AIMS/HYPOTHESIS: The aim of this systematic review was to synthesise the study findings on whether GLP-1 secretion in response to a meal tolerance test is affected by the presence of type 2 diabetes (T2D). The influence of putative moderators such as age, sex, meal type, meal form, and assay type were also explored.

METHODS: A literature search identified 32 relevant studies. The sample mean and SD for fasting GLP-1 TOTAL and GLP-1 TOTAL iAUC were extracted and used to calculate between-group standardised mean differences (SMD), which were meta-analysed using a random-effects model to derive pooled estimates of Hedges' g and 95 % prediction intervals (PI).

RESULTS: Pooled across 18 studies, the overall SMD in GLP-1 TOTAL iAUC between individuals with T2D (n = 270, 1047 ± 930 pmol·L -1·min) and individuals without T2D (n = 402, 1204 ± 937 pmol·L -1·min) was very small, not statistically significant and heterogenous across studies (g = -0.15, p = 0.43, PI: -1.53, 1.23). Subgroup analyses demonstrated an effect of assay type whereby Hedges' g for GLP-1 iAUC was greater in individuals with, versus those without, T2D when using ELISA or Mesoscale (g = 0.67 [moderate], p = 0.009), but not when using RIA (g = -0.30 [small], p = 0.10). Pooled across 30 studies, the SMD in fasting GLP-1 TOTAL between individuals with T2D (n = 580, 16.2 ± 6.9 pmol·L -1) versus individuals without T2D (n = 1363, 12.4 ± 5.7 pmol·L -1) was small and heterogenous between studies (g = 0.24, p = 0.21, PI: -1.55, 2.02).

CONCLUSIONS: Differences in fasting GLP-1 TOTAL and GLP-1 TOTAL iAUC between individuals with, versus those without T2D were generally small and inconsistent between studies. Factors influencing study heterogeneity such as small sample sizes and poor matching of groups may help to explain the wide prediction intervals observed. Considerations to improve comparisons of GLP-1 secretion in T2D and potential mediating factors more important than T2D diagnosis per se are outlined.

PROSPERO ID: CRD42020195612.