A. Consistent with a metabolically fasted state, decreased circulating
insulin levels will signal attenuation of biosynthetic reaction and aug-
mentation of catabolic reactions. The latter will provide necessary
fuels to sustain the energetic needs of the body. Elevation of counter-
regulator hormones (e.g., glucagon, epinephrine, and cortisol) will
synergistically stimulate catabolic processes. Increased proteolysis
(protein^amino acids) in tissues such as skeletal muscle and the liver
provide amino acids as a direct fuel source, as well as ketogenic and
gluconeogenic precursors (depending on the individual amino acid).
B IO C H E M IS T R Y PE A R L S
Twenty different amino acids are used for protein synthesis, each of
which is encoded by at least one codon (the three-nucleotide
genetic code) and differ only in their side-chain group.
Amino acids can be divided into one of two classes, essential and
nonessential amino acids. Essential amino acids cannot be syn-
thesized by humans.
Nitrogen balance means whether there is more or less nitrogen
(protein) ingested than excreted.
With prolonged caloric insufficiency, a large portion of the carbon
skeleton is used by the liver for the synthesis of either glucose or
ketone bodies, depending on the specific amino acid.
Rapidly dividing cells (e.g., lymphocytes, enterocytes) preferen-
tially use glutamine for energy and biosynthetic purposes.
Newsholme EA, Leech AR. Biochemistry for the Medical Sciences. New York: