The science of nutrition and weight loss revolves around creating a balance between calorie intake and expenditure. Weight loss occurs when calories consumed are fewer than those burned. **Macronutrients**—proteins, fats, and carbohydrates—each impact metabolism differently. Protein increases satiety and supports muscle retention, while fiber-rich foods enhance fullness and aid digestion. Healthy fats and controlled carbohydrate intake are crucial for sustained energy. **Metabolism**, influenced by genetics, activity level, and age, affects weight management. Effective weight loss combines a nutritious, balanced diet with regular physical activity and lifestyle changes for long-term success and overall health.