The science of nutrition and weight loss involves understanding how diet impacts body weight and overall health. Weight loss primarily occurs through creating a calorie deficit, where calorie intake is less than energy expenditure. **Macronutrients**—proteins, fats, and carbohydrates—play distinct roles in metabolism and satiety. Protein boosts metabolism and helps preserve muscle mass, while fiber-rich foods increase fullness. Healthy fats and controlled carbohydrate intake are essential for balanced energy. **Metabolic rate**, influenced by genetics and activity level, also affects weight management. Effective weight loss combines proper nutrition with physical activity and lifestyle changes for sustainable results and overall well-being.