Data Source and participants
The National Health and Nutrition Examination Surveys (NHANES) is an ongoing repeated cross-sectional study at the Centers for Disease Control and Prevention (CDC), conducted by the National Center for Health Statistics (NCHS). The aim of the NHANES is to investigate the general nutritional status and health of non- institutional population in the United States with a representative sample. Database in five sections (Demographics, Dietary, Examination, Laboratory, and Questionnaire) was collected by well-trained examiner every two years. NHANES program has been approved by NCHS the Ethics Review Board, and all participants have written informed consent. All NHANES data and information are publicly available at https://www.cdc.gov/nchs/nhanes/index.htm.
We performed an analysis based on data from two 2-year NHANES survey cycles: 1999–2000 and 2001–2002. We finally selected 2550 out of 3707 participants aged 60–85 years of NHANES in 1999–2002. We excluded the individuals with missing body composition measures (n = 723), missing questionnaires on cognitive function (n = 434).
Body Composition Measurement
All body compositions were measured by a dual energy X-ray absorptiometry (DXA) QDR-4500 Hologic scanner (Bed- ford, MA). The measurement limitation of the scanner on height was 192.5 cm in maximum and 136.4 kg on weight. Individuals beyond these ranges were considered missing related data. The NHANES reported the data of total skeletal muscle mass, appendicular lean mass (ALM), lean mass percent, fat mass, total body fat percent and bone mineral content. All measurements were operated by trained technicians normatively, and all metal objects (except false teeth and hearing aids) had to be removed during the measurement procedure. Individuals with other non-removable metal objects were forbidden from measurement.
ALM was the sum of muscle mass of all four upper/lower extremity limbs. In this research, according to the two FNIH definitions[2], we classified men whose ALM adjusted for BMI(ALMBMI) < 0.789 as sarcopenia, and women < 0.512, and men whose ALM < 19.75 kg as alternate sarcopenia, and women < 15.02 kg.
Cognitive Performance Assessment
The cognitive performance was assessed by Digit Symbol Substitution Test (DSST), version of the WAIS III (Wechsler Adult Intelligence Scale, Third Edition) in NHANES 1999–2002. The test was aimed at participants aged 60 and over, Proxy interviews were ineligible. The DSST is highly sensitive to neuropsychological dysfunction[15], it can measure many areas of cognitive function, in particular attention, cognitive and psychomotor speed, executive functions and visual scanning[16]. The DSST requires participant to draw the symbol under the corresponding number according to the provided key. The number of correct symbols in 120 seconds is the final score of the test. The maximum score is 133. Higher scores on the DSST indicated better cognitive performance.
Covariates
For covariates, continuous variables included age (year), poverty income ratio (PIR) and comorbidity index. Diabetes mellitus, congestive heart failure, coronary artery disease, chronic obstructive pulmonary disease (chronic bronchitis and/or emphysema) and hypertension, cancer consisted of comorbid conditions. The number of subjects reported conditions were then combined to generate an ordinal comorbidity index[17].
The Categorical variables included sex (male, female), race (Mexican American, other hispanic, non-hispanic white, non-hispanic black, other race), educational level (less than high school, high school or general educational development, above high school, unknown), physical activity (Less than moderate, moderate, Vigorous, unknown) and smoking status (never, former, current, unknown).
Statistical Analysis
All data was downloaded, merged and analyzed following the CDC guidelines (https://wwwn.cdc.gov/nchs/nhanes/tutorials/default.aspx). Marked variance was accounted and the proposed weighting methodology was used in our analyses. We also took sample weight into consideration and assigned it to each participant[18]. Continuous variables were presented as mean ± standard deviation (normal distribution) or median (quartile) (skewed distribution), and categorical variables were presented in frequency or as a percentage. The One-Way Anova (normal distribution), Kruscal Whallis H (skewed distribution) test and chi-square tests (categorical variables) were used to determine any statistical differences between the means and proportions between participants having sarcopenia or not. We conducted the following statistical analyses to explore the association between sarcopenia and cognitive performance. Firstly, adjusted only univariate and multivariate line regression analysis were employed. Three models were constructed and used in our analyses: model 1, no covariate was adjusted; model 2, we adjusted gender, age and race; model 3, we adjusted all covariates presented in Table 1, including gender, age, race, poverty income ratio, comorbidity index, educational level, physical activity and smoking status. Then we used a weighted generalized additive model (GAM) and conducted smooth curve fitting (penalized spline method) to explore the potential nonlinearity between sarcopenia and cognitive performance. In addition, we conducted subgroup analysis through weighted stratified line regression models. We perform an interaction test and hierarchical analysis using all covariates above and continuous covariables of them were converted into categorical variables according to their clinical cut points during the subgroup analysis. Besides, we also conducted sensitivity analysis to ensure the robustness of data analysis. All analyses described above were conducted to assess the association between alternative sarcopenia and cognitive performance.
Table 1
Baseline Characteristics of Participants
Sarcopenia | Overall | Yes | No | P value |
| (n = 2550) | (n = 641) | (n = 1909) | |
Mean (mean ± SD) | | | | |
Age (years) | 69.98 ± 7.25 | 71.43 ± 7.44 | 69.60 ± 7.15 | < 0.0001 |
DSST Scores | 47.62 ± 17.83 | 43.52 ± 17.40 | 48.70 ± 17.79 | < 0.0001 |
Proportion (%) | | | | |
Gender | | | | < 0.0001 |
Male | 44.28 | 52.04 | 42.22 | |
Female | 55.72 | 47.96 | 57.78 | |
Race | | | | < 0.0001 |
Mexican American | 2.03 | 3.93 | 1.53 | |
Other Hispanic | 4.49 | 6.71 | 3.91 | |
Non-Hispanic White | 84.32 | 84.3 | 84.32 | |
Non-Hispanic Black | 6.55 | 1.58 | 7.86 | |
Other Race | 2.61 | 3.48 | 2.38 | |
Education level | | | | 0.003 |
Less than high school | 25.74 | 31.53 | 24.21 | |
High school or General educational development (GED) | 28.64 | 28.6 | 28.66 | |
Above high school | 43.9 | 38.64 | 45.28 | |
Unknown | 1.72 | 1.23 | 1.85 | |
Poverty income ratio (PIR) | | | | 0.1554 |
Below poverty (< 1) | 10.12 | 12.18 | 9.58 | |
Above poverty (> 1) | 77.86 | 75.12 | 78.58 | |
Unknown | 12.02 | 12.7 | 11.84 | |
Physical activity | | | | < 0.0001 |
Less than moderate | 41.65 | 52.86 | 38.69 | |
Moderate | 32.61 | 27.98 | 33.84 | |
Vigorous | 18.74 | 10.18 | 21.01 | |
Unknown | 6.99 | 8.98 | 6.47 | |
Smoking status | | | | 0.7971 |
Never | 46.48 | 46.46 | 46.49 | |
Former | 42.98 | 43.8 | 42.77 | |
Current | 10.38 | 9.7 | 10.55 | |
Unknown | 0.16 | 0.04 | 0.19 | |
Comorbidity index | | | | 0.013 |
0 | 35.01 | 32.69 | 35.63 | |
1 | 42.08 | 40.48 | 42.51 | |
≥ 2 | 15.99 | 16.35 | 15.89 | |
Unknow | 3.63 | 5.18 | 3.22 | |
4 | 1.33 | 2.05 | 1.14 | |
6 | 1.96 | 3.26 | 1.61 | |
All statistical analyses were conducted using the statistical package R (http://www.R-project.org, The R Foundation) and Empower (R) (www.empowerstats.com; X&Y Solutions, Inc., Boston, MA). Two-sided p values < 0.05 was considered statistically significant.