Aims Conventional definitions of sarcopenia based on lean mass fail to capture low lean mass relative to fat mass, i.e., relative sarcopenia. Unlike percent body fat (%BF) and Quételet's (body mass) index (BMI, kg/m 2 ), definitions of obesity based on fat mass index (FMI, kg/m 2 ) are not confounded by lean mass. The objective is to determine the prevalence of sarcopenia, relative sarcopenia, and obesity in CKD, and determine if CKD is associated with relative sarcopenia and obesity, independent of demographics and comorbidities.Methods and Results DXA-derived appendicular lean mass index (ALMI, kg/m 2 ) and FMI were assessed in 13,980 NHANES participants. ALMI, FMI, and ALMI relative to FMI (ALMI FMI ) were expressed as sex-and race/ethnicity-specific standard deviation scores compared with young adults (T-scores) and by age (Z-scores). Sarcopenia was defined as ALMI T-score < -2, relative sarcopenia as ALMI FMI T-score < -2, and low lean mass relative to fat mass for age as ALMI FMI Z-score < -1. Obesity was defined using conventional BMI and %BF cutpoints and as sex-and race/ethnicity-specific FMI cutpoints. Glomerular filtration rate (GFR) was estimated using creatinine-(eGFR Cr ) and cystatin C-(eGFR Cys ). The prevalence of relative sarcopenia was higher than the prevalence of sarcopenia, especially in CKD stages 3b and 4 using eGFR Cr ; these CKD stages were associated with the highest FMI. CKD stage was independently associated with low ALMI FMI for age using eGFR Cys . BMI underestimated and %BF overestimated the prevalence of obesity compared with FMI. CKD was not independently associated with obesity by FMI. ConclusionsIn CKD, conventional definitions of sarcopenia underestimate muscle deficits and %BF overestimates the prevalence of obesity. CKD is independently associated with relative sarcopenia, but not excess adiposity.

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Background This study aims to assess the construct validity of a body composition-defined definition of sarcopenic obesity based on low appendicular lean mass relative to fat mass (ALMI FMI) and high fat mass index (FMI) and to compare with an alternative definition using appendicular lean mass index (ALMI) and percent body fat (%BF). Methods This is a secondary analysis of two cohort studies: the National Health and Examination Survey (NHANES) and the Health, Aging, and Body Composition study (Health ABC). Sarcopenic obesity was defined as low ALMI FMI combined with high FMI and was compared with a widely used definition based on ALMI and %BF cut-points. Body composition Z-scores, self-reported disability, physical functioning, and incident disability were compared across body composition categories using linear and logistic regression and Cox proportional hazards models. Results Among 14, 850 participants from NHANES, patients with sarcopenic obesity defined by low ALMI FMI and high FMI (ALMI FMI-FMI) had above-average FMI Z-scores [mean (standard deviation): 1.00 (0.72)]. In contrast, those with sarcopenic obesity based on low ALMI and high %BF (ALMI-%BF) had below-average FMI Z-scores. A similar pattern was observed for 2846 participants from Health ABC. Participants with sarcopenic obesity based on ALMI FMI-FMI had a greater number of disabilities, worse physical function, and a greater risk of incident disability compared with those defined based on ALMI-%BF. Conclusions Body composition-defined measures of sarcopenic obesity defined as excess adiposity and lower-than-expected ALMI relative to FMI are associated with functional deficits and incident disability and overcome the limitations of using %BF in estimating obesity in this context.

Background Conventional definitions of sarcopenia based on lean mass may fail to capture low lean mass relative to higher fat mass, that is, relative sarcopenia. The objective of this study is to determine the associations of sarcopenia and relative sarcopenia with mortality independent of co‐morbidities, and whether chronic kidney disease (CKD) and adiposity alter these associations. Methods Dual energy X‐ray absorptiometry‐derived appendicular lean mass index (ALMI, kg/m 2 ) and fat mass index (FMI, kg/m 2 ) were assessed in 14 850 National Health and Nutrition Examination Survey participants from 1999 to 2006 and were linked to death certificate data in the National Death Index with follow‐up through 2011. Sarcopenia was defined using sex‐specific and race/ethnicity‐specific standard deviation scores compared with young adults (T‐scores) as an ALMI T‐score < −2 and relative sarcopenia as fat‐adjusted ALMI (ALMI FMI ) T‐score < −2. Glomerular filtration rate (GFR) was estimated using creatinine‐based (eGFR Cr ) and cystatin C‐based (eGFR Cys ) regression equations. Results Three (3.0) per cent of National Health and Nutrition Examination Survey participants met criteria for sarcopenia and 8.7% met criteria for relative sarcopenia. Sarcopenia and relative sarcopenia were independently associated with mortality (HR sarcopenia 2.20, 95% CI 1.69 to 2.86; HR relative sarcopenia 1.60, 95% CI 1.31 to 1.96). The corresponding population attributable risks were 5.2% (95% CI 3.4% to 6.4%) and 8.4% (95% CI 4.8% to 11.2%), respectively. Relative sarcopenia remained significantly associated with mortality (HR 1.32, 95% CI 1.08 to 1.61) when limited to the subset who did not meet the criteria for sarcopenia. The risk of mortality associated with relative sarcopenia was attenuated among persons with higher FMI ( P for interaction <0.01) and was not affected by CKD status for either sarcopenia or relative sarcopenia. Conclusions Sarcopenia and relative sarcopenia are significantly associated with mortality regardless of CKD status. Relative sarcopenia is nearly three‐fold more prevalent amplifying its associated mortality risk at the population level. The association between relative sarcopenia and mortality is attenuated in persons with higher FMI.

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