Loss of the incretin effect (IE) in type 2 diabetes (T2D) contributes to hyperglycemia and the mechanisms underlying this impairment are unclear.

To quantify the IE impairment in T2D and to investigate the factors associated with it using a meta-analytic approach.

PubMed, Scopus, and Web-of-Science were searched. Studies measuring IE by the gold-standard protocol employing an oral glucose tolerance test (OGTT) and an intravenous glucose infusion at matched glucose levels were selected. We extracted IE, sex, age, body mass index (BMI), and hemoglobin A_1c, fasting values, and area under curve (AUC) of glucose, insulin, C-peptide, glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1). In subjects with T2D, we also recorded T2D duration, age at diagnosis, and the percentage of subjects taking antidiabetic medications.

The IE weighted mean difference between subjects with T2D and those with normal glucose tolerance (NGT) was −27.3% (CI –36.5% to –18.1%; P < .001; I² = 86.6%) and was affected by age (P < .005). By meta-regression of combined NGT and T2D data, IE was inversely associated with glucose tolerance (lower IE in T2D), BMI, and fasting GIP (P < .05). By meta-regression of T2D studies only, IE was associated with the OGTT glucose dose (P < .0001). IE from insulin was larger than IE from C-peptide (weighted mean difference 11.2%, CI 9.2-13.2%; P < .0001; I² = 28.1%); the IE difference was inversely associated with glucose tolerance and fasting glucose.

The IE impairment in T2D vs NGT is consistent though considerably variable, age being a possible factor affecting the IE difference. Glucose tolerance, BMI, and fasting GIP are independently associated with IE; in subjects with T2D only, the OGTT dose is a significant covariate.