Loss of Incretin Effect Is a Specific, Important, and Early Characteristic of Type 2 Diabetes

  1. Sten Madsbad, MD, PHD4
  1. 1Department of Biomedical Sciences, the Panum Institute, Copenhagen, Denmark
  2. 2Department of Internal Medicine F, Gentofte Hospital, Copenhagen, Denmark
  3. 3Department of Internal Medicine T, Bispebjerg Hospital, Copenhagen, Denmark
  4. 4Department of Endocrinology, Hvidovre Hospital, University of Copenhagen, Copenhagen, Denmark
  1. Corresponding author: Jens J. Holst, jjholst{at}sund.ku.dk

INCRETIN EFFECT IN TYPE 2 DIABETES

Whereas glucose-tolerant individuals are capable of adjusting their insulin secretion to their actual insulin sensitivity, people with type 2 diabetes are incapable of doing so (1). β-Cell failure is therefore the hallmark of this disease, although failure may be precipitated by the development of insulin resistance, typically as a consequence of obesity. In healthy subjects, a considerable part of the postprandial insulin response is due to the actions of the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Together, the two hormones are responsible for the so-called incretin effect, i.e., the amplification of insulin secretion that is observed when glucose is taken orally as opposed to infused intravenously to provide identical plasma glucose concentrations (2).

Although frequently ignored, the effect strongly depends on the dose of glucose (3). A convenient way of describing the effect is to calculate the gastrointestinally mediated glucose disposal (GIGD) (4). Here the amount of glucose required by intravenous infusion to copy the glucose excursions after the oral load is related to the oral load. Thus, if 25 g is required to copy a 75-g oral glucose load, the GIGD amounts to 100 × (75 – 25)/75 = 66%. In other words, mechanisms associated with and activated by the oral ingestion resulted in a disposal of 75 – 25 = 50 g of the ingested glucose. In healthy subjects, most of the GIGD is accounted for by the actions of the incretin hormones, but inhibition of hepatic glucose production by suppression of glucagon secretion, hepatic uptake of glucose from the portal vein, and gut-brain or liver-brain reflex activity may also play a role. GIGD is particularly useful in the study of oral glucose handling in C-peptide–negative patients with type 1 diabetes, where the classical incretin definitions have no meaning (4). In a …

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