The Thermodynamics of Metabolism
A calorie is a unit of energy. In nutrition, a calorie (specifically, a kilocalorie or kcal) is defined as the amount of heat energy required to raise the temperature of one kilogram of water by one degree Celsius.
Your body requires constant energy to support basic life functions and physical movement. This energy balance is governed by the first law of thermodynamics: energy can neither be created nor destroyed, only transformed. If you consume fewer calories than your body burns, your body draws from stored energy (such as fat tissue) to make up the difference.
Key Components of Calorie Balance
- Energy In: Calories consumed through food and beverages.
- Energy Out: Total Daily Energy Expenditure (TDEE), which includes your resting metabolic rate (BMR) and activity levels. Calculate your TDEE using our TDEE Calculator.