Reduced cardiorespiratory fitness has been reported in individuals with long-term type 1 diabetes. In a cross-sectional study published in the Journal of Diabetes, associations between body composition, substrate utilization, blood glucose control, and exercise performance were evaluated using cardiopulmonary exercise testing.

The study included 54 adults with type 1 diabetes and 54 age- and sex-matched controls, with 52% women. The median age was 39 years, the mean duration of diabetes was 21 ± 10 years, and median glycated hemoglobin (HbA1c) was 7.9%. Baseline physical activity, resting energy expenditure, respiratory quotient, and body composition were comparable between groups. Assessments included treadmill cardiopulmonary exercise testing, bioelectrical impedance analysis, and indirect calorimetry at rest and during exercise.

Maximal oxygen uptake (VO₂max) was lower in the type 1 diabetes group at 32 ± 9.2 mL/kg/min, compared with 39 ± 7.9 mL/kg/min in controls (P < 0.01). Maximum carbohydrate oxidation was also lower at 809 (interquartile range 614–1174) versus 1082 (interquartile range 863–1454) (P < 0.01).

Mediation analyses demonstrated sex-specific associations. In women, 73% of the association between diabetes status and VO₂max was mediated by carbohydrate oxidation, and 25% was mediated by heart rate reserve. In men, 78% of the association was mediated by carbohydrate oxidation, while 57% was mediated by phase angle.

These findings indicate that reduced carbohydrate oxidation is associated with lower cardiorespiratory fitness in long-term type 1 diabetes, with distinct mediating factors by sex.