Grant: Travel Awards - Institute Community Support

The metabolic syndrome (MetS) increases the risk of cardiovascular diseases and type 2 diabetes. Since fat-free mass (FFM) is a metabolically active tissue, it is generally recognized that a greater FFM has a protective effect on the risk of developing MetS. This assumption is often based on studies reporting FFM as a percentage of body weight (BW). However, recent work from our group suggests that greater absolute FFM could be deleterious to metabolic health. The objectives were thus to investigate if the conflicting results are explained by the various indices of FFM (absolute, relative to BW or relative to height). Secondly, we sought to assess the impact of FFM per se on the relative risk (RR) of MetS. A total of 5401 individuals (46.4 ± 16.2 y; 27.5 ± 5.0 kg/m2) were included in the analyses. Absolute FFM (kg), FFM index [FFMi; (kg/m2)] and FFM% [FFM/BW *100] were determined using DXA. Quartiles were determined in a sex and age-specific fashion. The ATP III definition was used to identify individuals with MetS. The RR of having the MetS was estimated with FFMi, corrected for FM index (FMi) and physical activity (PA). The reference group for RRs was the first quartiles (Q1) of FFMi in each subgroup. The prevalence of MetS was greater with increasing quartiles of FFM or FFMi, yet decreased with higher quartiles of FFM% (all p<0.001). Additionally, the RR of having MetS was greater compared to the reference group in all subgroups (all p<0.05), except for Q2 and Q3 in young men (p=0.347 and 0.095 respectively). These results first suggest that the indices of FFM greatly impacts the association with MetS. Secondly, the present study suggests a greater RR of having MetS with increasing FFMi, even when corrected for FMi and PA levels. Hence, the largely purported idea that a large FFM has a protective effect against metabolic abnormalities might stem from flawed conclusions. It would, therefore, be critical to re-examine the contribution of FFM to metabolic health.