section 17.1
Essential and Nonessential Amino Acids
333
commeal and kidney beans; and soybeans, peanuts, brown
rice, and bulgur (Chapter 12). Deficiency of vitamin B
12
(Chapter 38) may occur in persons on a pure vegetarian
diet. Although plant proteins used singly do not provide
all of the essential amino acids, their inclusion in the diet
provides nonessential amino acids that would otherwise
have to be synthesized at the expense of the nitrogen of
the essential amino acids. The following estimates of daily
protein needs are for persons on a Western diet: adults,
0.8 g/kg; newborns, 2.2 g/kg; infants (0.5-1 year), 2 g/kg.
During pregnancy and lactation protein intake above the
normal adult level is recommended (Appendix IV). These
protein requirements are valid only when energy needs
are adequately met from nonprotein sources. If intake of
carbohydrates and lipids is insufficient to meet the energy
expenditure, dietary protein is utilized to meet the energy
deficit and results in negative nitrogen balance.
Protein Energy Malnutrition
Two disorders of protein energy nutrition that are wide-
spread among children in economically depressed areas
are kwashiorkor (in Ghana, “the disease the first child gets
when the second is on the way”) and marasmus (from the
Greek “to waste away”).
Kwashiorkor
occurs after weaning and is due to inade-
quate intake of good-quality protein and a diet consisting
primarily of high-starch foods (e.g., yams, potatoes, ba-
nanas, maize, cassava) and deficient in other essential nu-
trients. Victims have decreased mass and function of heart
and kidneys; decreased blood volume, hematocrit, and
serum albumin concentration; atrophy of pancreas and in-
testines; decreased immunological resistance; slow wound
healing; and abnormal temperature regulation. Character-
istic clinical signs include edema, ascites, growth failure,
apathy, skin rash, desquamation, pigment changes and
ulceration, loss of hair, liver enlargement, anorexia, and
diarrhea.
Marasmus
results from deficiency of protein and energy
intake, as in starvation, and results in generalized wasting
(atrophy of muscles and subcutaneous tissues, emaciation,
loss of adipose tissue) Edema occurs in kwashiorkor but
not in marasmus; however, the distinction between these
disorders is not always clear. The treatment of marasmus
requires supplementation of protein and energy intake.
Protein energy malnutrition occurs with high frequency
(30%-50%) in hospitalized patients as well as in pop-
ulations in chronic care facilities as either an acute or
a chronic problem. These individuals suffer from inad-
equate nutrition due to a disease or depression, and are
susceptible to infections due to impaired immune func-
tion. Surgical patients with protein energy malnutrition
exhibit delayed wound healing with increased length of
stay in the hospital. Thus, protein energy malnutrition can
cause morbidity, mortality and also has economic con-
sequences. Acute stressful physiological conditions such
as trauma, bum, or sepsis can also precipitate protein en-
ergy malnutrition due to hypermetabolism caused by the
neuroendocrine system. Prompt diagnosis and appropriate
nutritional intervention is required in the management of
patients with protein energy malnutrition.
Measurements of the levels of serum proteins such as al-
bumin, transthyretin (also known as prealbumin), transfer-
rin and retinol-binding protein are used as biochemical pa-
rameters in the assessment of protein energy malnutrition
(Table 17-1). An ideal protein marker should have rapid
turnover and present in sufficiently high concentrations in
serum to be measured accurately. Transthyretin has these
properties; it is a sensitive indicator of protein deficiency
and is effective in assessing improvement with refeeding.
Transport of Amino Acids into Cells
Intracellular metabolism of amino acids requires their
transport across the cell membrane. Transport of L-amino
acids occurs against a concentration gradient and is an ac-
tive process usually coupled to Na+-dependent carrier sys-
tems as for transport of glucose across the intestinal mu-
cosa (Chapter 12). At least five transport systems for amino
acids (with overlapping specificities) have been identified
in kidney and intestine. They transport neutral amino acids,
acidic amino acids, basic amino acids, ornithine and cys-
tine, and glycine and proline, respectively. Within a given
carrier system, amino acids may compete for transport
(e.g., phenylalanine with tryptophan). Na+-independent
transport carriers for neutral and lipophilic amino acids
have also been described. D-Amino acids are transported
by simple diffusion favored by a concentration gradient.
Inherited defects in amino acid transport affect epithe-
lial cells of the gastrointestinal tract and renal tubules.
Some affect transport of neutral amino acids (.
Hartnup
disease),
others that of basic amino acids and ornithine
and cystine (
cystinuria
), or of glycine and proline (Chap-
ter 12).
Cystinosis
is an intracellular transport defect char-
acterized by high intralysosomal content of free cystine in
the reticuloendothelial system, bone marrow, kidney, and
eye. After degradation of endocytosed protein to amino
acids within lysosomes, the amino acids normally are
transported to the cytosol. The defect in cystinosis may
reside in the ATP-dependent efflux system for cystine
transport, and particularly in the carrier protein.
A different mechanism for translocation of amino
acids in some cells is employed in the
y-glutamyl cycle
(Figure 17.2). Its operation requires six enzymes (one
membrane-bound, the rest cytosolic), glutathione (GSH;
y-glutamylcysteinylglycine present in all tissue cells),
