This study aimed to assess the validity of the International Fitness Scale (IFIS) for evaluating cardiorespiratory fitness compared to the 6-minute walk test (criterion validity) and to examine their associations with cardiometabolic and body composition outcomes (construct validity) in adults with type 2 diabetes mellitus (T2DM). A cross-sectional analysis was conducted on 282 adults with T2DM (mean age 63.6 +- 8.1 years, 37.9% women). Self-reported fitness was assessed using IFIS, including overall fitness and cardiorespiratory fitness scores. Objective cardiorespiratory fitness was assessed using the 6-minute walk test. Associations with cardiometabolic (i.e., cardiovascular health score, metabolic dysfunction-associated steatotic liver disease [MASLD], high-sensitive C-reactive protein) and body composition outcomes (i.e., body mass index [BMI], visceral adipose tissue volume, and thigh fat-free muscle volume) were analyzed using ANCOVA and ROC curves, which were compared using DeLong tests. Both IFIS overall and cardiorespiratory fitness strongly correlated with performance in the 6-minute walk test (p<0.001 for ANCOVA models) and were associated with all cardiometabolic and body composition outcomes. ROC analyses showed at least similar predictive performance for IFIS overall and cardiorespiratory fitness scores compared to the 6-minute walk test. Furthermore, IFIS overall outperformed the 6-minute walk test for predicting poor cardiovascular health (AUCs: 0.678 [95% CI: 0.618-0.737] versus 0.586 [95% CI: 0.517-0.654]) as well as MASLD (AUCs: 0.630 [95% CI: 0.563-0.696] versus 0.519 [95% CI: 0.443-0.595]). This study proves that IFIS is a valid tool for assessing physical fitness in T2DM patients and that it conveys cardiometabolic and body composition outcomes at least as well as the 6-minute walk test.

The authors have declared no competing interest.

The EPSOMIP study was supported by ALF Grants, Region Ostergotland (ME, FI, PL, PN), unrestricted grants from GILEAD (ME) and Diapharma (SK), the Lion Research Grant from the Faculty of Medicine, Linkoping University (PN), and the Swedish Research Council (VR 2020-04826 to PL). This specific study was additionally supported by ALF Grants, Region Ostergotland (RO-999299 to PH).

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All recruitment processes and the collection of written informed consent were conducted in accordance with the World Medical Association Declaration of Helsinki (2018). Data collection adhered to the General Data Protection Regulation (EU) 2016/679 (GDPR) and complied with the International Conference on Harmonization Good Clinical Practice (ICH-GCP) guidelines. The EPSONIP study received approval from the Regional Ethical Board of Östergötland, Sweden (2018/176-31 and 2018/494-32) and is registered at clinicaltrials.gov (identifier: NCT03864510).

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The dataset used and/or analyzed during the current study is available from the corresponding author upon reasonable request.