Background: While Body Mass Index (BMI) is widely used in epidemiology, it does not differentiate between fat and lean mass, nor capture fat distribution. Furthermore, longitudinal evidence regarding how specific tissue compartments predict disease progression in populations undergoing rapid nutritional transitions remains scarce. This study investigated whether baseline body composition and central adiposity metrics predict the progressive worsening of weight-related complications.
Methods: This longitudinal study utilised five years (2015–2020) of follow-up data from 2,438 adults with overweight or obesity (BMI ≥ 25 kg/m²) in a rural Iranian cohort. Baseline exposures included Fat Mass Index (FMI), Fat-Free Mass Index (FFMI), and Waist-to-Height Ratio (WHtR). Weight-related complications were assessed annually using the Edmonton Obesity Staging System (EOSS), which categorises morbidity across medical, mental, and functional domains. Generalised Estimating Equations (GEE), adjusted for sociodemographic and lifestyle covariates, modelled longitudinal associations and tested for sex-based effect modification.
Results: Over the 5-year follow-up, the clinical severity of obesity significantly worsened across the cohort with no observed remission. In fully adjusted models, higher FMI and WHtR were independently associated with EOSS progression, whereas FFMI offered no protective metabolic advantage. A 0.1-unit increase in WHtR was associated with 43.9% higher odds of clinical deterioration, while a 1-unit increase in FMI increased odds by 7.1%. Significant sex interactions were observed; WHtR and FMI strongly predicted progression in females, whereas no significant associations were found in males, which is partially attributable to significantly fewer progression events in the male subgroup.
Conclusion: WHtR and FMI independently predict the longitudinal worsening of clinical obesity complications, with WHtR serving as the strongest relative predictor. Shifting clinical focus toward precise tissue distribution rather than generic mass offers significant advantages for risk stratification, helping anticipate end-organ damage and guide targeted interventions.