Essential Amino Acid Requirements

Essential amino acid requirements for humans recommended by FAO/WHO/UNU (21). An essential amino acid is one that caimot be synthesized by... 

Cottonseed. When compared with FAO/WHO/UNU essential amino acid requirements (see Table 3), cottonseed proteins are low in lysine, threonine, and leucine for 2 to 5-year-old children, yet meet all requirements for adults. 

Dietary protein provides essential amino acids. The quality of a protein is a measure of its ability to provide the essential amino acids required for tissue maintenance. Proteins from animal sources, in general, have a higher quality protein than that derived from plants. However, proteins from different plant sources may be combined in such a way that the result is equivalent in nutritional value to animal protein. 

Transaminases have been demonstrated in Hymenolepis spp., Anoplo-cephala magna, Raillietina cesticillus, T. taeniaeformis, M. expansa and Lytocestus indicus (39,669,796). Some 17 amino acids failed to act as amino acid donors in transamination reactions with Hymenolepis (796) and it appears that, compared with vertebrates, cestodes may have an extremely limited capacity for performing transaminations. This may reflect the fact that cestodes live in an environment rich in amino acids, in which case synthesis may play only a minor role in satisfying essential amino acid requirements. 

Essential amino acid requirements of man. Yale J. Biol. Med., 15, 723... 

The nonessential amino acids are synthesized by quite simple reactions, whereas the pathways for the formation of the essential amino acids are quite complex. For example, the nonessential amino acids alanine and aspartate are synthesized in a single step from pyruvate and oxaloacetate, respectively. In contrast, the pathways for the essential amino acids require from 5 to 16 steps (Figure 24.8). The sole exception to this pattern is arginine, inasmuch as the synthesis of this nonessential amino acid de novo requires 10 steps. Typically, though, it is made in only 3 steps from ornithine as part of the urea cycle. Tyrosine, classified as a nonessential amino acid because it can be synthesized in 1 step from phenylalanine, requires 10 steps to be synthesized from scratch and is essential if phenylalanine is not abundant. We begin with the biosynthesis of nonessential amino acids. 

TABLE 26.S Estimated Essential Amino Acid Requirements... 

The nonessential amino acids are synthesized by quite simple reactions, whereas the pathways for the formation of the essential amino acids are quite complex. For example, the nonessential amino acids alanine and aspartate are synthesized in a single step from pyruvate and oxaloacetate, respectively. In contrast, the pathways for the essential amino acids require... 

Chlorella contains protein, chlorophyll, dietary fiber, and large amounts of minerals and vitamins. The protein of Chlorella contains all the essential amino acids required for the nutrition of animals and humans. 

TABLE 26.5 Estimated Essential Amino Acid Requirement ... 

Essential Amino Acid Requirements and Casein Content for Children and Rats... 

The most abundant body proteins in birds are muscle proteins, collagen, keratin and, during the laying period, egg proteins. It is when these proteins are being synthesised most rapidly that amino acid deficiencies are likely to become apparent. The proportions of each essential amino acid required in... 

Murphy, M.E. (1993a). The essential amino acid requirements for maintenance in the White-crowned Sparrow, Zonotrichia leucophyrys gambelii. Canad. ]. Zool, 71, 2121-30. 

The dietary lysine requirement is calculated by assuming that the ileal digestibility of lysine is 0.84. The balance of the other essential amino acids required is expressed relative to lysine, where lysine equals 1.0 (Table 14.9).The minimum requirement for non-essential amino acids, and hence the total protein requirement, is calculated as 2.5 times the sum of the requirement for essential amino acids. 

Table 14.9 Example of the calculation of the essential amino acid requirement and the total protein requirement of a 65 kg intermediate-type growing pig retaining 0.172 kg protein per day...

Meeting the requirements of non-ruminants for a balanced supply of essential amino acids requires the use of expensive protein sources, such as fishmeal, or high levels of less well-balanced protein sources such as soya bean meal (see Chapter 13). Economics dictate that the latter option is usually taken, and, in order to meet the requirements for the limiting amino acid, an excess of total protein has to be supplied. This is deaminated and the nitrogen is excreted both processes require energy, and such protein oversupply is wasteful in terms of both protein and energy metabolism. In addition, the excreted nitrogen may be a source of pollution in the environment. 

After infancy the data are sparse indeed. Other than the limited data available from children age 10 to 12 years, the published estimates of amino acid needs are based upon extrapolation, assuming that amino acid needs change in proportion to somewhat better defined protein needs. While this seems logical, such data as are available suggest that essential amino acid requirements may fall faster than the total protein needs. 

Drugs can be found sometimes in foods and can participate in the metabolism. However, these substances are not normally exposed in humans at the dose at which they exert their beneficial effect. Some dietary supplements are drug-like when ingested in amounts that could never be achieved in the diet, even though they are essential nutrients when ingested in smaller quantities, for example, tryptophan is an essential amino acid required for metabolism and incorporation into proteins at low dose in humans. At high dose, it increases 5-hydroxytryptamine levels in brain and thus acts as a drug to treat insomnia. 

The requirements for essential amino acids for growth (expressed as proportion of total protein intake) are higher than the requirement to maintain N balance in adults, and younger children, with a faster growth rate, have a higher requirement for essential amino acids as a proportion of total protein than do older children with a lower rate of growth. Table 9.6 shows various estimates of essential amino acid requirements and the reference pattern of the amount of each amino acid that should ideally be present per gram of dietary protein. 

Early studies of essential amino acid requirements were based on the amounts required to maintain nitrogen balance in young adults. Interestingly, for reasons that are not clear, these relatively short-term studies did not show any requirement for histidine. More recent studies have measured the rate of whole-body protein turnover using isotopically labelled amino acids. These have shown that the maximum rate of protein turnover is achieved with intakes of the essential amino acids some threefold higher than are required to maintain nitrogen balance. What is not clear is whether the maximum rate of protein turnover is essential, or even desirable. 

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