The passage discusses human nutrition and the study of how food affects the human body. It explains that humans need to eat food to grow, reproduce, and maintain good health. Nutrients in food like carbohydrates, proteins, fats, vitamins, and minerals provide energy and allow important bodily functions to occur. Digestion breaks down food and absorbs nutrients, which are then used by tissues in the body. Carbohydrates, proteins and fats specifically provide energy, measured in calories. Nutrients are either essential and must come from food, or nonessential and can be produced by the body. Individual nutrient needs vary depending on factors like age and gender.