NUTRIENTS are components of food that are metabolically useful to the body. Nutrients may be considered essential, meaning they are required in the diet, or non-essential, meaning they are not required in the diet. A nutrient is considered essential when the body cannot synthesize it. Different species have different essential nutrients.

Most mammals, for example—including dogs and cats—can synthesize Vitamin C in their liver, so Vitamin C is non-essential, and not a required nutrient in their diets. On the other hand, mammals such as humans, primates, and guinea pigs cannot synthesize Vitamin C because their livers lack the necessary machinery, and therefore, Vitamin C is an essential nutrient for those mammals.

There are six broad categories of nutrients: Water, Carbohydrates, Proteins, Fats, Minerals, and Vitamins.

Each category has several functions in the body. Carbohydrates, proteins, and fat may be burned and used for energy.