Calorie balance refers to the relationship between calories consumed through food and drink and calories expended through physical activity and metabolic processes. When calorie intake equals calorie expenditure, weight remains stable. A calorie surplus leads to weight gain, while a calorie deficit results in weight loss. Maintaining a healthy calorie balance is crucial for overall health, body composition, and energy levels, as it helps regulate metabolic functions and prevents the adverse effects of both excess weight and undernutrition.