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

<title>Abstract</title> <p> <bold>Objectives</bold> To examine trends in body composition and sarcopenic obesity, and their associations with all-cause and cause-specific mortality as reflected in NHANES 1998-2018 data. <bold>Methods</bold> Data from 26,829 participants aged 20+ from the 1999–2018 NHANES were analyzed, with mortality risk assessed using the National Death Index through 2019. The study focused on trends in body composition and sarcopenic obesity over nearly 20 years, examining their associations with all-cause and cause-specific mortality. <bold>Results</bold> The mean age of participants ranged from 39.0 to 51.4 years, with women comprising 46.7–50.4% of the sample. Between 1999-2000 and 2017-2018, age- and sex-adjusted mean fat mass and trunk fat mass decreased by 1.6 kg and 1.4 kg, while fat-free mass and appendicular muscle mass increased by 1.5 kg and 0.7 kg, respectively. The fat-to-fat-free mass ratio and trunk fat-to-appendicular muscle mass ratio decreased by 0.05 and 0.08, respectively (all P &lt; 0.001). The prevalence of sarcopenic obesity decreased from 6.2% to 4.9% based on fat-to-fat-free mass, and from 6.2% to 3.6% based on trunk fat-to-appendicular muscle mass. Fat mass and trunk fat mass exhibited a J-shaped relationship with all-cause mortality, with lowest risk at a fat-free mass of 55–80 kg. Appendicular muscle mass below 22 kg was positively associated with mortality, while higher values were inversely related. Extremes in fat-to-muscle ratios were linked to increased mortality. Adjusted models showed that sarcopenic obesity, defined by either fat-to-fat-free mass or trunk fat-to-appendicular muscle mass, was associated with higher all-cause and cause-specific mortality compared to non-sarcopenic obesity. <bold>Conclusions</bold> This survey data from 1999 to 2018 shows long-term trends in body composition and sarcopenic obesity. Fat mass decreased while fat-free mass and appendicular muscle mass increased. Sarcopenic obesity associated with higher all-cause and cause-specific mortality. </p>