Twenty-Year Trends in Body Composition-based Obesity Phenotypes and Mortality Risk in US Adults

Technical Summary: Twenty-Year Trends in Body Composition-based Obesity Phenotypes and Mortality Risk in US Adults

Problem Statement
Obesity remains a critical global public health challenge, yet the traditional reliance on Body Mass Index (BMI) is increasingly recognized as insufficient. BMI fails to capture critical nuances regarding body fat distribution and muscle composition, leading to the misclassification of obesity phenotypes and suboptimal metabolic risk assessment. While the concurrent decline in muscle mass and increase in adiposity (sarcopenic obesity or adiposity-lean mass disproportion) is clinically significant, there is a lack of longitudinal data and standardized diagnostic criteria to elucidate long-term trends and their epidemiological trajectory. Furthermore, existing studies linking fat-to-muscle ratios to mortality have predominantly focused on elderly populations or utilized cross-sectional designs that preclude causal inference, leaving a gap in understanding these dynamics within the general adult population over extended periods.

Methodology
This study utilized a prospective cohort design constructed from eight cross-sectional datasets of the National Health and Nutrition Examination Survey (NHANES) spanning 1999–2018.

• Participants: The analysis included 26,829 US adults aged 20 years and older. Participants were excluded if they lacked body composition data or person-year information.
• Body Composition Assessment: Whole-body Dual-Energy X-ray Absorptiometry (DXA) was performed using Hologic fan beam densitometers. Key metrics included fat mass (FM), fat-free mass (FFM), appendicular muscle mass (AMM), and trunk fat mass.
• Phenotype Definition: Two distinct body composition-based obesity phenotypes were defined using age-standardized reference curves derived from NHANES 1999–2004 data:
  1. General Adiposity-Lean Mass Disproportion (GALMD): Defined as a fat-to-fat-free mass ratio exceeding the 95th percentile.
  2. Central Adiposity-Appendicular Muscle Disproportion (CAAMD): Defined as a trunk fat-to-appendicular muscle mass ratio exceeding the 95th percentile.
• Mortality Follow-up: Mortality status was ascertained via probabilistic matching with the National Death Index through December 31, 2019. The median follow-up was 11.3 years, resulting in 4,117 deaths (including 1,284 cardiovascular and 927 cancer deaths).
• Statistical Analysis: Analyses accounted for NHANES complex survey design (weights, stratification, clustering). Age- and sex-adjusted trends were assessed using linear and logistic regression. Associations between body composition indices/phenotypes and mortality were evaluated using multivariable Cox proportional hazards models, adjusting for demographics, socioeconomic factors, and lipid profiles. Restricted cubic splines were used to model non-linear associations.

Key Results

• Temporal Trends (1999–2018): Over the two-decade period, age- and sex-adjusted mean fat mass and trunk fat mass decreased (by 1.6 kg and 1.4 kg, respectively), while fat-free mass and appendicular muscle mass increased (by 1.5 kg and 0.7 kg, respectively). Consequently, the fat-to-fat-free mass ratio and trunk fat-to-appendicular muscle mass ratio declined significantly.
• Prevalence of Phenotypes: The prevalence of both GALMD and CAAMD decreased over time. GALMD prevalence dropped from 6.2% to 4.9%, and CAAMD from 6.2% to 3.6%. However, significant disparities existed: trends were favorable for Non-Hispanic White and Black populations, whereas Hispanic and Non-Hispanic Asian populations showed divergent patterns, with some groups experiencing increases in fat-to-muscle ratios.
• Mortality Associations:
  • Continuous Variables: Fat mass and trunk fat mass exhibited a reverse J-shaped relationship with all-cause mortality, with the lowest risk observed at a fat-free mass of 55–80 kg. Appendicular muscle mass below 22 kg was positively associated with mortality, while higher values were protective. Fat-to-muscle ratios showed U-shaped patterns, with extremes linked to increased mortality.
  • Phenotypes: Both GALMD and CAAMD were independently associated with higher all-cause and cause-specific mortality compared to non-disproportionate groups.
    • GALMD: Associated with a 29% increased risk of all-cause mortality (HR: 1.29) and a 57% increased risk of cardiovascular mortality (HR: 1.57).
    • CAAMD: Demonstrated even stronger associations, with a 54% increased risk of all-cause mortality (HR: 1.54), an 83% increased risk of cardiovascular mortality (HR: 1.83), and a 68% increased risk of cancer mortality (HR: 1.68).
  • Subgroup Analysis: CAAMD was particularly linked to increased all-cause mortality in women, individuals under 40, and Non-Hispanic Blacks.

Significance and Claims
The paper claims to provide the most recent national trend estimates of body composition and body composition-based obesity phenotypes in the US over a 20-year period. Its primary contributions include:

1. Epidemiological Evidence: It offers critical evidence that while overall adiposity-lean mass disproportion prevalence has declined in the US, significant racial, ethnic, and age-based disparities persist, particularly affecting Hispanic, Asian, and older populations.
2. Mortality Risk Stratification: The study establishes that body composition-based obesity phenotypes, specifically central adiposity-appendicular muscle disproportion, are robust predictors of all-cause and cause-specific mortality, often outperforming general adiposity measures in risk stratification.
3. Clinical Implications: The findings support the clinical significance of monitoring fat-to-muscle ratios rather than BMI alone. The authors suggest that the "central adiposity-appendicular muscle disproportion" phenotype represents a particularly high-risk group, warranting personalized prevention and treatment strategies, especially for women and younger adults where traditional cardiovascular risk factors may be less prevalent.
4. Methodological Advancement: By utilizing a large, nationally representative sample with extended follow-up and DXA-derived metrics, the study addresses gaps in previous literature regarding the temporal dynamics and mortality implications of these specific phenotypes in the general population.

The authors conclude that maintaining a lower trunk fat-to-appendicular muscle mass ratio is critical for reducing mortality risk, highlighting the need for population-specific metabolic risk thresholds that account for body composition rather than relying solely on BMI.