A method for preventing and/or treating vitamin B12 deficiency is taught. The method comprises administering a composition comprising pseudovitamin B12-producing bacteria, optionally in conjunction with mucin-degrading and/or propionate-producing bacteria, to a subject in need thereof. The method is particularly suitable for administration to subjects suffering from vitamin B12 deficiency due to metformin treatment for type-2 diabetes, and to subjects having undergone bariatric surgery.
