The effect of oral galactose on GIP and insulin secretion in man

The insulinotropic effect of 50 g galactose given orally to 5 normal volunteers on two occasions--once with and once without a period of hyperglycaemia produced by an intravenous glucose infusion--was studied. Oral galactose caused a rise in plasma GIP from fasting levels of 260 +/- 50 ng/l (mean +/- S.E.M.) to a maximum of 900 +/- 65 ng/l 30 min after ingestion, but in the presence of induced hyperglycaemia the GIP response was significantly diminished and delayed (maximum plasma GIP levels 595 +/- 110 ng/l at 45 min, p less than 0.05). The insulin response to galactose was greatly enhanced by IV glucose (mean area under plasma insulin curve with galactose alone 236.5 +/- 66.0, with galactose + IV glucose 451.9 +/- 81.6, p less than 0.025). The mean rise in plasma galactose was significantly lower in the presence of IV glucose (mean peak level 1.97 +/- 0.28 mmol/l with galactose alone, 0.69 +/- 0.16 mmol/l galactose + IV glucose, p less than 0.025). Oral galactose caused the release of GIP, which is powerfully insulinotropic in the presence of moderate hyperglycaemia. The lower plasma GIP and galactose levels observed following oral galactose in the presence of IV glucose may be accounted for either by postulating that insulin inhibits the absorption of oral galactose, or that insulin exerts a negative feed-back control on GIP release and accelerates galactose disposition in the body.