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To investigate whether indicators of obesity is associated with functional disabilities among elderly US women and men.
National Health and Nutrition Examination Survey (NHANES) 1999–2004, United States
1,684 elderly (≥60 years) women and 1,611 elderly men
In women, body mass index (BMI) and waist circumference were each related to higher prevalence of all measures of disabilities. Compared with the lowest quartile of waist circumference, the multivariate odds ratios (OR) and 95% confidence intervals of the highest quartile for having difficulties in functional domains were 2.4 (1.6, 3.6) for activity of daily living, 2.3 (1.6, 3.3) for instrumental activity of daily living, 2.6 (1.6, 4.1) for leisure and social activities, 4.8 (3.4, 6.9) for lower extremity mobility and 2.9 (2.1, 4.0) for general physical activity. In men, these associations were moderate: the corresponding ORs were 1.2 (0.8, 2.0), 1.3 (0.9, 2.1), 2.1 (1.2, 3.7), 1.8 (1.2, 2.7), and 2.1 (1.5, 2.8) respectively. Similar results were obtained for BMI. These associations could not be explained by the presence of major chronic conditions. When adjusted simultaneously, waist circumference appeared to be a better predictor than BMI of disabilities in women.
The results suggest that indicators of obesity are related to functional disabilities among elderly Americans.
Over the past decades, obesity epidemic has become a central public health issue in the United States (US). According to the most recent data from the National Health and Nutrition Examination Survey (NHANES 1999–2004), the prevalence of overall obesity as defined by body mass index (BMI) ≥ 30 kg/m2 was 30% or higher across most gender and ethnicity groups of US adults, and was particularly prevalent among non-Hispanic Blacks and women.(1) The prevalence of abdominal obesity as measured by waist circumference >102 cm in men or 88 cm in women was even higher (52.1%) and has been increasing at a higher rate than overall obesity.(2, 3) This high prevalence of obesity is accompanied by an increasing elderly population in the US, which makes it a public health priority to understand associations between obesity and age-related health issues.
Functional disabilities are common among elderly people. They contribute to higher morbidity and mortality in this already fragile population and substantially increase the health burden to the society. The public health impacts of disability are expected to increase as the population ages. Previous studies have suggested a relationship between overall obesity or BMI and risk of functional disabilities among the elderly.(4–14) Fewer studies,(4–7) however, have evaluated associations between waist circumference, as an indicator of abdominal obesity, and risk of functional disability, despite the fact that abdominal obesity and disproportional visceral fat accumulation are prevalent among elderly and may have independent health implications. Further, obesity increases the risk of many chronic diseases which in turn may contribute to functional disabilities; yet few data are available on whether the obesity-disability relationship is mediated by the presence of these chronic diseases.(6, 7) Therefore, by taking advantage of the data from the most recent NHANES survey (1999–2004), the authors investigated how waist circumference was associated with the prevalence of several measures of functional disabilities and if the associations between obesity and disability could be explained by the presence of several major chronic diseases.
NHANES is a population-based survey that was designed to collect nationally representative data on health and nutrition, using a complex, multistage probability sample of the US civilian and non-institutionalized population. The survey includes an in-home interview on general health status, disease history, and diet and lifestyle, and a health examination at a mobile examination center (MEC). All participants provided written consent and all interviews and examinations were carried out by trained technicians according to standard operation manuals (available at NHANES website: http://www.cdc.gov/nchs/nhanes.htm). In person interviews were conducted in either English or Spanish, using a computer-assisted personal interviewing system. Interview data were checked by NHANES field office staff for accuracy and completeness. The current study was limited to the elderly participants (≥60 years) of the NHANES 1999–2004 survey who completed the physical functioning section of the survey (n= 3,717). Among them, 1,684 women and 1,611 men had data on either BMI or waist circumference and were therefore included in the current analysis.
Anthropometric measures were primarily taken in a private room at NHANES MECs. Body weight was measured using a digital floor scale and standing height was taken with a wall-mounted stadiometer. BMI was calculated as weight in kilograms divided by height in squared meters (kg/m2). Waist circumference was measured using a steel measuring tape. Information on potential confounders (as listed in Table 1) was obtained from home interviews. Physical activity levels were defined as none, light to moderate, or intensive, based on the time spent on various leisure-time activities and the metabolic equivalent score of individual activities. The presence of chronic diseases was defined as ever being told by a doctor or other health professional as having each particular condition. These chronic diseases included diabetes, cardiovascular diseases (CVD: congestive heart failure, coronary heart disease, angina pectoris, myocardial infraction, or stroke, asked individually), chronic obstructive pulmonary disease (COPD: chronic bronchitis or emphysema, asked individually), arthritis, current asthma, cancer of any kind, and fracture and osteoporosis after age 50.
All participants aged 60 years or older were asked 19 questions to evaluate their functional status(15, 16). These questions were phrased to assess the individual’s level of difficulty in performing physical or mental tasks without using any special equipment. For each task, 4 levels of difficulties were allowed: “no difficulty”, “some difficulty”, “much difficulty” and “unable to do”. The items cover locomotion and transfers, household productivity, social integration, and manipulation of surroundings and were classified into five major domains: activities of daily living (ADL), instrumental activities of daily living (IADL), leisure and social activities (LSA), lower extremity mobility (LEM), and general physical activities (GPA), according to published definitions.(15, 16) Disability was defined as having any difficulty in performing one or more activities within a given domain. For functional domains other than LEM, there were few missing values on individual items within each domain. In this case, if participants answered “no difficulty” to the rest of the questions, they were coded as no difficulty for that particular domain. For LEM, 815 missed one item with the other being “no difficulty” and 1 missed both items; to keep consistent with other analyses, they were coded as having “no difficulty”. Nevertheless, sensitivity analyses were conducted by excluding individuals with any missing values from the models and the results were similar.
Analyses were conducted by gender. Waist circumference and BMI were defined as gender specific quartiles in the primary analysis. Additional analyses were also conducted according to standard cutoffs: BMI (kg/m2) <25, 25–29.9, 30–34.9, and ≥35; waist circumference > 88 cm for women or 102 cm for men (17) and the results were similar. Odds ratios (OR) of functional disabilities and 95% confidence intervals (CI) were obtained from logistic regression models, adjusting for known risk factors for disabilities including age, gender, ethnicity, education, physical activity level, smoking and alcohol consumption. Test for linear trend was conducted by including the gender specific median of each exposure category in the model as a continuous variable. To explore the potential role of chronic diseases in mediating relationships between obesity and disability, analyses were conducted to further adjust for the presence of each of the chronic conditions either collectively or individually. BMI and waist circumference were highly correlated and thus make it difficult to assess their independent associations with functional disabilities; nevertheless, analyses were conducted by first evaluating them individually and then simultaneously in the same regression models. As the NHANES weights apply to prevalence estimates of the entire population and this study aimed only to evaluate associations among certain elderly subset, no NHANES weights was adjusted in the analyses. All statistical analyses were conducted with the SAS software, version 9.1 (SAS Institute Inc., Cary, NC). Statistical tests were two-sided with p-values less than 0.05 as statistically significant.
Table 1 shows the characteristics of study participants according to quartiles of BMI and waist circumference distributions. As expected, these two obesity measures are closely correlated with a Pearson correlation coefficient of 0.84 (p < 0.001). In both men and women, higher BMI or larger waist circumference was associated with younger age, current nonsmoker, low physical activity, and the presence of some chronic diseases.
Both BMI (Table 2) and waist circumference (Table 3) were positively associated with all measures of functional disabilities in women; in men, they were associated with most measures of disabilities, but their relationships to ADL and IADL were less evident. Most chronic diseases were themselves related to disabilities: in men, COPD generally showed the strongest associations with disabilities with ORs ranging from 1.7 for having difficulty in GPA to 3.8 for having difficulty in IADL; in women, arthritis generally showed the strongest associations with ORs ranging from 2.6 for IADL to 3.2 for LEM. Nevertheless, adjusting for the presence of these chronic conditions, either collectively (Tables 2 and and3)3) or individually (data not shown), only slightly attenuated the relationships between indicators of obesity and functional disabilities. For example, the ORs of having difficulty in performing ADL between the extreme quartiles without adjusting for chronic conditions were 2.1 for BMI and 2.4 for waist circumference in women, and became 1.8 and 2.0 respectively after the adjustments.
When both obesity measures were put in the same models, most associations for BMI were substantially attenuated in women (Table 2), whereas most associations for waist circumference persisted (Table 3). The pattern was less clear in men.
In this cross-sectional analysis of elderly NHANES participants, indicators of overall and abdominal obesity were individually associated with functional disabilities among elder American women and men. These associations appeared to be independent of the presence of several major chronic conditions. Further, the data suggest that waist circumference may be a better predictor than BMI for certain functional domains in women.
Many epidemiological studies have found that higher BMI, as an indicator of overall obesity, was associated with a higher risk of disability. However, in elderly population, it is probably more important to evaluate the health impacts of abdominal obesity as body composition changes over the life span.(18) Loss of lean body mass and accumulation of visceral fat around abdominal area are more common in the elderly than in their younger peers.(18) Therefore, BMI may not be a good indicator of obesity in the elderly population due to disproportional lose of body lean mass. In NHANES, the prevalence of abdominal obesity as measured by waist circumference was higher among elderly (≥60 year) participants than middle aged adults,(ages 40–59) (3) while that for overall obesity (BMI) was similar or lower.(1) Moreover, the prevalence of abdominal obesity increased over calendar years across most age and ethnicity groups, particularly among individuals overweight but not obese (BMI: 25–29 kg/m2).(3) Previous studies have shown that abdominal obesity as measured by waist circumference was a better indicator of overall mortality and cardiovascular morbidity than BMI among the elders.(19–21)
Despite the importance of measuring abdominal obesity in health research, few epidemiological studies have evaluated indicators of abdominal obesity in relation to functional disability among elderly populations(4–7, 22, 23). Most of these studies found that waist circumference was related to higher prevalence of disabilities. Some further suggest that waist circumference may explain the observed associations between BMI and disabilities (6, 7, 23). Consistently, the current analysis suggests that waist conference may be a better indicator for disability than BMI in elderly women. The pattern in elderly men is less clear.
It is well known that obesity causes high mortality and contributes to higher risk of many chronic diseases, including diabetes, cardiovascular diseases, chronic pulmonary diseases, musculoskeletal illnesses, urinary incontinence, and certain cancers.(3, 18, 24) Many of these conditions may in turn increase the risk of developing functional disabilities. Interestingly, previous studies (6, 7) showed that adjusting for selected chronic conditions either did not change or slightly attenuated the associations between obesity and functional disabilities. More detailed analyses were conducted in the current analysis by adjusting for chronic conditions either collectively or individually; no chronic conditions seemed to explain the associations between indicators of obesity and disability. This suggests that obesity may relate to disability independent of these reported clinical conditions. On the other hand, obesity may relate to frailty syndrome, which in turn may lead to further development of disability among the elderly (25). This possibility should be evaluated in future studies.
Unlike most of the previous studies, this is a large national sample of the US elderly that includes different ethnic groups and social classes and therefore makes the study conclusions more generalizable to overall US elderly women and men. Further, anthropometric measures were conducted by trained technicians according to standard procedures and multiple measurements of functional disabilities were included in the analyses. The major limitation of this study is its cross-sectional nature, which makes a causal inference impossible. It is possible that obesity contributes to the development of functional disability; likewise it is also plausible that disability may lead to sedentary lifestyles which in turn result in weight gain and obesity. This may form a vicious cycle and leads to more severe problems. Finally, institutionalized individuals were not part of the NHANES survey, despite the fact that disabilities are more prevalent among institutionalized individuals.
In conclusion, the current study suggests that indicators of obesity is associated with higher prevalence of functional disabilities among elderly Americans and these associations could not be explained by the presence of major chronic conditions. These findings are of great public health significance in strategic planning for disability prevention and intervention.
This work was supported by the intramural program of the NIH, the National Institute of Environmental Health Sciences (NIEHS) and in part by the NIEHS contract (N01-ES-55547).
AUTHOR CONTRIBUTIONS: Dr. Chen contributed to the concept and design, analysis and interpretation of data, and preparation of the manuscript. Dr. Guo contributed to concept and design, analysis and interpretation of data, and critical revision of the manuscript.
SPONSOR’S ROLE: The Sponsor of this project has no role in the design, methods, subject recruitment, data collections, analysis and preparation of paper.
CONFLICT OF INTEREST: None, as declared in COI table.