Biology Reference
In-Depth Information
cycle intermediates such as
a
-ketoglutarate and oxaloacetate, all of which are
gluconeogenic precursors of glucose. The literature contains reports of both
positive and negative impact of amino acids, ingested with or without glu-
cose, on glycemic regulation.
Studies have reported that the intravenous administration of amino acids
decrease glucose disposal, induce hyperinsulinemia and hyperglycemia, and
potentially lead to insulin resistance (
Ferrannini & Mari, 1998; Krebs et al.,
2002
). Intravenous administration of individual amino acids and mixtures of
amino acids were accompanied by large increases in blood glucose with
acute increases in plasma amino acids (
Floyd et al., 1966a
)
. A decreased glu-
cose uptake and increased secretion of insulin and glucagon were also
observed.
Gannon and Nuttall (2010)
pointed out the importance of study-
ing the effect of amino acids on the glucose concentration when adminis-
tered orally rather than intravenously, since some amino acids are
metabolized by the intestinal cells and liver, and others pass through the liver
into the peripheral circulation.
Early studies showed that alanine given to obese subjects prior to ther-
apeutic starvation resulted in an increased insulin concentration and
decreased glucose concentration (
Genuth, 1973
)
. When oral doses of gly-
cine were given to normal and diabetic adults, a moderate reduction in
blood sugar concentrations was observed in both groups (
Cochrane et al.,
1956
). Leucine administered orally resulted in a decrease in blood glucose
in most of the subjects and an increase in plasma insulin (
Fajans et al.,
1963
). Phenylalanine (7 g) when ingested by normal-weight men increased
glucagon and insulin concentrations but showed no change in plasma glu-
cose concentrations
(
G¨ ttler et al., 1978
). These studies conclusively show
that amino acids have different effects on glucose and insulin. They also
suggest that amino acids have a greater effect on insulin secretion than on
glucose modulation.
In an earlier study, gelatin, when ingested with glucose, strongly
potentiated a glucose-stimulated increase in insulin in type 2 diabetics
(
Gannon et al., 1988
)
. A study was later carried out to determine if gly-
cine (the major amino acid in gelatin) stimulated insulin secretion or
reduced glucose response when ingested with glucose (
Gannon et al.,
2002b
). The results showed that glycine when ingested with glucose
did not significantly affect the plasma glucose concentration, similar to
earlier studies (quoted by
Cochrane et al., 1956
). When glycine was
ingested with glucose, the insulin peak occurred later and was slightly less
than when glucose was ingested alone (with glucose, there was a rapid rise
