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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 62:859-865 (2007)
© 2007 The Gerontological Society of America

High Body Mass Index and Physical Impairments as Predictors of Walking Limitation 22 Years Later in Adult Finns

Sari Stenholm, Päivi Sainio, Taina Rantanen, Seppo Koskinen, Antti Jula, Markku Heliövaara and Arpo Aromaa

Department of Health and Functional Capacity, National Public Health Institute, 1 Turku and2 Helsinki, Finland.
3 The Finnish Center for Interdisciplinary Gerontology, Department of Health Sciences, University of Jyväskylä, Finland.

Address correspondence to Sari Stenholm, MSc, Department of Health and Functional Capacity, National Public Health Institute, Peltolantie 3, FI-20720 Turku, Finland. E-mail: sari.stenholm{at}ktl.fi


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
Background. Our aim was to study the effects of high body mass index (BMI) and physical impairments in midlife on later life walking limitation.

Methods. Primarily middle-aged persons (aged 32–72 years) with no walking limitation at baseline (n = 840) were followed-up for 22 years as a part of the Mini-Finland Follow-up Survey. Incident walking limitation (walking speed < 1.2 m/s or difficulty in walking 0.5 km) was predicted by measured BMI, handgrip strength, squatting test, and self-reported running difficulties.

Results. Twenty-one percent of the participants developed walking limitation. After adjustment for multiple potential confounders, high BMI, low handgrip strength, impaired squatting, and running difficulties were significant predictors of incident walking limitation. The odds ratio (OR) of walking limitation was 4.55 (95% confidence interval [CI], 1.32–15.74) for squatting difficulties and 2.39 (95% CI, 1.26–4.55) for major running difficulties as compared to participants with no difficulties. The corresponding ORs for handgrip strength and BMI were 0.56 (95% CI, 0.38–0.81) and 1.39 (95% CI, 1.10–1.75) per an increment of 1 standard deviation. For persons in the highest BMI tertile who had two or more physical impairments, the adjusted risk of walking limitation was 4.5 times higher in comparison to normal weight persons with no physical impairments.

Conclusions. In primarily middle-aged persons, BMI and simple tests of physical impairment strongly predicted the development of walking limitation 22 years later. In addition, physical impairments coexisting with high BMI predisposed to later life walking limitation more than high BMI alone. Therefore, increasing physical fitness by physical activity and promoting weight loss in middle age may prevent mobility limitation and subsequent disability in old age.

MOBILITY is essential for human functioning. Difficulties in mobility often predict the onset of further disability (1) and mortality (2,3). However, very little long-term follow-up information exists about early risk factors of mobility limitations.

Obesity is a growing public health problem in developed countries. Although there is evidence that excess weight in middle age predicts late-life disability (4–8), we are aware of only one long-term longitudinal study about the effect of obesity on mobility limitations (9). In addition, Houston and colleagues (10) found that recalled weight at age 25 is significantly associated with functional limitations in late adulthood, but in their study no information about possible functional limitation at baseline was available.

Another important area related to mobility that can be modified by proper interventions is physical fitness. Results from the Honolulu Heart Program showed that low muscle strength in middle age predicts disability 25 years later (11) and mortality over 30 years (12). Impaired pulmonary function has been found to predict disability 15–25 years later (13). The long-term predictive effect of cardiovascular fitness on mobility limitations has not been studied, but there is evidence that poor cardiovascular fitness precedes the metabolic syndrome (14) and mortality (15). In addition to obesity and physical impairments, certain chronic conditions in middle age, such as osteoarthritis (4,6,16), back pain (6), and hypertension (6,7,13,17) seem to predict poor functioning in old age.

It is particularly appropriate to study early risk factors of walking limitation with simple tests and self-report questions because they can be easily applied in healthcare settings. This study aimed at examining the effects of high body mass index (BMI) and physical impairments in midlife on incident walking limitation 22 years later in initially nondisabled persons. Also the effect of coexisting physical impairments with high BMI was studied.


   METHODS
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 Methods
Results
 Discussion
 References
 
Study Population
The Mini-Finland Follow-up Survey comprises survivors from the population-based Mini-Finland Health Examination Survey (1978–1980) (18) who lived in seven selected large municipalities in 2000–2001. Altogether, 1278 persons were invited to take part in the follow-up study, which was carried out as part of the Health 2000 Survey (19). The present study was limited to persons reporting no walking limitation (were able to walk 500 m) and not pregnant at baseline, and who were 55 years old or older at follow-up (n = 954). Only 11.9% (n = 114) of the eligible sample were unable or unwilling to participate in the follow-up. Among the 840 persons included in the analyses, there were 245 participants aged 32–39 years, 333 aged 40–49 years, 197 aged 50–59 years, and 65 aged 60–72 years at baseline.

Details of the design and implementation of the Mini-Finland Health Examination Survey (18,20) as well as Health 2000 Survey (19) have been reported elsewhere. All the participants signed a written informed consent, and the study was approved by the Ethical Committee for epidemiology and public health in the hospital district of Helsinki and Uusimaa in Finland.

Baseline Measurements
BMI.-- Body weight and height were measured in light indoor clothing without shoes. BMI was defined as weight divided by the square of height (kg/m2). The cut-points for BMI tertiles were 24.1 and 26.5 kg/m2 in men and 22.7 and 26.1 kg/m2 in women. The highest tertile is referred to as overweight. Standard cut-points for normal weight (BMI < 25 kg/m2), overweight (BMI 25–29.9 kg/m2), and obesity (BMI ≥ 30 kg/m2), suggested by the World Health Organization, were also used (21).

Measurement of physical impairments.-- Overall muscle strength was estimated by handgrip strength (22), which was measured in kiloponds (kp) using a handheld dynamometer based on strain gauge sensors (Bruel-Kjaer Type 1526; Denmark) (23). Grip strength was measured in the seated position with elbow flexed at a 110°–140° angle, and the width of the handle was adjusted for the participant's hand size. The best result of the stronger hand was used in the analyses. There was a high correlation between the test–retest results (r = 0.91–0.93, n = 449) (23). The cut-points for gender-specific tertiles were 50 and 58 kp in men and 27 and 33 kp in women. The lowest tertile is referred to as impaired strength.

General fitness was determined by the answer to the question: "Can you run a longer distance (about 0.5 km)?" The four response categories were: without difficulties, with minor difficulties, with major difficulties, or not at all. The last two categories were combined to represent impaired fitness. Lower limb performance was assessed with a squatting test (24). Participants were asked to squat and stand up once. They were allowed to gently take support from a table to keep their balance. A trained nurse observed and evaluated the performance: normal (thighs at least in horizontal level), impaired (thighs above horizontal level, but lean angle more than 45°), or unable (thigh lean angle less than 45°). For this study, the categories impaired and unable were merged. Repeatability over 3 months was moderate both for running ({kappa} = 0.59, n = 392) (23) and squatting ({kappa} = 0.66, n = 793) (25).

Assessment of potential confounders.-- Factors reported in the literature as predictors of mobility limitation were considered as potential confounders (26), and the association between these variables and walking limitation were ascertained before the analyses (Table 1). Specially trained physicians diagnosed chronic diseases during a clinical examination by combining the results of measurements and biochemical analyses with data from interviews and questionnaires, and by using current good treatment practice as a reference (18,27). For this study, diagnoses of chronic bronchitis, angina pectoris, heart failure, hypertension, diabetes, low back syndrome, and knee osteoarthritis were included.


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  		Table 1. Baseline Characteristics of Participants With or Without Walking Limitation 22 Years Later.

 
The basic questionnaire elicited information on habitual leisure time physical activity level with three alternatives: regular physical exercise (e.g., running, biking, gymnastics), occasional physical exercise or physically active hobbies (e.g., gardening, hunting, outdoor recreation), and little physical exercise (e.g., reading, watching television). Also, average weekly alcohol consumption (g/wk) during the preceding month was determined. The limit for heavy alcohol use was set at 280 g/wk for men and 140 g/wk for women. The answers were checked and completed by trained nurses during the examination. Smoking behavior was determined in an interview, and participants were classified as never smokers, ex-smokers, or current smokers. Level of education was based on the highest completed degree reported in the interview, and was classified as basic (usually no more than 8 years of education), intermediate (9–12 years), and higher education (> 12 years).

Follow-Up Measurements
Maximal walking speed was measured over a distance of 6.1 meters (28) using a stopwatch. Participants were instructed to "walk to the end of the course as fast as you can," starting from a standstill. A walking aid was allowed if the person normally used one. The reliability of the walking speed test, measured with intra-class correlation coefficient, was moderately good (r = 0.77). Self-reported walking difficulty was assessed using the question: "Are you able to walk about half a kilometer without resting?" The four response options were: without difficulty, with minor difficulties, with major difficulties, and not at all.

Participants were considered to have a walking limitation if their walking speed was < 1.2 m/s (n = 120) or if they were unable to finish the test (n = 11). Speed of 1.2 m/s was chosen as a proxy for the ability to cross a street safely at a traffic light (29). For persons who did not participate in the walking test (n = 78) or did not get a result for technical reasons (n = 20), self-reported difficulty walking 500 meters was considered to indicate walking limitation. Of all persons with both measured and self-reported information on walking limitations, 73% of those reporting difficulty in the 500 meter walk had walking speed < 1.2 m/s.

Statistical Analysis
To compare the baseline characteristics of participants with and without incident walking limitation, chi-square test, Fisher exact test, and t test were used. Interaction effects between gender and BMI, handgrip strength, running, and squatting were studied. Furthermore, age-adjusted and gender-specific incidences of walking limitation in different BMI categories were calculated, and the linearity of the association was tested with the generalized linear model (GLM) procedure of the SAS statistical package. Multivariate analyses with logistic regression models were used to estimate the risk of walking limitation according to baseline BMI, handgrip strength, running, and squatting. All analyses were adjusted for age, gender, education, baseline physical activity, smoking status, alcohol use, and physician-diagnosed chronic diseases. In addition, the models including running and squatting were adjusted for BMI. The models including handgrip strength were adjusted for body height and weight, because handgrip strength correlates strongly with body size (30). The association of BMI, as well as handgrip strength, with walking limitation was found to be linear (tested with the SAS-GLM procedure); they were therefore used as continuous variables in logistic regression models. All analyses were performed using the SAS System for Windows, version 9.1 (SAS Institute, Inc., Cary, NC).


   RESULTS
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 Abstract
 Methods
 Results
Discussion
 References
 
Over the 22-year follow-up, walking limitation developed in 176 persons (21% of the study group), of whom 125 were women and 51 men (Table 1). High BMI was a significant predictor of walking limitation in both genders. The age-adjusted incidence of walking limitation increased linearly with increasing BMI (Figure 1). Because there was no interaction between BMI and gender (p =.29), the results are shown as combined for both genders. After adjustment for potential confounders, the odds ratio (OR, with 95% confidence interval [CI]) per an increment of 1 standard deviation (SD) for BMI was 1.39 (1.10–1.75) (Table 2).


Figure 01
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  		Figure 1. The age-adjusted incidence of walking limitation by baseline body mass index (BMI) and gender (and 95% confidence intervals). Incident cases and total number of subjects in each BMI category are shown within the bars

 

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  		Table 2. Risk of Walking Limitation 22 Years Later According to Baseline BMI and Physical Impairments.

 
Physical impairments in middle age (participants aged 32–72 years) predicted later life walking limitation (Table 2). The age- and gender-adjusted risk of walking limitation was approximately fourfold among participants who had major running or squatting difficulties compared to participants who had no difficulties. In addition, poor handgrip strength was a significant predictor of walking limitation (OR = 0.56 per an increment of 1 SD, 95% CI, 0.40–0.78). The associations between physical impairments and walking limitation remained statistically significant after further adjustment for education, baseline lifestyle factors, and chronic diseases. Concerning the confounding factors, age, education, physical activity, and chronic bronchitis had an independent effect on walking limitation among all the physical impairments.

To study the effect of the physical impairments, with or without coexisting overweight, on walking limitation, participants were categorized into six groups according to BMI and physical impairments. The highest BMI tertile is referred to as overweight and the lowest handgrip strength tertile to as impaired strength. Impaired running and squatting represents the other physical impairments (Table 3). The age- and gender-adjusted risk of walking limitation among participants who at baseline were in the highest third of BMI and had two or more physical impairments was 6.4 times that of the participants who were neither overweight nor physically impaired. After adjustment for potential confounders, ORs attenuated, but remained high among participants who were in the highest third of BMI and had, in addition, at least two physical impairments (OR = 4.43, 95% CI, 1.72–11.36). In addition, the participants who were not overweight, but who had two or more impairments had 3.4 times higher risk (95% CI, 1.46–8.03) for later life walking limitation compared to participants who were neither overweight nor physically impaired.


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  		Table 3. Risk of Walking Limitation 22 Years Later According to the Number of Physical Impairments With or Without Coexisting Overweight at Baseline.

 
There was a statistically significant interaction between BMI and 500 meters' run (p =.04). Walking limitation emerged less frequently in participants who were in the lowest BMI tertile regardless of their running ability. In contrast, the incidence of walking limitation increased strongly among participants in the highest BMI tertile who had running difficulties at baseline. Other physical impairments at baseline also tended to increase the risk of walking limitation, particularly among the persons in the highest BMI tertile, but the interactions were not statistically significant (data not shown).


   DISCUSSION
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 Abstract
 Methods
 Results
 Discussion
References
 
This population-based study provides evidence that high BMI and simple tests of physical impairments in midlife predict incident walking limitation 22 years later even after adjustment for potential confounders. In addition, two or more physical impairments coexisting with high BMI increased the risk of later life walking limitation more than did high BMI alone.

The present study is one of the first in which the long-term effect of BMI, a surrogate of body fat (31), on incident walking limitation is investigated among men and women with a wide age range. Thus, the findings add to our knowledge about the predictive value of BMI on mobility limitation beyond self-reported data (9). The effect of BMI on walking limitation may partially be mediated through medical conditions. For example, excess body weight can cause mechanical stress on lower limb joints, causing knee and hip osteoarthritis (32). In addition, obesity is a known risk factor for diabetes, hypertension, and cardiovascular diseases (21), which in turn predict mobility limitations (33,34).

We are not aware of earlier longitudinal studies on the effects of general fitness and lower limb performance on later life functioning. Our results emphasize the importance of good physical fitness in middle age in regard to future mobility. There are different mechanisms by which physical impairments may affect future disability. Impairments may precede chronic conditions or they may indicate a subclinical disease (12), which predicts impaired functioning. For example, low muscle strength has shown to precede knee osteoarthritis (35) and metabolic disorders, such as insulin resistance and subsequent diabetes (36). The effects of poor general fitness may be mediated through cardiovascular diseases, because low physical fitness is associated with many cardiovascular disease risk factors (37). Therefore, we adjusted the models for the presence of chronic conditions at baseline. This attenuated the effects, which still remained highly significant. Consequently, the higher prevalence of chronic conditions among persons with physical impairments at baseline does not fully explain the effect. In contrast, it is possible that diseases that have emerged after the baseline examination may have an effect on incident walking limitation. Unfortunately, our only follow-up data on incident diseases came from the final examination. It would have been valuable also to have data on each participant's health status over the whole follow-up period. Having this information would have enabled us to study the pathway leading to later life walking limitation. Similarly, the effect of weight change on later life walking limitation was not reported in this study. However, it was found in the additional analysis that weight change over 22 years was not a statistically significant predictor of walking limitation. Despite adjusting for weight change, high BMI remained associated with walking limitation (data not shown). Nonetheless, our results unequivocally demonstrate the considerable predictive power of BMI and physical impairments on future walking limitation.

Physical impairments and obesity as predictors of later life walking limitation are a complex combination, as persons who have physical impairments are likely to avoid physical activity, thus gaining weight as a result of reduced energy expenditure. Obesity itself may lead to avoidance of physical activity, resulting in impaired physical fitness. There is evidence that obese persons have lower aerobic capacity (38) and lower relative muscle strength (39) than normal-weight persons.

The present study indicates that both coexistence of physical impairments with high BMI and two or more coexisting physical impairments without overweight predict walking limitation over two decades. Up to now, only few studies have investigated the effect of coimpairment on disability or mortality. Rantanen and coworkers (40,41) found that co-occurrence of strength and balance impairments in older women is associated with severe walking disability. In addition, they found that the combination of low muscle strength and high BMI predicts mortality over 30 years (12). It is important to study coimpairments as predictors of functional decline, because with increasing age the proportion of people with many impairments will increase (42). Early intervention to prevent coimpairments, or a single impairment, may have the potential to reduce mobility limitation in older people.

The major strength of our study is the long follow-up period of 22 years. There are not many long-term prospective studies about high BMI and physical impairments as risk factors for old-age functional limitation and disability (4–7,9,11,13). In addition, our sample consists only of persons who did not report any walking limitations at baseline, which supports the causality between impairments and functional limitations. Unfortunately, walking speed was not measured at baseline. Even if the participants were able to walk 500 meters, they may have had different levels of physical functioning at baseline, which may have affected, at least in part, the studied relationships. It was also important to have baseline information about lifestyle factors and chronic diseases to control for potential confounders. Finally, most of our explanatory factors were objectively assessed at baseline, and the outcome variable "walking limitation" was mainly based on objective measurements.

Nonparticipation and the selection in relation to either the risk factors or the outcome may reduce the validity of results in a prospective study like the current one. As only large municipalities were selected to the follow-up examination, and because a substantial proportion of the initial cohort had died during 22 years, the effects of selection could not be controlled in the current study. However, previous studies have shown that low handgrip strength (12,43) as well as obesity (44–46) predict mortality. Thus, it is likely that the selection from the baseline examination to the invitation in the reexamination has likely led to conservative rather than pronounced risk estimates in the current study. In addition, those survivors who were invited, but were lost to follow-up were older, had higher BMI, lower handgrip strength, and more medical conditions at baseline in comparison with persons who remained in the study. Furthermore, the prevalence of difficulties at baseline in running 500 meters and squatting was higher among the dropouts than among the participants. As nonparticipants generally tend to have more mobility difficulties than participants do (47), we expect that our results may rather underestimate than overestimate the associations of BMI and physical impairments with walking limitation.

The simple tests used in this study captured relevant information among middle-aged persons to predict future walking limitation. There are plenty of measurements and questionnaires about functional impairments and limitations for the older population, but simple and valid instruments for identifying early risk factors of functional decline among the middle-aged population are scarce. Therefore, the findings of this study are important regarding the feasibility of the measurements and questions in primary healthcare, developing countries, and large-scale epidemiological studies. Further research is needed about the predictive power of these tests on more severe disability, difficulties in activities of daily living (ADLs), and mortality. In addition, the underlying causes of physical impairments should be recognized for the purpose of planning preventive interventions against physical decline.

Conclusion
High BMI and physical impairment determined by simple tests strongly predicted incident walking limitation 22 years later in initially nondisabled, primarily middle-aged persons, suggesting that high body weight and low physical fitness in midlife may turn into functional limitation in later life. Therefore, increasing physical fitness by means of physical activity and promoting weight loss in overweight middle-aged persons can be expected to prevent mobility limitation and subsequent disability in old age.


   Acknowledgments
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 Methods
 Results
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This study was funded by the Juho Vainio Foundation, Finland.


   Footnotes
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Decision Editor: Darryl Wieland, PhD, MPH

Received June 27, 2006

Accepted November 9, 2006


   References
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