Impaired Cardiorespiratory Fitness in Type 1 Diabetes Is Associated With Metabolic Inflexibility and Specific Factors According to Sex.

Subjects with type 1 diabetes had significantly lower VO2max compared to age- and sex-matched controls.

• VO2max was 32 ± 9.2 mL/kg/min in cases versus 39 ± 7.9 mL/kg/min in controls (p < 0.01).
• 54 cases and 54 controls were evaluated, 52% female, with age 39 (28–46) years.
• Disease duration was 21 ± 10 years and HbA1c was 7.9 (7.3–8.7).
• The study design was observational and cross-sectional, with participants undergoing treadmill cardiopulmonary exercise testing.

Maximum carbohydrate oxidation during exercise was significantly lower in persons with type 1 diabetes compared to controls.

• Maximum carbohydrate oxidation was 809 (614–1174) in cases versus 1082 (863–1454) in controls (p < 0.01).
• Indirect calorimetry was used to measure energy substrate use at rest and during exercise.
• The authors interpreted this as evidence of altered metabolic flexibility due to low substrate availability.
• Baseline physical activity, resting energy expenditure, respiratory quotient, and body composition were similar between groups.

In women with type 1 diabetes, the effect of diabetes status on VO2max was mediated primarily by carbohydrate oxidation and heart rate reserve.

• 73% of the effect of diabetes status on VO2max in women was mediated by carbohydrate oxidation.
• 25% of the effect of diabetes status on VO2max in women was mediated by heart rate reserve.
• Mediation analysis was used to identify these pathways.
• These findings suggest sex-specific determinants of cardiorespiratory fitness impairment in type 1 diabetes.

In men with type 1 diabetes, the effect of diabetes status on VO2max was mediated primarily by carbohydrate oxidation and phase angle.

• 78% of the effect of diabetes status on VO2max in men was mediated by carbohydrate oxidation.
• 57% of the effect of diabetes status on VO2max in men was mediated by phase angle.
• Phase angle is a measure derived from bioelectrical impedance analysis reflecting body composition and cellular health.
• The mediating role of phase angle in men but heart rate reserve in women highlights distinct sex-specific pathways.

Baseline physical activity, resting energy expenditure, respiratory quotient, and body composition were similar between persons with type 1 diabetes and controls.

• Bioelectrical impedance analysis was used to assess body composition.
• Indirect calorimetry was used to assess resting energy expenditure and respiratory quotient.
• Similarity of these baseline characteristics suggests that the observed differences in CRF and carbohydrate oxidation are not explained by differences in physical activity level or resting metabolism.
• Groups were paired by sex and age.

The authors attributed impaired cardiorespiratory fitness in type 1 diabetes to metabolic inflexibility and altered substrate availability during intense exercise.

• Lower carbohydrate oxidation was interpreted as reflecting altered metabolic flexibility.
• The authors suggested this 'could explain the earlier fatigue during intense exercise' observed in this population.
• Multivariate linear regression models and mediation analysis were used to evaluate determinants of CRF.
• The study focused on persons with uncomplicated, long-term type 1 diabetes to isolate the effect of diabetes itself from complications.