Managing insulin dosing around exercise remains a clinical challenge in type 1 diabetes (T1D). A double-blind, laboratory-controlled, randomized crossover study published in the Diabetes, Obesity, and Metabolism, evaluated the effects of dose reductions of ultra-rapid–acting insulin aspart (URA-IAsp) and rapid-acting insulin aspart (IAsp) on blood glucose during moderate-intensity exercise.

Forty-three adults with T1D completed four laboratory-controlled visits. Participants injected either a 50% or 75% reduced dose of URA-IAsp or IAsp with a standardized breakfast 60 minutes before 45 minutes of cycling at approximately 61% V̇O2peak. The same insulin type and dose were repeated four hours later with lunch. Venous blood samples were collected frequently to assess glucose and insulin concentrations. The primary endpoint was change in blood glucose from exercise start to end.

Blood glucose declined during exercise across all conditions. The decline with 50% URA-IAsp (−4.0 ± 2.8 mmol/L) was similar to the other conditions (all p>0.05). However, the 50% IAsp dose (−5.1 ± 3.0 mmol/L) resulted in a greater glucose decline compared with both the 75% reduced-dose URA-IAsp (−2.8 ± 3.3 mmol/L) and 75% reduced-dose IAsp (−3.4 ± 3.3 mmol/L) conditions (both p<0.05). Differences in insulin concentrations reflected dose rather than insulin type.