairs thyroid deiodinase conversion and disrupts reproductive axis hormone pulsatility.

Glycogen stores in human skeletal muscle (~400–600g) and liver (~80–100g) drive high-intensity glycolytic output. Concentrating complex and simple carbohydrates in the peri-workout window (pre- and post-training) elevates skeletal muscle perfusion, suppresses glucocorticoid-mediated muscle catabolism, and accelerates recovery through insulin-dependent GLUT4 translocation.

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