In this study, we examined pre-exercise insulin reductions on consequent metabolic and dietary patterns for 24 h after running in individuals with type 1 diabetes. Seven participants self-administered their Full rapid-acting insulin dose or 75%, 50% or 25% of it, immediately before consuming a 1.12-MJ meal. After 2 h, participants completed 45 min of running at 70% peak oxygen uptake ([Vdot]O(2peak)). Blood glucose and insulin were measured for 2 h before and 3 h after exercise. Blood glucose, diet, and administered insulin were self-recorded for 24 h after exercise. Data were analysed using repeated-measures analysis of variance. Pre-exercise peak insulin concentrations were greatest with the Full dose and consequently elicited the lowest blood glucose concentrations (P < 0.05). Blood glucose decreased under all conditions with exercise, with the fall with the Full dose (-6.1 mmol . l(-1), s(x) = 0.4) greater than with 25% insulin (-3.2 mmol . l(-1), s(x) = 0.4; P < 0.05). There was little change in blood glucose from 0 to 3 h post-exercise under all conditions (P > 0.05). Blood glucose at 3 h post-exercise was greatest with the 25% dose. Over the next 21 h, blood glucose area under the curve was greater with the 25% dose compared with all other trials despite consuming less energy and fewer carbohydrates (P < 0.05). A 75% reduction to pre-exercise insulin results in the greatest preservation of blood glucose, and a reduced dietary intake, for 24 h after running in individuals with type 1 diabetes.