4
Nutrition and Metabolism of Proteins and
Amino Acids
Naomi K Fukagawa and Yong-Ming Yu
Key messages- Protein is the most abundant nitrogen-containing compound in
the diet and the body. Proteins are formed when L-α-amino acids
polymerize via peptide bond formation. - Amino acids have similar central structures with different side-
chains determining the multiple metabolic and physiological
roles of free amino acids. - Indispensable (essential) amino acids cannot be synthesized
by humans from materials ordinarily available to cells at a
speed commensurate with the demands of human growth and
maintenance. - The requirements for indispensable amino acids can be defi ned
as “the lowest level of intake that achieves nitrogen balance or
that balances the irreversible oxidative loss of the amino acid,
without requiring major changes in normal protein turnover and
where there is energy balance with a modest level of physical
activity.” For infants, children, and pregnant and lactating
women, requirements would include protein deposited and
secretion of milk proteins. - “Conditionally” indispensable amino acids are those for which
there are measurable limitations to the rate at which they can be
synthesized because their synthesis requires another amino acid
and because only a number of tissues are able to synthesize
them, and probably only in limited amounts. The metabolic
demands for these amino acids may rise above the biosynthetic
capacity of the organism. - Protein and amino acid requirements are determined by the pro-
cesses of protein synthesis, and maintenance of cell and organ
protein content, as well as the turnover rates of protein and
© 2009 NK Fukagawa and Y-M Yu.
amino acid metabolism, including synthesis, breakdown, inter-
conversions, transformations, oxidation, and synthesis of other
nitrogen-containing compounds and urea. These processes are
infl uenced by genetics, phase of life cycle, physical activity,
dietary intake levels, how energy needs are met, route of delivery
of nutrients, disease, hormones, and immune system products.- Protein and amino acid requirements can be determined by nitro-
gen excretion and balance, factorial estimations, and/or tracer
techniques. - Existing recommendations on requirements differ by various
authorities because of a lack of data when some were formu-
lated, different interpretations of data, and different criteria for
judging adequate intakes. - The United Nations plans to publish new recommendations for
protein and amino acids in the near future. Those made by the
Institute of Medicine, US National Academies of Science, in 2002
are cited in this chapter. - Apparent protein digestibility, measured in the past as the differ-
ence between nitrogen intake and fecal nitrogen output, under-
estimates “true” digestibility because fecal nitrogen is derived, in
part, from endogenous nitrogen sources. - Tracer techniques have shown that “true” digestibility of most
dietary proteins is high. The quality of food protein can be
assessed as the protein digestibility-corrected amino acid score. - Animal protein foods generally have higher concentrations of
indispensable amino acids than plant foods. Lysine is often the
most limiting amino acid, followed by sulfur amino acids (methio-
nine and cystine) and tryptophan and threonine.
4.1 Introduction
Protein is the most abundant nitrogen-containing
compound in the diet and in the body. It is one of the
fi ve classes of complex biomolecules present in cells
and tissues, the others being DNA, RNA, polysaccha-
rides, and lipids. The polymerization of L-α-amino
acids through synthesis of peptide bonds contributes
to the formation and structural framework of pro-
teins. These may contain two or more polypeptide
chains forming multimeric proteins, with the indi-
vidual chains being termed subunits. Proteins are the
workhorses in cells and organs and their building
blocks are the amino acids, which are joined together