Is Disability Obesity's Price of Longevity?

The long-term effect of the increasing obesity epidemic is one of the most closely watched and actively debated issues in public health today.¹ Because of the diverse consequences of obesity, there is concern that it could reverse many of the public health successes that have occurred in recent decades and could erode the overall health status of people in the United States. The strongest validation of this fear to date is the increase in the prevalence and incidence of diabetes, which have unabatedly paralleled obesity trends.^{2 - 4} The effect of obesity on overall mortality, cardiovascular disease (CVD), and related risk factors, however, has been weaker and may have diminished over time.^{4 - 7} The differing influence of obesity on diabetes and CVD reveals the complex nature of chronic disease epidemics and the degree to which their effects can be influenced by the capacity of public health to respond.

In this issue of JAMA, Alley and Chang⁸ highlight disability as the potential cost of a longer life with obesity, and barring an effective public health response, perhaps one of obesity's most important and persistent effects into the future. Based on data from the National Health and Nutrition Examination Survey (NHANES) over a 16-year period, the authors found that higher body mass index (BMI) in adults aged 60 years or older was strongly associated with disability prevalence as measured by self-reported functional limitations in mobility and the ability to perform daily physical and social tasks. Disability prevalence did not increase markedly until BMI levels of 30, but prevalence increased steeply thereafter; for example, patients with class III obesity (BMI >40) had 5 to 9 times the odds of functional impairment compared with persons with normal weight. Class III obesity has quadrupled over the past 25 years to the current prevalence of 5% of the adult US population and accounts for about one-fourth of the secular increase in diabetes prevalence.^{9 - 10}

The most important finding in the study by Alley and Chang was that obese persons during 1999-2004 were more likely to be disabled than obese persons during the previous decade. This finding contrasts with the general reduction in disability reported for older adults in the population.¹¹ The authors speculate that this increased disability may be due to the average obese person of the current cohort having spent more years obese than in previous cohorts. It is also possible that because of declining mortality rates, the obese segment of the population is now composed of more people with multiple chronic conditions who in previous decades would have died at a younger age. Increased longevity among disabled persons would also bring an increased risk and opportunity to become obese, further increasing the joint prevalence of obesity and disability.¹²

The observations by Alley and Chang parallel a growing literature documenting the effect of obesity on disability and demonstrating that the excess risks of disability are far greater than obesity's excess risk of mortality.^{13 - 15} Several conditions appear to play a role, including diabetes, musculoskeletal disorders (eg, arthritis, back pain, gait disturbances), coronary heart disease, heart failure, and depression.^{13 ,16 - 18} More direct, nondisease pathways between obesity and disability include inflammation, fatty infiltration of the muscle, and reduced muscle strength to body mass ratio, flexibility, and cardiorespiratory fitness.^{16 - 17}

Studies modeling the simultaneous effect of obesity on morbidity and mortality show that among older obese persons, years of active life are traded for more years in a disabled state.^{14 - 15} A recent study suggested that a 65-year-old woman with a BMI higher than 35 could expect about 2 fewer remaining years of life than a lean counterpart, but about 4 fewer years in an active state.¹⁴ Other corroborating analyses suggest that disability levels among middle-aged adults may have increased over time and that this increase is mainly explained by increases in the prevalence of obesity.¹⁹ Regardless of whether they are driven by declining mortality or increasing number of years being obese, these findings are discouraging if they reflect the experience of obese Americans. Disability erodes quality of life and strongly predicts subsequent functional deterioration and increased health care utilization and costs.¹⁷ For older adults, loss of independence may seem more frightening than the classic clinical conditions that precede it.

The ability of clinical and public health systems to offset the effects of obesity appears to have varied across 3 key potential links between obesity and disability: CVD, diabetes, and arthritis. Although adiposity may increase CVD risk even in the overweight and moderately obese range, this effect appears to have been compensated by multiple factors in the public health response to CVD, including health education and promotion efforts, reduced saturated fats in commonly eaten foods, reduced use of tobacco, and better identification and medical management of risk factors.^{4 ,7 ,20} By selectively giving greater attention to obese patients, clinicians may have further reduced risks, but with increased health care costs.²¹ Consequently, an obese person today has lower risks from elevated blood pressure, lipid disorders, and smoking-related conditions than their leaner counterparts in previous decades.^{4 ,7} The net effect of these trends is reflected by recent statistical models suggesting that obesity trends have thus far slowed the rate of improvements in CVD mortality only modestly,²⁰ if at all.²²

The influence of obesity on diabetes trends appears different, however. Prevalence and incidence of diabetes have increased, largely due to the increase in numbers of very obese persons.^{3 ,9} Lifestyle interventions can reduce type 2 diabetes incidence in high-risk overweight people, but effective implementation of such programs is slow in coming. As a result, rates of many diabetes complications expressed per unit of the entire population have increased despite improved treatment of persons with type 2 diabetes.²

Obesity-mediated arthritis trends may similarly reflect insufficient public health efforts. Arthritis prevalence is about twice as high among obese persons as among those with normal weight.¹⁶ A separate report using NHANES data showed that although current cohorts have similar arthritis prevalence as did previous cohorts, obesity may now play a proportionately greater role.²³ While other causes of arthritis may have decreased, the increase in obesity may have taken their place to forestall a reduction in arthritis in general. Simulation models proposed 10 years ago forewarned much of the current scenario, suggesting that decreases in commonly fatal conditions such as cancer and CVD would actually increase disability prevalence and that declines in arthritis and diabetes would be essential to reduce disability over time.¹⁸ Unfortunately, the 2 latter phenomena have not occurred.

Disability represents in part the collective effects of multiple obesity-related conditions, which bodes poorly for any simple clinical or public health solutions to modify the obesity-associated disability trends. This challenge is compounded by the lack of commonly practiced interventions directly aimed at reducing disability in at-risk populations. Structured exercise and weight loss programs may be among the most promising unifying interventions because they appear to help prevent type 2 diabetes, reduce arthritis symptoms, and improve physical functioning—ie, they can reduce each of the outcomes of obesity that have persisted over time.^{24 - 25} Thus, these findings make a compelling case to overcome the barriers of integrating effective lifestyle and exercise programs into health systems and communities. In the end, however, reducing the effect of obesity on morbidity by simply altering its course or accommodating its presence may never have an impact equal to a successful public health strategy to prevent obesity.