Role of Proteins in Insulin Secretion and Glycemic Control - Advances in Food and Nutrition Research - page 35

accompanying increase in insulin response, which was a different response to
that seen with leucine ( Kalogeropoulou et al., 2008 ) and isoleucine ( Nuttall,
Schweim and Gannon, 2008 ) . Lysine ingestion with glucose also decreased
glucagon concentrations ( Kalogeropoulou et al., 2009 ) . It appears that lysine
attenuates blood glucose through noninsulin-mediated pathways.
The metabolic response to proline with and without glucose in
nondiabetic subjects was reported by Nuttall et al. (2004) . Proline ingestion
without glucose increased plasma proline concentrations 13-fold with no
change or a slight decrease in circulating glucose. However, plasma proline
concentrations are decreased by 50% when proline was coingested with glu-
cose. Ingestion of proline with glucose also attenuated the glucose response
and did not affect the insulin response compared with glucose alone. Proline
also facilitated a glucose-stimulated decrease in glucagon concentration.
Therefore, proline appears to be having a noninsulin-mediated hypoglyce-
mic effect.
Ingestion of phenylalanine alone increased glucagon in healthy subjects
but had only a modest effect on insulin ( Nuttall et al., 2006 ). It did not affect
blood glucose concentrations compared to water. Ingestion of phenylala-
nine with glucose showed that the plasma glucose area response was
decreased by 66% and insulin area responses were the sum of the responses
to phenylalanine alone and glucose alone. Phenylalanine seems to notably
attenuate the glucose-induced rise in plasma glucose when ingested with
glucose.
In an attempt to determine the metabolic effects of glutamine, Greenfield
et al. (2009) fed glutamine þ water, glucose þ water and water to normal,
obese, and obese subjects with impaired glucose tolerance over three sepa-
rate days in random order. The glucose þ water as expected showed a mar-
ked increase in plasma insulin concentrations for all three groups.
Glutamine þ water increased insulin and glucagon significantly in normal,
obese, and obese subjects with impaired glucose tolerance but did not affect
blood glucose concentrations in all three groups ( Greenfield et al., 2009 ) .
However, the insulin and glucagon response to glutamine þ water was
greatest in obese subjects with impaired glucose tolerance, followed by obese
and then normal subjects. This concurs with other studies showing that
amino acid-mediated insulin and glucagon expression is hyperstimulated
in those with impaired glucose tolerance.
The effect of coingesting amino acids and/or protein with carbohydrate
mixtures on blood glucose was studied by a few investigators. A mixture
of amino acids (leucine,

isoleucine, valine, lysine, and threonine) was