Our objective was to investigate the relationship between diet composition and amount of substrate with microbial fermentation end products in a dual-flow continuous culture system. A meta-analysis was performed using data from 75 peer-reviewed studies. To derive the linear models, the MIXED procedure was used, and for nonlinear models the NLMIXED procedure was used. Significance levels to fit the model assumed for fixed and random effects were P ≤ 0.05. Independent variables were: dietary neutral detergent fiber (NDF) and crude protein (CP), and fermenter dry matter intake (FDMI), whereas dependent variables were: total volatile fatty acids (VFA) concentration, molar proportions of acetate, propionate, and butyrate, true ruminal digestibility (%) of organic matter (OM), CP, and NDF, ammonia nitrogen (NH₃-N) concentration and flows of non-ammonia nitrogen (NAN), bacterial-N, dietary-N, and efficiency of microbial protein synthesis (EMPS). Ruminal digestibilities of OM, NDF, and CP decreased as FDMI increased (P < 0.04). Total VFA linearly increased as FDMI increased (P < 0.01); exponentially decreased as dietary NDF increased (P < 0.01); and was quadratically associated with dietary CP (P < 0.01), in which total VFA concentration was maximized at 18% dietary CP. Molar proportion of acetate exponentially increased (P < 0.01) as dietary NDF increased. Molar proportion of propionate exponentially decreased as dietary NDF increased (P < 0.01). Bacterial-N quadratically increased and dietary-N exponentially increased as FDMI increased (P < 0.01). Flows of bacterial-N and dietary-N linearly decreased as dietary NDF increased (P < 0.02), and dietary-N flow was maximized at 18% CP. The EMPS linearly increased as dietary CP increased (P < 0.02). Overall, the analysis of this data set demonstrates evidences that the dual-flow continuous culture system provides valuable estimates of ruminal digestibility, VFA concentration, and nitrogen metabolism.