Strong positive correlations were found between the
early GLP-1 response (iAUC 0–15 min) and postprandial
insulin release up to 30 min, as well as negative correlations
to blood glucose response (iAUC 0–90 min), an indication
that early GLP-1 release may be important for
the modulation of glycemia following both human and
bovine milk. In support of such an opinion, we recently
observed that ingestion of whey protein and free amino
acid caused an early GLP-1 secretion that reduced postprandial
glycemia in absence of a hyper-insulinemic peak
[31]. In addition to GLP-1, the human milk also resulted
in a small elevation of early phase GIP that was significantly
higher than after the WWB reference meal at 7.5
min. GIP may be present in human milk [32], and the
response we found might result from a higher intake of
this incretin, and could possibly attenuate the postprandial
insulin response and facilitate glycemic regulation.
Although, little is known about to what extent ingested
GIP can maintain activity following exposure to the
acidic environment in the stomach, it could be hypothesized
that ingestion of intrinsic GIP could possibly contribute
to the incretin effect of human milk. In previous
studies of incretin responses we have preferentially seen
increase in GIP after whey ingestion [5]. Thus, it is interesting
that the whey meal, as well as the human milk
meal, in this study significantly increased both GLP-1
and GIP response compared to all the other meals. GLP-
1 secretion following whey ingestion has previously been reported by Hall et al. [33]. However, in that study the
test meals contained 38E% of fat, which may have influenced
GLP-1 secretion. Both GLP-1 and GIP have been
identified as strong insulinotropic agents [13,34], and a
release of both these incretins after whey ingestion may
add to its insulinogenic properties. Interestingly, it was
recently reported that whey protein inhibits dipeptidyl
peptidase IV (DPP-4), an enzyme that inactivates GLP-1
[35]. Consequently, whey protein may both induce GLP-
1 secretion as well as prolong the activity of this
incretin