The association between dietary protein intake and metabolic syndrome: a GRADE-assessed systematic review and meta-analysis of observational studies

Abstract

Objectives: The primary aim of this meta-analysis is to assess the association of dietary protein with the risk of metabolic syndrome (MetS) in observational studies. In addition, the secondary aim is to evaluate the effectiveness of protein intake on MetS components. Methods: An Initial search was conducted from PubMed, Web of Science (WOS), and Scopus until May 2024. Cohort, cross-sectional, and case-control studies were included, and their quality and certainty were evaluated by the Newcastle – Ottawa Quality Assessment Scale (NOS) and the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) tools, respectively. Result: Based on our meta-analysis, we found that plant protein (PP), and animal protein (AP) had an inverse association with MetS (OR: 0.77, 95 CI: 0.69, 0.87, P < 0.001; I2 = 93.0; Pheterogeneity < 0.001), (OR: 0.92, 95 CI: 0.86, 0.98, P = 0.012; I2 = 83.5; Pheterogeneity < 0.001), respectively. Besides, there was no association between total protein (TP) and MetS (OR: 0.90, 95 CI: 0.82, 1.00, P < 0.051; I2 = 91.3; Pheterogeneity < 0.001) as the primary outcomes. Furthermore, TP, AP, and PP had a negative association with MetS components, except TP-WC (OR: 0.78; 95 CI: 0.55, 1.12; P = 0.178; I2 = 80.0; Pheterogeneity < 0.001), TP-FBS (OR: 0.93; 95 CI: 0.82, 1.05; P = 0.231; I2 = 91.0; Pheterogeneity < 0.001), TP-BP (OR: 0.86; 95 CI: 0.76, 0.96; P = 0.008; I2 = 87.9; Pheterogeneity < 0.001), AP-FBS (OR: 1.04, 95 CI: 1.00, 1.07, P = 0.061; I2 = 29.6; Pheterogeneity >0.001), PP-FBS (OR: 0.94, 95 CI: 0.86, 1.03, P = 0.207; I2 = 72.2; Pheterogeneity =0.001). Conclusion: Current evidence suggests that PP and AP intake may be associated with reduced risk of MetS as the primary outcome. However, in specific contexts, such as some of the secondary outcomes, results showed no reaction, e.g., TP-WC, TP-FBS, TP-BP, AP-FBS, PP-FBS. Besides, due to the high heterogeneity, methodological quality, and significant bias in PP-MetS and PP-TG, recommendations must be made cautiously. Finally, no definitive conclusions can be drawn regarding a causal or uniform protective relationship. Trial registration: Prospero ID 1020957. © The Author(s) 2025.