The main source of energy for human survival is food. Human bodies require energy for various functions such as metabolism, physical activity, and maintaining body temperature. This energy primarily comes from macronutrients found in food: carbohydrates, proteins, and fats. Each of these macronutrients provides calories, which are measured in kilocalories (kcal). Carbohydrates are often the body's preferred source of energy, as they can be quickly converted into glucose, which is essential for cellular functions. Additionally, vitamins and minerals from food play crucial roles in metabolic processes that help the body efficiently utilize this energy.