Existing scholarship on prison diets has emphasised the role of food and its restriction as a key aspect of the deterrent system of prison discipline introduced in the 1860s. Here we suggest that a strong emphasis was placed on dietary regulation after the establishment of the reformist, but also ‘testing’, separate system of confinement in the mid-nineteenth century. While the impact of diet on the physical health of prisoners was a major concern, we argue that the psychological impact of food was also stressed, and some prison administrators and doctors argued that diet had an important protective function in preserving inmates’ mental wellbeing. Drawing on a wide range of prison archives and official reports, this article explores the crucial role of prison medical officers in England and Ireland in implementing prison dietaries. It highlights the importance and high level of individual adaptations to dietary scales laid down centrally, as a means of utilising diet as a tool of discipline or as an intervention to improve prisoners’ health. It examines the forays of some prison doctors into dietary experiments, as they investigated the impact of different dietaries or made more quotidian adjustments to food intake, based on local conditions and food supplies. The article concludes that, despite central policies geared to establishing uniformity and interest in new scientific discourses on nutrition, a wide range of practices were pursued in individual prisons, mostly shaped by practical rather than scientific factors, with many prison medical officers asserting their autonomy in making dietary adjustments.