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pancreatic cells ( Landgraf, Landgraf-Leurs, & H¨ rl, 1974 ) . Similarly, Milner
(1970) observed no insulin secretion when pieces of rabbit pancreas were
incubated in 5-mMphenylalanine. Interestingly, the study showed that phe-
nylalanine inhibited insulin secretion in the presence of glucose. Although
in vitro studies show poor effects, the majority of human studies show an
effect of phenylalanine on insulin secretion also at physiological doses found
in food. One review highlighted that phenylalanine stimulated the greatest
insulin response of all amino acids ( Gannon & Nuttall, 2010 ).
Proline is a readily absorbed amino acid that is found in higher quantities
in dairy, meats, and collagen. Nuttall, Gannon, and Jordan (2004) investi-
gated the effect of physiological levels of proline on insulin secretion. They
fed eight young healthy participants 1 mM/kg lean body mass of proline and
measured serum insulin over the following 150 min. The authors found that
proline only very slightly increased insulin levels and had no impact on glu-
cagon. While no studies have been conducted to investigate higher doses of
proline on insulin, the data so far suggest that proline alone at levels com-
monly found in food has little impact on insulin secretion.
Floyd et al. (1966b) evaluated the insulinogenic potential of valine (30 g),
methionine (30 g), histidine (30 g), threonine (7.5, 8.5, 15, and 22.5 g), and
tryptophan (2.5, 5, and 7.5 g) when they were infused intravenously to 2-10
healthy subjects. The authors found that all these amino acids had little effect
on insulin secretion. In an effort to determine the effect of ornithine on insu-
lin secretion, Bucci, Hickson, Wolinsky, and Pivarnik (1992) administered
oral boluses of 40, 100, and 170 mg/kg body weight of L -ornithine to
12 body builders. Serum insulin was not significantly affected by ornithine
at all three levels. While this suggests that ornithine has no effects on insulin
secretion, further studies are needed to confirm it. For instance, citrulline
(metabolizable from ornithine) has been shown to potentiate insulin secre-
tion from b -cells through nitric oxide production ( Nakata & Yada, 2003 ).
In vitro and animal studies also provide indicative data on the effect of
amino acids on insulin secretion. Kuhara, Ikeda, Ohneda, and Sasaki
(1991) gave intravenous infusions of 17 amino acids each at doses of
3 mM/kg body weight to sheep and measured insulin, glucagon, and
growth hormone secretions. While leucine was the most insulinogenic, ala-
nine, glycine, and serine showed the next highest stimulation. The authors
concluded that straight chain amino acids were more insulin and glucagon
secretory than short-chain amino acids, and that BCAAs were insulinogenic
but had no effect on glucagon. In agreement, Hutton, Sener, and Malaisse
(1980) showed that rat islet cells produced insulin when they were exposed
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