The Obesity-Arthritis Link – As Solid as Previously Thought?

Osteoarthritis (OA) is the progressive degeneration of joint cartilage that affects 10 percent of Canadian adults.  OA causes tremendous pain and disability as damaged joint tissues are unable to repair themselves normally, resulting in a breakdown of cartilage and bone.  In fact, a report by the Arthritis Alliance of Canada from 2011 reported that more than 4.4 million Canadians live with OA.  The connection between obesity and OA seems obvious – every extra kilogram of body weight adds an extra load of about four kilograms on your knees, for instance, and it is this wear and tear that spurs the joint corrosion process.  Simple, right? The problem with the above reasoning is that OA of the hand, containing non-weight-bearing joints, is also twice as common in obese people.  This question pits traditionalists who view OA as a straightforward mechanical issue, and those who believe that inflammatory signals triggered by conditions like obesity can play a central role. As evidence for the latter perspective accumulates, sports like running are receiving a second look.  Running, often seen as a sport which is perfect for inflicting the wear and tear that causes OA, may not have the damaging effect many believe.  A new long term study of 90,000 people published in Nature Reviews: Rheumatology suggests that running may actually help, not hurt, your joints.  The crux of this new view is that fat tissue is not just an inert substance that accumulates in inconvenient locations on our bodies.  Instead, it is metabolically active, a rich source of pro-inflammatory endocrine factors – which can trigger changes throughout the body, including in joints.  One example of the endocrine factors produced by fat tissue is leptin, which is best known as an appetite hormone.  Researchers at the University of Michigan recently measured leptin levels in 543 middle-aged women.  The results, forthcoming in the journal Arthritis Case & Research, show that subjects with elevated leptin levels were more likely to develop knee OA during the 10 year follow-up period, even after adjusting the results to control for body weight.   It is clear that mechanical factors also play a key role in the disease.  For example, one of the clearest risk factors for OA is sustaining an acute joint injury.  This is why studies of soccer and hockey players invariably reveal an increased risk of developing osteoarthritis later in life.  However, when athletes who have suffered acute injuries are removed from the equation, the risk essentially disappears.  Even more interesting is that studies now show that the traditional view of cartilage as an inert substance that cannot respond to extra demands is incorrect.  Regular exercise can actually lead to a thickening of the cartilage layer, whereas inactivity results in thinning cartilage.      With the above in mind, another large-scale study soon to be published in Medicine & Science in Sports and Exercise should not be surprising.  The Lawrence Berkeley National Laboratory in California has been following a large cohort of runners since 1991, and the latest research coming out of this ambitious project has analyzed OA risk in 90,000 of them.  This piece of research has found that those running more than 12 km per week were 18.1 percent less likely to develop OA and 35.1 percent less likely to require a hip replacement during the study’s seven-year follow, compared with those running less than that amount.  Even those that ran considerably more – 863 subjects ran more than 96 km per week – had similarly reduced rates of osteoarthritis.  These findings replicate previous studies. Further interpretation of this research shows that the protective effect of running was almost entirely due to its role in reducing BMI (Body Mass Index – the ratio of height to weight).  Those who ran (or walked) more weighed less, and consequently had healthier joints.  While we don’t yet know whether it is lower body weight or less fat-induced inflammation that is more important, one thing is most certainly clear: avoiding physical activity, even high impact activities like running – to “preserve” your joints is a losing strategy.   Reference: Hutchinson, Alex.  “Debating the obesity-arthritis link”, The Globe and Mail, March 4 2013.