Final answer:
Essential nutrients must be consumed through diet as they cannot be synthesized by the body in adequate amounts; they are crucial for maintaining physiological functions.
Step-by-step explanation:
Essential nutrients are best described as **nutrients** that must be obtained from food because the body is unable to manufacture them or create enough of them to meet physiological needs. These nutrients are vital for energy, body building, and controlling various body processes. There are six major classes of nutrients: carbohydrates, proteins, lipids, water, vitamins, and minerals. Within these, there are both essential and nonessential nutrients. Essential nutrients, such as certain amino acids, fatty acids, and various vitamins and minerals, are crucial because they cannot be sufficiently synthesized by the human body and hence must be included in our diet.