Doctors have long known that obesity goes hand-in-hand with osteoarthritis, the type of arthritis that occurs when wear-and-tear on joints damages the cartilage pads on the ends of bones. Obesity rates are 54 percent higher among adults with arthritis than those without.
It makes perfect sense, after all – an obese person is carrying more weight, and therefore putting more stress on his or her joints. For example, your knee takes a load of three to six times your body weight every time you take a step. However, this doesn’t explain why osteoarthritis is so common in non-load-bearing joints, like the ones in our fingers.
According to new research from Duke Medicine, published in the Annals of the Rheumatic Diseases, the relationship between obesity and osteoarthritis may be more complicated than just a matter of mechanics. In fact, it may all come down to what you eat, not how much.
One of These Fats Is Not Like the Others
A previous study by the research team found a connection between leptin, the body’s satiation or "stop-eating" hormone, and arthritis.
To test the theory that diet influences arthritis risk, the scientists induced osteoarthritis in the mice by injuring their knees. Injury accounts for about 10 to 15 percent of arthritis cases among humans. Then, the researchers put mice on a diet that was high in either saturated fats, omega-6 fatty acids, or omega-6 fatty acids supplemented with a small amount of omega-3 fatty acids.
“Fatty acids are classified into several types, depending on their chemical structure,” explained Farshid Guilak, a professor of orthopedic surgery and director of orthopedic research at Duke University Medical Center and principal investigator on the study, in an interview with Healthline. “Saturated fats are most commonly found in animal products such as pork or beef, whereas polyunsaturated fatty acids such as omega-3 and omega-6 fatty acids are found in vegetable and plant oils.
"However, one of the greatest sources of omega-3 fatty acids is fish oil. There is growing evidence that saturated fats can contribute to different diseases such as heart disease, for example by raising bad cholesterol levels, while omega-3 fatty acids can have an anti-inflammatory effect in the body.”
The Devil’s in the Diet
Unsurprisingly, the mice on all three high-fat diets became obese. But obesity and arthritis severity were not tied to the mice’s activity levels; their weight gain, and any resulting arthritis, was caused by diet alone.
“We showed a very high fat diet that was rich in saturated fats or omega-6 fatty acids significantly worsened the arthritis that occurs after a knee injury,” said Guilak. “However, if the diet had a small supplement of omega-3 fatty acids, the joints were protected.” In fact, the obese mice on the omega-3 diet had the same joint health as control mice.
“One of the most surprising findings of the study was that the type of fat played a more important role in osteoarthritis than the amount of weight gained," Guilak said. "These findings show that the relationship between diet, obesity, and arthritis is much more complex than just increased joint loading due to greater body weight, but likely involves inflammatory factors related to the diet and other metabolic conditions associated with obesity.”
Inflammation might be the key. Omega-3 fatty acids help regulate the body’s inflammation system, while the fat cells present in obesity make inflammation worse. Inflamamation also interacts with the body’s healing system. As part of their study, Guilak’s team tested how well the mice on different diets healed.
They removed a small piece of cartilage from each mouse’s ear, a common practice used to tell mice apart in experiments, and then measured how well each ear healed. The mice on the omega-3 fatty acid diet saw their wounds heal faster, more cleanly, and with less scarring. Interestingly, the faster their wounds healed, the less arthritis they had. This suggests that there might be common factors involved in skin healing and the body’s maintenance of cartilage in damaged joints.
“It is now becoming well recognized that obesity is associated with increased systemic inflammation that can potentially inhibit wound healing and exacerbate osteoarthritis progression,” Guilak said. “The results of this study, as well as several of our previous studies, suggest that this inflammation can play a more important role than weight gain in causing osteoarthritis.”
Should We Change Our Diets?
There’s a wealth of data on the benefits of omega-3 fatty acids, as well as the dangers of too much saturated fat. But rather than avoiding saturated fats altogether, maintaining the right ratio of fats, it seems, is the trick.
“It is hard to know what the ‘ideal’ ratio is, but we know that diets associated with relatively low heart disease and metabolic issues, such as Mediterranean or Native American diets, have very low omega-6 to omega-3 ratios of 4:1 or even 1:1,” Guilak said. “However, over the past few decades, our Western diet has increased the ratio of omega-6 to omega-3 fatty acids to levels of 15:1 or even 25:1.”
However, Guilak cautions that people looking to treat their osteoarthritis shouldn’t go out and buy fish oil capsules just yet. He said, “While this study is promising, it is still too early to recommend big dietary changes, and we need to do perform careful clinical studies to show that these effects are present in people too.”