Food energy is the amount of usable energy the human body can obtain from food, measured in kilocalories (kcal) or kilojoules (kJ). It comes from the macronutrients: fat provides about 9 kcal per gram, while protein and carbohydrate each provide about 4 kcal per gram, and alcohol about 7 kcal per gram. Fibre and most micronutrients contribute little or no energy. Energy values on nutrition labels are calculated from composition using standard conversion factors, sometimes adjusted for digestibility. Balancing energy intake against expenditure — basal metabolism plus physical activity — is the foundation of weight management: sustained surplus leads to fat gain, sustained deficit to weight loss. Public health guidance frames recommended intakes around typical energy needs.