Medical doctors and scientists have long recognized a connection between being overweight and developing osteoarthritis. It turns out, however, that the connection is not as straightforward as was thought.
It had been assumed that the primary factor causing damage to the joints was simply the mechanical effect of the increased impact experienced with every step taken by an overweight person. But while that would explain symptoms in the hips, knees and feet, it does not account for the fact that obese people frequently also experience arthritis in their hands. According to Duke University’s Farshid Guilak, Ph.D., “This indicated that something besides just body-weight level affected their joints.”
What Causes Arthritis In An Obese Person – The Duke Study
Now, a team from Duke University has released a study in the journal Arthritis & Rheumatism indicating that leptin, a naturally-occurring hormone linked to body fat and metabolic levels, may play a determining role in whether an obese person (or mouse) will develop osteoarthritis. The trigger may not be weight, but a more complicated chemical or systemic metabolic effect.
Funded by the National Institute for Arthritis, Musculoskeletal and Skin Diseases and by the Arthritis Foundation, the report—“Extreme obesity due to impaired leptin signaling in mice does not cause knee osteoarthritis”—was written by Timothy M. Griffin, now with the Oklahoma Medical Research Foundation’s Free Radical Biology and Aging Program, with Janet L. Huebner and Virginia B. Kraus of the Duke Department of Medicine, and Dr. Guilak, director of orthopedic research in Duke’s Department of Surgery.
The Role Of Leptin in Arthritis – From Mouse to Man
The Duke researchers studied the effects of leptin deficiency and obesity in mice. “With obesity and osteoarthritis, there are good similarities between humans and mice,” notes Dr. Guilak. “We were completely surprised to find that mice that became extremely obese had no arthritis if their bodies didn’t have leptin.”
Leptin has been in the news before because of the key role it plays in body weight control. It affects many physical functions—appetite, metabolism, energy balance, immune response, fertility and prostate growth. But it had not previously been tied to the condition of joint cartilage.
The Duke study started out by asking whether excess fat in the body instigates an immune system imbalance that leads to excess production of cytokine—a protein associated with the joint inflammation of arthritis.
Comparing leptin-deficient and overweight mice to normal mice, the researchers looked specifically at how their knee bones were affected. They found that the cytokine levels in the overweight but leptin-free mice did not change. Despite their obesity, the mice that were deficient in leptin did not develop arthritis.
Exactly how or why is more difficult to pinpoint. Many of the interrelated physical processes implicated in osteoarthritis—weight, sex hormones, bone metabolism, and inflammation—are influenced by leptin. Lead author Griffin notes that for that reason “it’s difficult to isolate which pathway is being altered to prevent the development of osteoarthritis.”
However, the study clearly suggests that leptin plays an important role in both skeletal and immune system changes associated with osteoarthritis. More research will be needed to reach an understanding of exactly how controlling leptin might alter or prevent the onset of osteoarthritis in human beings.
But while these findings suggest that body fat, in and of itself, may not be a risk factor for joint degeneration, it remains true that whatever role leptin plays, losing weight is still one of the best methods of mitigating the discomfort of osteoarthritis. Notes Guilak, “if you are obese and lose just 10 pounds, pain decreases significantly."