Thursday, February 2, 2012

Large Study Finds Obesity and Pain Linked

Research Update Small-scale studies have demonstrated a positive relationship between body weight, particularly obesity, and self-reported pain of various types. Now, a study of more than 1-million Americans confirms the presence of higher rates of pain among the heaviest of individuals. The exact reasons for this, which comes first, pain or obesity, and whether losing weight is beneficial are still unclear.

Arthur A. Stone, PhD, and Joan E. Broderick, PhD, — researchers at Stony Brook University in New York — report in an early online edition of the journal Obesity on an assessment of 1,010,762 randomly-selected adult Americans interviewed via telephone by the Gallop Organization between 2008 and 2010 [Stone and Broderick 2012]. This was part of a much larger survey, the Gallup Healthways Wellbeing Index, and the current study focuses on a subset of those questions that collected data on height, weight, and other demographics. The respondent sample for this study was 50.5% female, primarily white (85%), and most had completed high school or higher education (94%).

Participants also were asked, if they felt pain “a lot of the day, yesterday (yes or no),” and “In the last 12 months, have you had any of the following, or not? Neck or back pain condition that caused recurring pain? Knee or leg condition that caused recurring pain? Other condition that caused recurring pain?”

BMI Classifications Body Mass Index (BMI) was calculated for each participant and categorized according to World Health Organization criteria (Table at left). In this population, 37% of respondents were classified as low-to-normal BMI (<25), 38% were overweight, including pre-obese (25 to <30), and the remaining 25% were in one of the three obese classifications. [BMI = weight in kg / height in meters squared, or BMI = weight in lbs x 703 / height in inches squared.]

BMI and “pain yesterday” were significantly and directly associated even when demographic variables — age, gender, race, education, and smoking — were controlled. Specifically, compared with Low-Normal BMI participants as the index group, and reflected in odds ratios, reports of pain were 20% higher in the Overweight/Pre-Obese group, 68% higher in the Obese I group, 136% higher in the Obese II group, and 254% higher in the Obese III group.

Those associations of pain and BMI were similar for both men and women, although there was a sharper increase in rate of pain among females, and the association also was stronger in older age groups, particularly those aged 60 to 69 years. Adjusting the data to control for pain-related medical conditions experienced during the 12-months prior to the survey reduced the magnitude of pain-BMI relationships; however, the associations of pain and BMI remained substantial for the Obese groups.

The authors conclude that BMI and daily pain are positively correlated in the United States. In particular, people who are obese are considerably more prone to having daily pain and, interestingly, the pain that obese individuals reported was not driven exclusively by musculoskeletal pain, which individuals carrying excess weight might typically experience. Therefore, the data suggest that obesity alone may cause pain, aside from the presence of painful diseases. The association is robust and holds after controlling for several pain conditions and across gender, age, and race.

COMMENTARY: In a prior UPDATE [here] we noted that obesity, defined as BMI ≥30, appeared to play a critical role in arthritis: roughly 30% of obese adults had been diagnosed with arthritis compared with 20% of persons categorized as overweight and only 17% in adults of normal or lower-than-normal weight.

Other UPDATES [here and here] reported on research finding that overweight and obese women — especially those who do not exercise — are at higher risk for developing fibromyalgia syndrome; however, both studies had limitations and flaws. In fact, some researchers claim that relying on the BMI alone for group allocation may have little external validity since it fails to take into account individual factors, such as location of body fat, muscle mass, bone structure, age, and ethnic factors. Furthermore, there is little convincing evidence to date confirming a cause-effect relationship or that weight loss alone in overweight or obese patients reduces their pain.

Still, the large-scale study by Stone and Broderick described above is of importance in its portrayal of a strong relationship between BMI and pain. This is of special concern, since a newly reported study in JAMA suggests that excess weight is becoming endemic in the American population [Flegal et al. 2012]. The researchers found that in 2009-2010 the age-adjusted average BMI for both men and women was nearly 29 (in the pre-obese range), and the prevalence of obesity was roughly 36% in the adult population. This is less than the 25% obesity prevalence in the Stone and Broderick study but, either way, the data suggest excess weight may be a significant contributing factor to pain in America (and probably worldwide).

Both studies relied on data reservoirs from large-scale studies that were not specifically designed to prospectively examine the BMI-pain hypotheses in question, and we have cautioned previously many times that such data-mining approaches have many inherent limitations and sources of bias. For example, Stone and Broderick acknowledge that, with such a large sample size, even small differences between groups could easily achieve statistical significance but might be meaningless from a clinical perspective; therefore, they focused on the magnitude of the differences in their study expressed as odds ratios.

However, in the Stone and Broderick study, as with other data-mining research along these lines, there were important questions that could not be answered by the methodology and available data:

  • Which came first, pain or the excess weight condition? Many types of pain can limit any form of exercise, which might facilitate weight gain that places further strain on weight-bearing anatomy to exacerbate pain, or possibly amplifies effects of other conditions sometimes associated with pain (eg, peripheral diabetic neuropathy).

  • Conversely, the researchers propose an interesting hypothesis that having excess body fat might trigger complex physiological processes influencing the development of pain-generating inflammation. However, no clinical data were collected in the Gallup survey to address this possibility.

  • Additionally, depression, often experienced by obese individuals, also may be linked to pain; although, this variable was not specifically examined in the study.

  • Questions to assess pain in the Gallup survey were not designed to capture details about the severity, frequency, or exact duration of self-reported pain. Similarly, the questions did not evaluate the type of pain, such as nociceptive versus neuropathic, specific diagnosed pain conditions, or therapies that respondents might have used for their conditions. All of these factors could be important for evaluating the consequences of excess body weight on pain, or vice versa.

Achieving a better understanding of obesity-pain relationships seems to call for an end to the data-mining studies that have been so popular and the implementation of prospective, controlled trials that begin with the “right” questions and collect necessary data to answer those. Meanwhile, as the population ages and obesity continues to be problematic in society, these two factors alone could become quite significant contributors to the already high prevalence of chronic pain in the U.S. and in many other countries.

> Flegal KM, Carroll MD, Kit BK, et al. Prevalence of Obesity and Trends in the Distribution of Body Mass Index Among US Adults, 1999-2010. JAMA. 2012;307(5):491-497 [
abstract here].
> Stone AA, Broderick JE. Obesity and Pain Are Associated in the United States. Obesity. 2012(Jan 19); online ahead of print [
abstract here]. Note: some data in the online abstract are inconsistent with correct data in the text, but this may be amended in the final print edition.

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