The Engine of Metabolism
Your body burns calories every second — powering heartbeat, breathing, brain activity, and cellular repair. This baseline energy cost is your Basal Metabolic Rate (BMR), and it accounts for 60–75% of total daily energy expenditure. The Mifflin-St Jeor equation, published in 1990, remains the gold standard for BMR estimation, outperforming older Harris-Benedict equations in clinical validation studies.
Energy Balance Determines Weight Change
Weight change follows thermodynamics: consume more energy than you expend and you store the excess; consume less and your body taps reserves. One kilogram of body fat stores approximately 7,700 kcal of energy. This simulation models the trajectory of weight change based on your daily calorie balance, letting you see weeks and months into the future.
Activity Multipliers Explained
TDEE multiplies BMR by an activity factor ranging from 1.2 (desk job, no exercise) to 1.9 (professional athlete or heavy manual labor). Most people overestimate their activity level — a 30-minute gym session three times per week typically corresponds to 1.375, not the 1.55 many assume. Accurate self-assessment is the single most impactful variable in calorie planning.
Beyond Simple Arithmetic
Real metabolism is more complex than a linear equation. As you lose weight, BMR decreases (less tissue to maintain). Metabolic adaptation can reduce expenditure by 10–15% beyond what weight loss alone predicts. Macronutrient composition affects the thermic effect of food — protein costs 20–30% of its calories to digest versus 5–10% for carbohydrates. This simulator provides a first-order approximation; long-term results require periodic recalculation.