BACKGROUND: Cardiorespiratory fitness and fatness (notably central obesity) are mediating factors of the metabolic syndrome (MetS) and consequent cardiovascular disease (CVD)/mortality risk. The fitness-fatness index (FFI) combines these factors and has been reported to be a better indicator of CVD and all-cause mortality risk, beyond the capacity of either fitness or fatness alone. OBJECTIVE: This study sought to investigate the effects of different exercise intensities on FFI in adults with MetS. METHODS: This was a sub-study of the 'Exercise in the prevention of Metabolic Syndrome' (EX-MET) multicentre trial. Ninety-nine adults diagnosed with MetS according to the International Diabetes Federation criteria were randomized to one of the following 16-week exercise interventions: (i) moderate-intensity continuous training (MICT) at 60 to 70% HRpeak for 30 min/session (n = 34, 150 min/week); (ii) 4x4 min bouts of high-intensity interval training at 85 to 95% HRpeak, interspersed with 3-min active recovery at 50 to 70% HRpeak (n = 34, 38 min/session, 114 min/week); and (iii) 1x4 min bout of HIIT at 85- to 95% HRpeak (n = 31, 17 min/session, 51 min/week). Cardiorespiratory fitness (peak oxygen uptake, VO2peak) was determined via indirect calorimetry during maximal exercise testing and fatness was the ratio of waist circumference-to-height (WtHR). FFI was calculated as VO2peak in metabolic equivalents (METs) divided by WtHR. A clinically meaningful response to the exercise intervention was taken as a 1 FFI unit increase. RESULTS: Seventy-seven participants completed pre and post testing to determine FFI. While there was no significant between group difference (p = 0.30), there was a small group x time interaction effect on FFI (F[2,73] = 1.226; partial-eta2 = 0.01), with numerically greater improvements following HIIT (4HIIT +16%; 1HIIT +11%) relative to MICT (+7%). There was a greater proportion of participants who had a clinically meaningful change in FFI following high-volume HIIT (60%, 15/25) and low-volume HIIT (65%, 17/26) compared to MICT (38%, 10/26), but with no significant between-group difference (p = 0.12). A similar trend was found when a sub-analysis comparing the FFI between those with type 2 diabetes (MICT 33%, 3/9; high-volume HIIT 64%, 7/11; and low-volume HIIT 58%, 7/12) and without type 2 diabetes (MICT, 41%, 7/17; high-volume HIIT, 57%, 8/14; low-volume HIIT, 71%, 10/14). CONCLUSION: Although there were no statistically significant differences detected between groups, this study suggests that the response to changes in FFI in adults with MetS may be affected by exercise intensity, when numerical differences between exercise groups are considered. Further research is warranted. TRIAL REGISTRATION NUMBER AND DATE OF REGISTRATION: ClinicalTrials.gov NCT01676870; 31/08/2012.

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