Access to complex carbohydrates can generate ecological memory in gut microbial communities, but it’s unclear if the same is true for simple sugars. We tested whether lactose could “train” a human gut microbiome model to improve lactose metabolism and whether prior exposure leaves lasting ecological effects. A faecal community derived from six healthy donors was grown with lactose over three successive daily transfers (“training period”), followed by a two-day “break” period and re-exposure to lactose on day six. The response of this community was compared to a community that had not encountered lactose. Rather than enhancing lactose metabolism, the training unexpectedly resulted in a slower-growing state with a reduced rate of lactose consumption and acidification. Lactose also decreased alpha diversity and drove substantial compositional restructuring, enriching taxa predicted to possess lactose-metabolising ability, particularly lactic acid bacteria and Bifidobacterium, while depleting many other taxa. Among the genera that were significantly reduced, Dialister was the only genus that increased in abundance following the lactose break period. Recovery after the break period depended on the extent of this taxonomic depletion, with greater losses shifting the community to an alternative stable state. Association analysis linked Limosilactobacillus fermentum to this slower community growth phenotype. The findings show how simple carbohydrate exposure can have a lasting ecological imprint on a gut model community when significant taxonomic losses occur.