Role of Proteins in Insulin Secretion and Glycemic Control - Advances in Food and Nutrition Research - page 10

shown to induce skeletal muscle insulin resistance and reduce glycogen syn-
thesis by stalling glucose transport and phosphorylation ( Krebs et al., 2002 ) .
Elevated amino acid levels also activate intermediates important for protein
synthesis initiation such as p70 S6 Kinase and PHAS-I in the presence of
insulin ( Patti, Brambilla, Luzi, Landaker, & Kahn, 1998 ) . Increased amino
acid levels have further been shown to induce gluconeogenesis in the liver
( Nuttall, Ngo, & Gannon, 2008 ) . This was most evident when amino acids
were fed in an impaired insulin secretion state as it showed the increased
plasma glucose levels

in the absence of peripheral disposal ( Krebs

et al., 2003 ) .
Protein quality appears to notably affect the magnitude of the insulin
response. Early work carried out by Floyd and colleagues showed that con-
suming 500 g of either ground beef or chicken liver sharply increased plasma
insulin levels ( Floyd et al., 1966b ) . Studies have shown that proteins from
various sources elicit different effects on insulin. A beef meal showed signif-
icantly higher plasma insulin levels than a cod fillet meal which was attrib-
uted to differences in their amino acid composition and also the differences
in protein digestibility ( Soucy and Le Blanc, 1999 ). In an attempt to deter-
mine postprandial responses to different protein sources, Bowen, Noakes,
and Clifton (2006) fed 72 healthy lean and obese men 50 g of whey protein,
soy protein, and gluten and measured insulin, ghrelin, cholecystokinin
(CCK) and glucagon-like peptide-1 (GLP-1). They found that all the pro-
teins equally suppressed ghrelin, CCK, and GLP-1 but had different effects
on insulin secretions. This is suggestive of proteins' directs effects on insulin
secretion. While all proteins failed to secrete insulin as much as glucose did,
the lowest expression was observed for gluten. Soy and whey proteins
showed higher and similar secretions. Nuttall and Gannon (1990) fed seven
healthy males 50 g of cottage cheese and egg white and measured insulin,
glucagon, and C-peptide and found that insulin, C-peptide, and glucagon
secretions were 50% less for egg white compared to cottage cheese. The
study also estimated the amount of metabolized protein in the test meals
and found that only 47% of the egg white had been metabolized compared
to 70% in cottage cheese. This data suggest that the lower hormonal response
to egg white was due to its lower digestibility. Previous studies on the bio-
logical value have also shown a low digestibility for egg protein and that it
depended on energy intake and maintenance ( Inoue, Fujita, & Niiyama,
1973 ). Krezowski, Nuttall, Gannon, and Bartosh (1986) fed 50 g of lean
hamburger to healthy participants and showed that protein alone stimulated