Biology Reference
In-Depth Information
4.2. Effect of amino acids on insulin secretion
A greater volume of research has looked at the effects of amino acids on insu-
lin secretion. This is reasonable as the impact of proteins on glucose homeo-
stasis have been shown to be mediated through the effects of their
constituting amino acids (
Bos et al., 2003
)
. The amino acid composition
of dietary proteins differ markedly (
Table 1.1
). Therefore, the protein type
ingested will determine the composition of postprandial circulating amino
acids and thereby the metabolic response to the protein meal.
Amino acids appear to affect insulin secretion and glucose homeostasis in
unique ways. Early work carried out in the 1960s showed that both single
and combined boluses of amino acids significantly increased insulin secretion
(
Fajans, Floyd, Knopf, &Conn, 1967; Floyd et al., 1966b
)
. While the major-
ity of amino acids influence insulin secretion, some have been suggested
to be notably insulinogenic (phenylalanine, arginine, lysine, alanine,
leucine, and isoleucine;
Newsholme, Brennan, Rubi, & Maechlen, 2005;
Nuttall & Gannon, 1991
)
. Amino acids are postulated to stimulate insulin
through different mechanisms (
Henquin & Meissner, 1981
)
. Cationically
charged amino acids stimulate insulin by polarizing the plasma membrane,
and amino acids cotransported with Na
þ
has been shown to stimulate insulin
secretion by depolarizing the plasma membrane via Na
þ
transport and acti-
acid oxidation can also increase ATP stores and thus activate Ca
þ
channels
leading to insulin release. In the following section, the insulinogenic poten-
tial of individual amino acids is discussed.
Although several studies have shown that alanine plays a notable role in
gluconeogenesis (
Felig et al., 1970
)
, a limited number of studies have inves-
tigated its effect on insulin secretion in humans. Studies have shown that oral
administration of alanine increases insulin secretion in normal, diabetic, and
obese subjects
(
Genuth, 1973; Genuth & Castro, 1974
). These effects were
seen when obese individuals were given 50 g/day, and when normal and
diabetic subjects were given 0.1 and 0.5 g/kg body weight of alanine per
T¨ljedal, 1971
) and that it induces insulin secretion from them (
Dunne,
et al., 1996
). These studies showed that alanine induces insulin by depolar-
ization of the membrane through cotransportation with Na
þ
. Other studies
have shown that alanine does not induce insulin secretion from rat
b
-cells
