ABSTRACT
BACKGROUND: protein influences acute postprandial-glucose and -insulin responses but the effects of dose, protein-type and health-status are unknown. OBJECTIVE: to determine the acute effect of adding protein to carbohydrate on postprandial responses and identify effect modifiers. METHODS: we 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. RESULTS: in 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. CONCLUSIONS: in 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.
