Nutrition is the study of how the body uses food for health and involves what, how much, and when a person eats. The six essential nutrients are carbohydrates, fats, proteins, vitamins, minerals, and water. Carbohydrates, fats, and proteins provide calories and energy, while vitamins and minerals regulate body functions. Portion sizes, hunger, appetite, calories, and food guides are important concepts in understanding nutrition. Obesity can increase risks for health conditions like heart disease and diabetes.