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Health and Social Services Centre, Sherbrooke University Institute of Geriatrics, Faculty of Physical and Sports Education, University of Sherbrooke, Quebec, Canada.
Address correspondence to Martin Brochu, PhD, Centre de recherche sur le vieillissement, Institut universitaire de gériatrie de Sherbrooke, 1036, rue Belvédère Sud, Sherbrooke (Québec) Canada J1H 4C4. E-mail: martin.brochu{at}usherbrooke.ca
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Methods. A structural equation modeling strategy was used to test an explanatory model of the association between body composition and physical capacity. The concept of physical capacity was assessed by walking speed at a normal pace and the one leg stand test. To test the model, 904 men and women between 67 and 84 years old were studied. Body composition was measured by dual-energy x-ray absorptiometry (DXA). Confounding factors related to body composition and physical capacities were included in the explanatory model (physical activity level, age, gender, and number of reported diseases).
Results. The final model showed that physical capacity can be represented by a factorial first-order model including generic measures of walking speed and balance. Moreover, our results showed that percentage of FM was significantly associated with physical capacity (p <.01), whereas no such association was observed with FFM. Other variables such as physical activity level, number of self-reported diseases, and age were associated with physical capacity (all p <.01). Overall, the proposed model explained 48% and 57% of the variance observed in men and women when using the one leg stand and the walking speed at normal pace tests as measures of physical capacity.
Conclusion. FM was significantly and inversely correlated with physical capacity, whereas FFM was not associated when controlled for other potential confounding variables. More studies are needed to investigate the effect of different levels of obesity on physical capacity in older individuals.
Aging is also accompanied by significant changes in body composition. We now know that fat mass (FM) increases and fat-free mass (FFM) decreases with age, even when body weight (3) and physical activities (4) remain stable. Variations in FM and FFM have been found to be related to lower PC in elderly persons (5). However, there is some uncertainty regarding whether high FM or low FFM is more related to declines in PC. Some studies reported that low FFM was associated with incapacity (6,7), whereas others reported that FM is the major contributor (8,9).
It is difficult to get a clear picture of the association between body composition and PC because of differences in methodologies used. For example, studies have used self-reported measures such as activities of daily living (ADL), instrumental activities of daily living (IADL), and the Medical Outcomes Study 36-item survey (SF-36) to evaluate PC limitations (10,11). In contrast, other studies have used direct measures such as walking speed, leg extension power, and the chair stand test to measure PC (12,13). Despite the fact that questionnaires have been shown to be easy to administer and generally useful for measuring PC at a population level (14), tasks related to PC are complex and known to be influenced by psychological status, judgment, mood state, fitness level, and strength (15). These latter factors may explain the low correlations observed (r < 0.60) between self-reported and direct measures of PC (16,17). One other major problem relates to the methods used to determine body composition. Studies have used body mass index (BMI) (9), prediction equations (18), bioimpedance (7), or waist circumference (19).
The aim of this study was to investigate the contribution of FM and FFM to PC when taking into account other confounding variables such as physical activity level, age, and the number of self-reported diseases (13). To overcome limitations in the literature, we used (i) direct measures of body composition and PC with a large cohort of older men and women, and (ii) structural equations with latent variables (two or more measures combined to build a specific concept analyzed with Linear Structural Relationship [LISREL] software).
| METHODS |
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Inclusion criteria were: French or English speaking, willing to commit for a 5-year period, able to walk without help, free of disabilities in ADLs, no cognitive impairment, able to walk 300 meters and to climb 10 stairs without rest, and able to sign an informed consent form. Exclusion criteria were: class II heart failure; chronic obstructive pulmonary disease requiring home oxygen therapy or oral steroids; inflammatory digestive disease; and cancer treated by radiation therapy, chemotherapy, or cancer surgery in the 5 years prior to enrollment. All participants signed an informed consent document approved by the ethics committees of both institutions.
For the purpose of the present study, data from 465 women and 439 men were analyzed because only participants from the Sherbrooke area underwent measurement using dual-energy x-ray absorptiometry (DXA). Two objectives tests were used to measure PC: walking speed at normal pace and one leg stand test. This article uses baseline data of the longitudinal study.
Assessment of PC
PC measurements used in this study were adapted from a previous study that had used similar validated tests (20). The tests used in the present study were chosen for their validity and accessibility and were supervised by trained research assistants.
Walking speed (normal pace).-- Participants walked twice at their usual pace over a 4-meter course (21). Time in milliseconds was recorded between the 1st and the 4th meter with a sport watch. The best trial was used for data analysis. Distance (m) divided by time (ms) was used to calculate walking speed.
Balance (one leg stand).-- Without shoes, participants stood 1 meter from the wall with one foot raised about 6 inches off the ground (22). During the procedure, they kept their hands on their hips and stood on one leg. The maximum time for the test was 60 seconds. The time was stopped when participants used the wall to maintain their balance or when they put their foot down. The mean performance on each side after two attempts was recorded, and the mean score for both legs was used in the analysis.
Body Composition
FM and FFM were measured using DXA (GE Lunar Prodigy; Madison, WI). Test–retest measures in our laboratory yielded a mean absolute difference of 5.7% and 1.1% for FM and FFM, respectively. Percent body fat and appendicular skeletal mass index (ASMI, kg/m2) were then calculated. ASMI (muscle mass of both legs and arms divided by height [m2]) was used to assess ASMI. This index has been reported to be a good way to quantify the association between muscle mass and functional limitations (23).
Physical Activity Level
Physical activity level was assessed using the Physical Activity Scale for the Elderly (PASE), which is validated in older populations (24). Daily activity was then recorded as a global performance according to the intensity and time of reported activities.
Sum of Self-Reported Diseases
Diseases were reported using a modified version of the Older American Resources and Services questionnaire (25). Participants were asked to answer "yes" (1) or "no" (0) if they had been diagnosed by a physician for 25 common diseases (e.g., diabetes, osteoporosis, high blood pressure). Participants also had the possibility of adding other diseases at the end of the questionnaire. The total number of diseases was recorded by adding the number of positive answers.
Data Analysis
Data are presented as means ± standard deviation (SD). Unpaired t tests were used to compare men and women. LISREL was used to estimate coefficients in a set of structural equations to test the basic hypothesis, which is the fit of our model with the present literature. In simple terms, this software allows simultaneous analysis of two or more dependent variables (such as walking speed and balance) as only one concept (PC, as proposed in the present study), and control for confounding variables found in the scientific literature.
First, we tested the homogeneity of variances and the normality of the distribution across variables. Correlation matrices were calculated with PRELIS, a LISREL utility program. Second, we tested the invariance hypothesis (
= 
+
) to determine if there a relationship between the study variables and the literature data with a chi-square test. The plausibility of the hypothetical model was evaluated by the strategy of structural equation proposed by Jöreskog and Sörbom (26). The ML (maximum likelihood) method was used to estimate the results as proposed by Rivera and Satorra (27). Root mean square error of approximation (RMSEA) was used to guide the overall fit of the model (28). Finally, the 95% statistical threshold was used in our analyses to evaluate the structural coefficient. All lambdas x (
x) and theta-delta (
) were fixed at 1, whereas theta-epsilon (
), lambda y (
y) were set free, except for Models 1 and 2, in which they were fixed at 1.
| RESULTS |
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Based on the LISREL procedure, the chi square was not significant in all models (p >.05). This result indicates that the three models studied fit with the literature. Furthermore, the adjustment index RMSEA also indicates that the three models are in agreement with the literature (p <.05). Models 1 and 2 were completely saturated (p = 1.00) because there was only 1 "y" variable analyzed (Table 2).
| DISCUSSION |
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Our observations also showed that FM had a comparable effect on PC in both men and women, as shown by the correlation reported. This observation seems to be in agreement with those of Buchner and colleagues (31), who also reported no relationship between gender and walking speed in a population with characteristics similar to those in our population. However, contrary to the present study, others reported that percentage of FM affects physical capacities differently in men and women (32). For example, Jensen and Friedmann (33) showed (when using self-reported questionnaires) that only obese women had an increased risk of having physical limitations. Knowing that correlations between questionnaires and direct measures of PC are generally weak (16,34), discrepancies between our results and those of other published studies must be interpreted with caution. Furthermore, in the present study, adiposity level was measured with DXA, whereas others used BMI (35,36).
Some limitations of the present study need to be addressed. First, our cohort was composed of independent and generally healthy older individuals, which limits our ability to generalize to other populations. This may lead to underestimation of the observed correlations between FFM and PC impairment previously reported by Buchner and colleagues (31). In fact, based on the walking speed threshold of 0.8 m/s proposed by Lauretani and colleagues (37) to define poor PC, only 5.4% of our participants had a slower average walking speed. Similarly, Janssen and colleagues (38) proposed a threshold of 7.26 kg/m2 in men and 5.45 kg/m2 in women as a minimum ASMI to have normal functional capacities. Despite the fact that PC was assessed with self-reported questionnaires in their study, if we consider those gender thresholds, only 16 men and 9 women in the present study were below the proposed values. Second, the cross-sectional design precludes conclusions regarding cause and effect relationships. Finally, some may argue that the use of percentage of FM to assess obesity levels may not be appropriate because two individuals can present similar relative values despite large differences in absolute terms. However, we obtained the same results when using other measures of body composition in the model such as %FFM as well as total FFM and FM.
Despite the limitations reported, the present study used a robust study design in a large and well-characterized cohort composed of men and women between 68 and 82 years old. In addition, instead of using questionnaires, we used valid, reliable, and direct measures of PC. Furthermore, the two physical tests studied are reliable (39) and commonly used to measure PC (6,7,40). Finally, body composition was measured with DXA. Taken together, we consider that the methodology used strengthens our results.
Conclusion
After controlling for potential confounding variables reported in the literature, percentage of FM was significantly associated with the PC concept whereas FFM was not when both measures of body composition were included in the same model. Our results may suggest that weight management interventions would be helpful to maintain or improve PC in older men and women. However, because obesity and aging are associated with long-term disability (41,42), institutionalization (43), and premature death (2), intervention and prospective studies should be conducted to investigate the long-term effects of the level of obesity on PC in older obese individuals.
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Received November 13, 2006
Accepted March 9, 2007
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