A calorie is a unit of energy, used to measure how much energy your body gets from food and drink, and how much it spends running its vital functions and physical activity.
Where do calories come from?
Calories come from three main macronutrients: protein (4 calories per gram), carbohydrates (4 calories per gram), and fat (9 calories per gram). Alcohol also contains calories (7 per gram) but isn't considered a core macronutrient.
How does your body determine its daily needs?
Your body burns calories even while you sleep, through what's known as Basal Metabolic Rate (BMR) — the energy needed to run your heart, breathing, and other vital functions. Add in daily activity and exercise, and you get your Total Daily Energy Expenditure (TDEE) — the number your calorie intake should be compared against.
Why are calories the foundation of any weight goal?
Simply put: eat more calories than your body burns, and you'll gain weight. Eat less, and you'll lose it. This principle is called "energy balance," and it governs the outcome on the scale regardless of which diet you follow.
That's why knowing your accurate daily calorie needs is the first and most important step before starting any journey to lose weight, gain weight, or build muscle.