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Older views of the nutritional classification of amino acids categorized them into two groups: indispensable (essential) and dispensable (nonessential). The nine indispensable amino acids /(histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine)/ are those that have carbonskeletons that cannot be synthesized to meet body needs from simpler molecules in animals, and therefore must be provided in the diet. ...The definition of dispensable amino acids has become blurred as more information on the intermediary metabolism and nutritional characteristics of these compounds has accumulated. ...Dispensable amino acids /have been divided/ into two classes: truly dispensable and conditionally indispensable. Five amino acids /(alanine,aspartic acid, asparagine, glutamic acid, and serine)/ are termed dispensable as they can be synthesized in the body from either other amino acids or other complex nitrogenous metabolites. In addition, six other amino acids, including /arginine, cysteine, glutamine, glycine, proline, and tyrosine/, are conditionally indispensable as they are synthesized from other amino acids or their synthesis is limited under special pathophysiological conditions. This is even more of an issue in the neonate where it has been suggested that only alanine, aspartate, glutamate, serine, and probably asparagine are truly dietarily dispensable.

NAS, Food and Nutrition Board, Institute of Medicine; Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). National Academy Press, Washington, D.C., pg. 593-594, 2009. Available from, as of March 10, 2010: http://www.nap.edu/catalog/10490.html

The term conditionally indispensable recognizes the fact that under most normal conditions the body can synthesize these amino acids to meet metabolic needs. However, there may be certain physiological circumstances: prematurity in the young infant where there is an inadequate rate at which cysteine can be produced from methionine; the newborn, where enzymes that are involved in quite complex synthetic pathways may be present in inadequate amounts as in the case of arginine, which results in a dietary requirement for this amino acid; or pathological states, such as severe catabolic stress in an adult, where the limited tissue capacity to produce glutamine to meet increased needs and to balance increased catabolic rates makes a dietary source of these amino acids required to achieve body nitrogen homeostasis. The cells of the small intestine become important sites of conditionally indispensable amino acid, synthesis, with some amino acids (e.g., glutamine and arginine) becoming nutritionally indispensable under circumstances of intestinal metabolic dysfunction. /Amino acids/

NAS, Food and Nutrition Board, Institute of Medicine; Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). National Academy Press, Washington, D.C., pg. 594, 2009. Available from, as of March 10, 2010: http://www.nap.edu/catalog/10490.html

When diets high or low in protein are given, there is a gain or loss of body protein over the first few days, before re-equilibration of protein intake with the rates of oxidation and excretion. This phenomenon has led to the concept of a "labile protein reserve," which can be gained or lost from the body as a short-term store for use in emergencies or to take account of day-to-day variations in dietary intake. Studies in animals have suggested that this immediate labile protein store is contained in the liver and visceral tissues, as their protein content decreases very rapidly during starvation or protein depletion (by as much as 40 percent), while skeletal muscle protein drops much more slowly. During this situation, protein breakdown becomes a source of indispensable amino acid needs for synthesis of proteins critical to maintaining essential body function. This labile protein reserve in humans is unlikely to account for more than about 1 percent of total body protein. Thus, the immediately accessible stores of protein (which serve as the source of indispensable amino acids and amino nitrogen) cannot be considered in the same light as the huge energy stores in the form of body fat; the labile protein reserve is similar in weight to the glycogen store. /Amino acids/

NAS, Food and Nutrition Board, Institute of Medicine; Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). National Academy Press, Washington, D.C., pg. 595, 2009. Available from, as of March 10, 2010: http://www.nap.edu/catalog/10490.html