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Arthritis, exercise and obesity: some unexpected results

October 8th, 2009

A couple of recent studies worth noting, one surprising, the other not.

First, there’s a new Cochrane Systematic Review recommending exercise as a form of treatment for rheumatoid arthritis, to improve “functional ability.” It’s already fairly well-established that exercise is helpful for osteoarthritis, which is the more common (about 1 in 10 people will get it) and better known form of arthritis. But the evidence about exercise and rheumatoid arthritis, which is less common (about 1 in 100 people will get it), is a little thinner on the ground, so the Cochrane review is interesting.

“Based on the evidence in this study, we would recommend aerobic capacity training combined with muscle strength training as routine practice for RA patients,” said lead researcher Emalie Hurkmans of the Leiden University Medical Center in Leiden, Netherlands. “But we need more research to establish the recommended length and type of exercise programs, whether patients need to be supervised and if these programs are cost effective.”

All of this makes the second study a little more surprising. In a mouse study, Duke University researchers found that obesity, on its own, does not cause osteoarthritis. Because of the strong link between arthritis and obesity as a risk factor (one of the reasons exercise is so important), researchers have assumed that the extra weight puts strain on joints, which then leads to degeneration and ultimately arthritis.

But in the new study, researchers used mice that either didn’t have or couldn’t process the hormone leptin, which helps to regulate appetite. No matter how fat these mice got, they didn’t have an elevated rate of arthritis. Mice with normal leptin levels, on the other hand, developed significant knee arthritis when they got just half as fat as the leptin-free mice.

So what’s going on? It’s not clear, since leptin influences factors like body weight, inflammation, sex hormone levels and bone metabolism, all of which could affect the development of arthritis. What it implies is that loading your joints with extra weight isn’t what causes arthritis — the strong link between obesity and arthritis appears to be chemical instead.

“We knew from other studies that obese people got arthritis in their hands, too, which don’t bear weight. This indicated that something besides just body-weight level affected their joints,” [one of the researchers said].

Just to reiterate: the link between arthritis and obesity is as strong as ever. It’s just the mechanism that isn’t as simple as we thought.

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