Effects of a 50% versus 100% pre-prandial insulin bolus reduction to improve glycemic safety during postprandial continuous and intermittent exercise in adults with type 1 diabetes treated with multiple daily injections

Objective: This study aimed to evaluate the effect of a 50% reduction in preprandial bolus insulin (50%-B) on plasma glucose (PG) responses during postprandial exercise of continuous moderate intensity (CONT) and intermittent high intensity (INT) in individuals with type 1 diabetes (T1D). Methods: Sixteen adults with T1D (31% male), treated with multiple daily insulin injections (MDI), participated in a randomized crossover study comprising four experimental conditions, separated by a washout period of at least 48 hours. Participants performed two 30-minute, preceded by a 3-minute warm-up without weights:• CONT: continuous cycling at 60% of maximal aerobic power (MAP).• INT: 2-minute intervals alternating between 40% and 80% of MAP, repeated for 7 intervals, with the last interval adjusted so that the total exercise time is exactly 30 minutes. Each exercise modality was performed under two insulin conditions: a full preprandial bolus (100%-B) and a 50% reduction (50%-B). Plasma glucose, insulin, and cortisol were measured before, during, and after exercise. Linear mixed models were used to analyze temporal changes and condition effects. Results: Blood glucose decreased significantly over time for both exercise types (p < 0.001). During CONT, the decline in PG was similar between doses (Δ100%-B: -3.01 ± 2.96 vs. Δ50%-B: -2.82 ± 2.28 mmol/L; p = 0.18), However, the nadir PG was higher with 50%-B compared to 100%-B (8.59 ± 4.07 vs. 5.69 ± 3.06 mmol/L, respectively; β = +2.91 mmol/L; p = 0.026), and hypoglycemia was less frequent (2 vs. 18 episodes; p = 0.028). During INT, PG decreased less with 50%-B than with 100%-B (Δ: -2.03 ± 1.63 vs. -3.62 ± 2.76 mmol/L; p = 0.022), with no hypoglycemic episodes under 50%-B compared to six with 100%-B. Mean PG remained higher with 50%-B across both exercise types (p < 0.01). Plasma insulin decreased over time (p = 0.038) regardless of bolus condition, while cortisol increased more during INT with 100%-B than with 50%-B (p = 0.02). Conclusions: Reducing the preprandial bolus insulin by 50% effectively attenuates exercise-induced declines in plasma glucose and substantially reduces hypoglycemia risk, particularly during intermittent high-intensity exercise. These results emphasize the clinical relevance of personalized insulin adjustments to enhance metabolic safety during exercise in individuals with T1D.