Backgroundprotein influences acute postprandial-glucose and -insulin responses but the effects of dose, protein-type and health-status are unknown. Objectiveto determine the acute effect of adding protein to carbohydrate on postprandial responses and identify effect modifiers. Methodswe searched MEDLINE, EMBASE and Cochrane databases through 30 July 2023 for acute, crossover trials comparing acute postprandial-responses elicited by carbohydrate-containing test-meals with vs without added protein in adults without-diabetes or with type-2 (T2DM) or type-1 (T1DM) diabetes. Group data were pooled separately using generic inverse-variance with random-effects models and expressed as ratio-of-means with [95% CIs]. Risk-of-bias and certainty-of-evidence (GRADE) were assessed. Resultsin 154 trial-comparisons of animal-, dairy- and plant-proteins (without-diabetes, n=22,67,32; T2DM, n=14,16,3), each gram-protein/gram-carbohydrate (g/g) reduced glucose-area-under-the-curve (AUC) less in T2DM than in those without-diabetes (-10% vs -50%, P<0.05) but increased insulin-AUC similarly (+76 vs +56%, respectively). In subjects without-diabetes, each g/g of dairy- and plant-proteins reduced glucose-AUC by 52 and 55% and increased insulin-AUC by 64 and 45% (all P<0.05). Animal-proteins significantly reduced glucose-AUC by 31% and increased insulin-AUC by 37% (pooled effects), but without a significant dose-response. In T2DM animal-protein reduced glucose-AUC by 13% and increased insulin-AUC by 105%, with no significant dose-response. Dairy-protein reduced glucose-AUC by 18% (no dose-response), but each g/g increased insulin-AUC by 34% (P<0.05). In T1DM protein increased glucose-AUC by 40% (P<0.05, n=5). Data-source (reported vs calculated) and study-methodology-quality significantly modified some outcomes and contributed to high between-study heterogeneity. Conclusionsin people without-diabetes, adding dairy- or plant-protein to a carbohydrate-containing meal elicits physiologically significant reductions in glucose-AUC and increases insulin-AUC. Animal-protein may slightly reduce glucose-AUC and may increase insulin-AUC. In T2DM, protein may not have such large and consistent effects. Further research is needed to determine if the effects of protein differ by health status and protein-source. Registration: PROSPERO CRD42022322090. Funding: General Mills.