Calories are essential because they provide the energy the body needs to perform vital functions necessary for life. This includes basic metabolic activities such as breathing, circulating blood, building and repairing cells, and maintaining body temperature. Calories come from macronutrients in food—carbohydrates, fats, and proteins—that the body breaks down to release energy. Without sufficient calories, the body wouldn't have the fuel to sustain these functions, and survival would be impossible. Additionally, calories support physical activity and growth, and excess calories are stored as fat for future energy use when food intake is low. Thus, calories are critical for maintaining health, energy balance, and proper body function.