Classes of nutrients Nutrient classes can be categorized as either macronutrients (needed in relatively large amounts) or micronutrients (needed in smaller quantities). The macronutrients are carbohydrates, fats, fiber, proteins, and water. The micronutrients are minerals and vitamins. The macronutrients (excluding fiber and water) provide structural material (amino acids from which proteins are built and lipids from which cell membranes and some signaling molecules are built) and energy. Some of the structural material can be used to generate energy internally, and in either case it is measured in joules or calories (sometimes called "kilocalories" and on other rare occasions written with a capital C to distinguish them from little 'c' calories). Vitamins, minerals, fiber, and water do not provide energy but are required for other reasons. A third class dietary material, fiber (i.e. non-digestible material such as cellulose), seems also to be required for both mechanical and biochemical reasons, though the exact reasons remain unclear.

Malnutrition Malnutrition refers to insufficient, excessive, or imbalanced consumption of nutrients by an organism. Diseases of malnutrition in humans are often associated with nutritional imbalances or excessive consumption. Although there are more organisms in the world that are malnourished due to insufficient consumption, increasingly more organisms suffer from excessive over-nutrition; a problem caused by an over abundance of sustenance coupled with the instinctual desire (by animals in particular) to consume all that it can.

Malnutrition and disease Malnutrition increases the risk of infection and infectious disease, and moderate malnutrition weakens every part of the immune system. Protein and energy malnutrition and deficiencies of specific micronutrients (including iron, zinc, and vitamins) increase susceptibility to infection. Lower energy and impaired function of the brain also represent the downward spiral of malnutrition, as animals are less able to perform the tasks they need to in order to acquire food and reproduce.

Starvation Starvation mode is a state in which the body responds to prolonged periods of low energy-intake levels. During short periods of energy abstinence, the body will burn primarily free fatty acids from body fat stores. After prolonged periods of starvation, the body has depleted its body fat and begins to burn lean tissue and muscle as a fuel source. Ordinarily, the body responds to reduced energy intake by burning fat reserves first, and only consumes muscle and other tissues when those reserves are exhausted. Specifically, the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in muscle and liver cells. After prolonged periods of starvation, the body will utilize the proteins within muscle tissue as a fuel source.