High-amylose maize starch (HAMS) can lead to succinate accumulation in the rat colon depending on the colonic microbiota. Since succinate is primarily produced via the vitamin B12 (VB12)-dependent succinate pathway, limited VB12 availability in the colon may impair fermentation. While a portion of dietary VB12 may reach the colon, most of it is absorbed in the upper gastrointestinal tract, potentially resulting in an insufficient supply for colonic bacteria. This study aimed to determine the minimum caecal VB12 concentration required to prevent succinate accumulation and to assess whether dietary cobalt (Co), a structural component of VB12 and its analogues, promotes microbial VB12 analogue synthesis. Sprague-Dawley male rats were used in three experiments. In Experiment 1, HAMS-fed rats were given diets with increasing VB12 doses. Caecal succinate concentrations decreased dose-dependently, with a predicted threshold of 74 pmol/g VB12 required to prevent accumulation. In Experiment 2, rats were fed HAMS diets with varying Co levels. Co supplementation significantly increased VB12-equivalent concentrations, measured by microbiological assay, from 27 to 915 pmol/g without altering cobalamin concentrations, suggesting enhanced microbial synthesis of VB12 analogues. Caecal succinate levels decreased with increasing Co intake, mimicking the effects of dietary VB12. In Experiment 3, rats were fed HAMS diets with or without high-dose Co to confirm these effects and assess microbiota changes. Co supplementation restored the abundance of Akkermansia, which utilises VB12 and its analogues. These findings suggest that maintaining sufficient colonic VB12—through direct supplementation or Co-stimulated microbial production—may help mitigate HAMS-induced succinate accumulation and support balanced colonic fermentation.