One of the limitations of the isolated beta cell islet studies is that, largely, they fail to take account of other factors that modulate insulin secretion in vivo. Such factors include the incretin hormones glucagon-like-peptide-1 (GLP-1) and glucose-dependent insulinotrophic peptide. It has been reported in both healthy humans (Thomsen et al., 1999) and those with type 2 diabetes (Thomsen et al., 2003) that olive oil intake caused increased GLP-1 response compared with butter intake. Furthermore, postprandial plasma GLP-1 concentrations were increased more after an oral fat test containing MUFAs compared to PUFAs and SFAs (Beysen et al., 2002). Recent work has suggested that fatty acids may modulate the effects of GIP on GLP-1 and thereby insulin secretion. Experiments using an isolated ileal L cell model suggest that improvements in glycaemic response seen in MUFA compared with SFA fed rats may be due to increased GLP-1 receptor activation in response to increased GIP secretion (Rocca et al., 2001).
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