Role of Proteins in Insulin Secretion and Glycemic Control - Advances in Food and Nutrition Research - page 34

synergistically increase plasma insulin concentrations but not affect blood
glucose ( Floyd et al., 1970 ) . Later studies also showed similar findings.
Kalogeropoulou et al. (2008) showed that leucine when ingested with glu-
cose modified glycemia, insulin, and glucagon response in healthy subjects.
On its own, leucine did not affect serum glucose concentrations but mod-
estly increased insulin. When leucine was ingested with glucose, a strong
attenuation in glucose response (by 50%) was seen. A synergistic effect of
ingested glucose with leucine on insulin secretion was observed whereby
the insulin area response increased by an additional 66%. Leucine alone
increased glucagon concentration while coingestion with glucose attenuated
the decrease in expected glucagon when glucose alone was ingested
( Kalogeropoulou et al., 2008 ) . Leucine stimulated pancreatic insulin release
and therefore decreased plasma glucose concentrations when ingested with
glucose. This synergistic effect could either be a direct effect of leucine or
through a leucine-stimulated release of gut incretins. In vitro studies have also
shown that leucine is an insulin secretagogue ( Newsholme et al., 2005; Xu,
Kwon, Cruz, Marshall, & McDaniel, 2001 ) and improves blood glucose
clearance ( Nishitani et al., 2002 ).
Isoleucine when ingested by healthy subjects also showed similar results
as leucine ( Nuttall, Schweim, et al., 2008 ). When isoleucine was ingested
alone, it decreased glucose but did not affect insulin. When isoleucine
was ingested with glucose, a lower plasma concentration of the amino acid
was observed. However, insulin expression was increased by 43% more than
that following ingestion of glucose alone. The glucose increase was less and it
decreased more rapidly following isoleucine þ glucose, compared to glucose
alone. Consequently, the glucose area response was markedly attenuated and
isoleucine had little effect on the glucagon concentration. Isoleucine alone
does not appear to stimulate insulin and glucagon but seems to decrease glu-
cose concentrations. But when ingested with glucose, insulin secretion was
stimulated and glucose response decreased. This suggests that isoleucine
modulates blood glucose both through insulin-dependent and -independent
mechanisms.
Lysine ingestion showed no increases in blood glucose in healthy subjects
in one study ( Floyd et al., 1966a ). Another recent study also showed only a
slight decrease in serum glucose in healthy subjects ( Kalogeropoulou et al.,
2009 ). But the authors observed an increase in glucagon and insulin secretions.
Floyd and colleagues observed that lysine was also potent in stimulating
the release of insulin ( Floyd et al., 1966a ) . When lysine was ingested with
glucose, there was a striking attenuation of the glucose response without an