Scientists have found that any perceived link between obesity and arthritis in animals is caused by more than just wear and tear from added weight. A new study conducted at Duke University suggests that when there is an absence of the appetite hormone leptin, this can determine arthritis in lab mice, regardless of their weight.

According to the Centers for Disease Control and Prevention, osteoarthritis affected 13.9 percent of adults over 25 years of age and 33.6 percent of people over the age of 65 in 2005.

Farshid Guilak, PhD, director of orthopedic research in the Duke Department of Surgery, commented on the lack of association between obesity and the development of osteoarthritis.

“We were completely surprised to find that mice that became extremely obese had no arthritis if their bodies didn’t have leptin. Although there was some earlier evidence that leptin might be involved in the arthritis disease process, we didn’t think that there would be no arthritis at all,” said Guilak in a statement.

The researchers found that the joints from the overweight mice actually looked healthier than the joints of the control mice. Yet in another study, the mice that gained half as much weight, but still produced leptin, usually developed some degree of osteoarthritis in the knee.

Leptin – a well-known appetite regulator – influences many factors associated with osteoarthritis, such as sex hormone levels, inflammation, bone metabolism and body weight.

The Duke researchers wanted to see whether or not an increase in body fat caused an balance of the immune system prompting cytokines and other chemicals to cause inflammation in the joints.

They discovered that the leptin-free obese mice did experience some change in the knee bones, but did not necessarily develop osteoarthritis. The levels of cytokines remained relatively unchanged, suggesting that leptin may play dual roles in osteoarthritis by being involved in the regulation of both the immune and skeletal system.

The findings of the study are published in the journal Arthritis & Rheumatism.