Association of Physical Activity to the Risk of Obesity in Adults with Physical Disabilities

Masaru Teramoto, PhD, MPH

Assistant Clinical Professor, Department of Health Sciences, College of Nursing and Health
Professions, Drexel University, 1601 Cherry Street, MS 9503, Philadelphia, PA 19102, USA, Tel. +1-267-359-5718, Fax: +1-267-359-5722; E-mail: Masaru.Teramoto@drexel.edu

INTRODUCTION

Obesity is a major health concern in most countries today.¹ Currently in the U.S., more than one-third of adults are reported to be obese.² Obesity is associated with a number of chronic diseases, such as coronary heart disease, stroke, type 2 diabetes and certain types of cancer.³ The annual medical spending attributed to obesity in the U.S. was estimated to be close to $150 billion in 2008, which was nearly doubled since 1998.⁴ Therefore, it is essential to raise awareness of the importance of reducing the risk of obesity in the general public.

One of the populations requiring much attention with respect to the risk of obesity is adults with physical disabilities. Physical disability is defined as a significant deviation or loss of body functions and structures, limiting physical activity and participation.^5,6 According to the U.S. Census Bureau,⁷ approximately 41.5 million adults had some types of physical disabilities in 2010. It was reported that those with physical disabilities used a wheelchair, cane, crutches, or walker; had difficulty in walking, climbing stairs, lifting; and had conditions contributing to an activity limitation, including arthritis or rheumatism, back or spine problem and broken bone or fracture.⁷ Research shows that the prevalence of obesity in this population is 1.2- to 3.9- times higher than among those without physical disabilities.⁸ Due to limited mobility, total energy expenditure of those with physical disabilities is generally low,⁸ increasing the likelihood of the accumulation of excess body fat and the loss of muscle mass and bone mineral content. Consequently, adults with physical disabilities are considered to be at increased risk of obesity-attributable diseases, including type 2 diabetes and coronary heart disease.^9,10

Physical activity plays a major role in reducing the risk of obesity and diseases associated with obesity.¹¹ Currently, it is recommended that people should engage in at least 150 minutes per week of moderate-intensity physical activity or 75 minutes per week of vigorous-intensity physical activity to gain substantial health benefits.¹² However, due to limited mobility and functional capacity, adults with physical disabilities cannot engage in physical activity in the same manner as people without disabilities can. According to Healthy People 2010,¹³ 56% of people with disabilities report no leisure-time physical activity compared to 36% of people without disabilities. To help those with physical disabilities engage in physical activity, adapted physical activity programs are available, so that people with various disabilities can enjoy and compete together.

Although adults with physical disabilities are also recommended the same amount of physical activity as that of the general population,¹² it is not fully understood how much physical activity is needed in order to decrease the risk of obesity for this population. The current paper discusses how physical activity is associated with the risk of obesity in adults with physical disabilities.

PHYSICAL ACTIVITY ON BODY MASS INDEX

BMI (kg/m²) classifies individuals into different weight categories and is widely used in epidemiological studies. The World Health Organization¹⁴ defines BMI of less than 18.50 kg/m² as underweight, between 18.50 and 24.99 kg/m² as normal range, greater than or equal to 25.00 kg/m² as overweight and greater than or equal to 30.00 kg/m² as obese. It has been shown that higher BMI values are correlated with greater risks of diseases, including type 2 diabetes, hypertension and cardiovascular disease.¹⁵

Past studies reported that individuals with physical disabilities who were physically active did not meet the BMI criteria for obesity (Table 1). Two studies found that active individuals with Spinal Cord Injury (SCI) or Multiple Sclerosis (MS) were overweight (BMI = 25.3 and 26.0 kg/m² respectively).^16,17 Lussier et al.¹⁸ examined two female wheelchair athletes competing nationally and internationally. They reported BMI values of 16.5 and 19.2 kg/m² for each athlete, the lowest among the selected studies. In the other studies, BMI values of active individuals with physical disabilities were within the normal range.^{19,20,21,22,23,24,25,26}

These findings in the reviewed studies differ from those in the literature indicating that adults with physical disabilities have a higher risk of obesity.^27,28 However, it has been well documented that regular physical activity helps achieve and maintain optimal weight and BMI among the general population.²⁹ In addition, three studies in table 1 compared BMI between individuals with physical disabilities who were physically active and disabled or able-bodied ones who were sedentary/inactive.^17,23,25 The studies indicated that BMI of those with physical disabilities who engaged in regular physical activity was lower compared to those who were sedentary/inactive regardless of whether they had disabilities or not. In particular, Slawta et al.¹⁷ reported that mean BMI values of individuals with MS who engaged in light- to moderate-intensity and heavy-intensity physical activity were 26.0 and 23.1 kg/m² respectively, whereas BMI of those with MS who were inactive was 30.4 kg/m². The results of this study may indicate a dose-response relationship between physical activity levels and BMI values among adults with physical disabilities. This dose-response relationship has been observed in the general population.^30,31 Therefore, physical inactivity, not physical disabilities, appears to be a major determinant of BMI and obesity in this population.

It should be noted that there is a limitation associated with using BMI to determine whether or not people are obese. BMI does not account for the composition of body mass. For example, individuals who have a large muscle mass (e.g., strength athletes) may be classified as overweight or obese by BMI when they are actually not in terms of the amount of fat mass. In addition, adults with physical disabilities could undergo drastic changes in fat and fat-free mass components following injury as a result of disuse atrophy.^10,32 In fact, a decrease in muscle/fat-free mass and an increase in fat mass after SCI have been frequently observed.^33,34 Furthermore, recumbent length, often used instead of height when individuals with physical disabilities cannot stand and maintain straight posture, is not likely to provide accurate BMI values. Hence, it is not clear whether the BMI standards used for the general population should also be used for adults with physical disabilities. McDonald et al.³³ suggest that traditional BMI standards would underestimate obesity in people with SCI and thus it would be necessary to develop new BMI criteria for this population. However, BMI is practical, easy to obtain and suitable for large epidemiological studies that are currently lacking and will need to be conducted for special populations. This future research can help develop new BMI standards for special populations, if traditional BMI criteria are not appropriate for them.

PHYSICAL ACTIVITY ON BODY COMPOSITION

Body composition is commonly expressed as %BF. Excessive body fat indicated by high %BF is associated with diseases, including cardiovascular disease, stroke, hypertension and diabetes.^3,35 There are no universal standards of %BF; however, Gater and Clasey³² suggest that healthy male and female adults should have %BF of less than 18% and 33%, respectively, while %BF of greater than 22% (males) and 35% (females) are considered obese. Typical techniques to estimate %BF includes: Hydrostatic Weighing (HW), Dual-energy X-ray Absorptiometry (DXA), Skinfold measurements (SKF), whole-body counting of potassium (40K), Total Body Electrical Conductivity (TOBEC) and Bioelectrical Impedance Analysis (BIA).

Past studies that assessed %BF of physically active individuals with physical disabilities are listed in table 1. Slawta et al.¹⁷ found relatively high %BF (30.8-37.6%) among older females with MS (average age of 45-50 years) who engaged in various intensities of physical activity. This observation may have been partly due to the characteristics of the participants in the study, because %BF is generally higher among women than among men and tends to increase with age.³⁶ On the other hand, two studies reported low %BF (15.6 and 17.4%, respectively) of competitive/elite athletes with SCI or poliomyelitis.^25,37 Ide et al.²⁶ conducted anthropometric measurements, including %BF, of more than 800 wheelchair marathon racers for 10 years. The study found an average %BF of 18.1-18.4%, even though the gender of the racers is not specified. Compared with BMI, more variability of the data exists in %BF across the studies. In the reviewed studies, the most common methods used to estimate %BF were DXA^{9,21,22,23,24} and SFK,^17,20,25,26 followed by HW^16,18,19, along with TOBEC³⁷ and 40K.¹⁸

Several studies in table 1 compared %BF between the groups with different physical activity levels. The majority of the studies found lower %BF among active individuals with physical disabilities compared to those with or without disabilities who were sedentary/inactive.^17,23,25,37 The results of these studies may indicate that regular physical activity is effective in improving body composition of adults with physical disabilities, as this relationship has been demonstrated in the general population.²⁹ However, when compared to able-bodied individuals who were active/athletes, %BF of individuals with physical disabilities who were active/athletes was much higher.^9,19,22,25 Specifically, Jones LM et al.⁹ found that highly active individuals with SCI had %BF of 27.5% compared to 17.8% among age-, height- and weight-matched able-bodied controls with similar activity levels. Also, two elite female wheelchair athletes had very low BMI values (16.5 and 19.2 kg/m² respectively) but their %BF was around 30% estimated by HW or 40K.¹⁸ The findings in these studies indicate that, in spite of engaging in regular physical activity, adults with physical disabilities may have difficulty in increasing or maintaining muscle mass because of the limited ability to contract muscles. This assumption may explain the general trend seen in the reviewed studies that active individuals with physical disabilities had a normal range of BMI but higher %BF. By failing to maintain muscle mass, an individual could lose weight resulting in a lower BMI, but without a concomitant reduction in body fat, %BF would become higher.

Caution must be taken when interpreting the results of the selected studies. All of the studies used a two-component (2-C) model to estimate %BF, though the DXA is sometimes referred to as a three-component (3-C) model.³⁸ A 2-C model divides the body into two parts, fat and fat-free body, whereas a 3-C model divides the body into three parts (e.g., fat, bone mineral and bone-free lean tissue) and a four-component (4-C) model divides the body into four parts, fat, mineral, water and protein.³⁸ More assumptions derived from the reference body must be made when using a 2-C model to estimate %BF compared with using multi component models, such as 3-C and 4-C models.³⁸ Therefore, any deviations in the components of fat-free body (i.e., water, mineral and protein) from the reference body would increase the measurement errors.³⁸ For example, potential changes in the composition of fat-free body following injury among individuals with SCI are well known.^10,32 Hence, there is a greater possibility of underestimating or overestimating %BF of individuals with physical disabilities when a 2-C model is used.

In order to obtain an accurate estimation of %BF among individuals with physical disabilities, a 4-C model is recommended.³² However, a 4-C model requires more measurements than a 2-C or 3-C model and thus is expensive and time-consuming. If a 4-C model is not feasible, DXA may be superior to 2-C model methods for estimating %BF of paralyzed individuals because the DXA model measures bone mineral content of the body to estimate %BF. It has been shown that bone mineral density is altered in people who are wheelchair-bounded or paralyzed.^39,40 Therefore, DXA could minimize the measurement errors associated with the variations in bone mineral density between the reference body and individuals with physical disabilities. SKF, on the other hand, does not seem to be an appropriate technique for predicting %BF of adults with physical disabilities, including those with SCI.^19,41 However, SKF may be used to assess subcutaneous fat deposits that can be compared within and between individuals. It would be particularly useful to develop practical but valid and reliable techniques for estimating %BF of individuals with physical disabilities. One such example is the work of P.S Kocina⁴² who used a multicomponent model and developed a BIA prediction equation to estimate %BF of individuals with SCI.

EFFECTIVE VOLUME OF PHYSICAL ACTIVITY TO REDUCE THE RISK OF OBESITY

The volume of physical activity necessary to prevent obesity among adults with physical disabilities is not clearly defined. However, health experts recommend that 60 min of daily physical activity is needed to prevent weight gain in the general population.⁴³ Until the guidelines specific to the special populations are developed, it seems prudent that this volume of physical activity recommended for able-bodied individuals be applied to individuals with physical disabilities, as well.

There is the limited number of studies that quantified the volume of physical activity and examined the association between physical activity levels and the prevalence of obesity among adults with physical disabilities (Table 1). Two studies reported that male adults with SCI (mean age = 35.6 and 34.7 years, respectively) whose BMI values were within the normal range (22.6-23.2 kg/m²) participated in recreational activities at least 3 days per week and spent a total of 8-10 hr per week for the activities.^21,22 Mojtahedi et al.²³ found that young male athletes with SCI (mean age = 22.5 years) engaging in a combined 12 hr of sport-specific activities and 3 hr of resistance training per week had mean BMI of 22.2 kg/m² and %BF of 25.1%. Other studies have indicated that an hour of exercise per day for 2-3 days per week could achieve favourable BMI and %BF among people with SCI.^24,37 According to Slawta et al.¹⁷ higher intensity of physical activity seemed to be correlated with lower BMI and %BF among active individuals with MS. In addition, the study suggests that physical activity comparable to walking pace above 4 mph may be necessary to achieve a normal range of BMI among people with Ribeiro et al.²⁴ found that wheelchair basketball players with poliomyelitis who practiced the minimum of 1 hr/day for 3 days/week had mean BMI of 23.0 kg/m² and %BF of 25.2%. Zwiren and Bar-Or²⁵ reported favourable BMI (21.0 kg/m²) and %BF (17.4%) values among wheelchair athletes with poliomyelitis or paraplegia who were competing internationally.

CONCLUSIONS AND IMPLICATIONS FOR FUTURE RESEARCH

Physical activity can reduce the risk of obesity and diseases associated with obesity by helping achieve and maintain optimal BMI and body composition. It appears from current literature that adults with physical disabilities can also lower the risk of obesity to some extent by participating in regular physical activity, as well. Individuals with physical disabilities who are physically active tend to have a normal range of BMI and their BMI could be lower than that of sedentary people with or without disabilities. However, caution must be used when interpreting BMI data from this population, because it does not take into account the composition of the body mass; individuals with physical disabilities likely have a reduced muscle mass and an increased fat mass. %BF of active adults with physical disabilities appears to be lower compared to sedentary/inactive counterparts but could be still higher compared to active, able-bodied people. This higher %BF among active adults with physical disabilities could be due to the measurement errors associated with estimating %BF of the special populations or due to the assumption that reported physical activity levels among those with physical disabilities are not sufficient to increase/maintain muscle-mass. Because of the limited number of studies, we were unable to identify the optimal volume of physical activity that can reduce the risk of obesity among adults with physical disabilities. However, even 2-3 hr of physical activity per week may positively affect BMI and body composition for adults with physical disabilities. In addition, there is likely a dose-response relationship such that higher volume of physical activity seems to further reduce the risk of obesity among this population. Until further research is conducted, the physical activity guideline for the general population to prevent obesity (i.e., 60 min/day) may also be recommended to adults with physical disabilities.

Clearly, there is a limited number of existing studies that examine the association between physical activity levels and the prevalence of obesity among adults with physical disabilities. Moreover, recent literature on this topic is certainly scarce and some of the studies reviewed in this paper were published more than 10 years ago. Further studies, especially large epidemiological studies, will be needed to determine how different type/mode, intensity, time and frequency of physical activity affect BMI and %BF in those with physical disabilities. Because of the potentially misleading conclusions drawn from BMI in this population, future studies should include the assessment of body composition. To facilitate future studies, it will be necessary to develop accurate and practical techniques for estimating %BF of individuals with physical disabilities. Past studies have focused mainly on people with SCI; therefore, adults with physical disabilities other than SCI will need to be studied in the future, as well. Individuals with physical disabilities are at increased risk for becoming obese, but engaging in regular physical activity and maintaining active lifestyles will likely lessen the prevalence and severity of obesity and obesity-related diseases among them. Therefore, it is imperative to develop general physical activity recommendations for the special populations to help them reduce the risk of obesity and improve the quality of life.

CONFLICTS OF INTEREST

The authors declare that there is no conflict of interest regarding the publication of this paper.

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