New research shows that people with diabetes and knee osteoarthritis (KOA) are more likely to experience more pain as a result of their condition compared with people with diabetes alone. The study, published in February 2020 in Arthritis Care & Research, found that the increased pain was present even after controlling for obesity status, sex, and the severity of the disease according to imaging tests.
These finding are interesting because we tend to think about osteoarthritis as a mechanical problem, says Joshua F. Baker, MD, assistant professor of medicine and rheumatologist at Penn Medicine in Philadelphia. “We know that weight is really important in the development of osteoarthritis, and ‘wear and tear’ can be greater because of that excess weight, but there may be also metabolic changes related to obesity that could influence osteoarthritis. It’s reasonable to think that diabetes and other metabolic problems might actually make your arthritis worse and give you more symptoms,” says Dr. Baker.
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The number of people with type 2 diabetes continues to climb; according to the Centers for Disease Control and Prevention (CDC), it’s estimated that more than 34 million people age 18 and older have diabetes. Knee osteoarthritis is on the rise as well, as shown in a study published August 2017 in the journal PNAS, which found that the prevalence of knee osteoarthritis has more than doubled in prevalence since 1940, rising from 6 to 16 percent; current estimates show that almost 20 percent of people in the United States over age 45 have KOA.
Although the two conditions share key risk factors, such as having a body mass index (BMI) over the healthy range and being older, research published in Diabetes Care showed that having type 2 diabetes increases the risk for developing severe osteoarthritis independent of either of those factors.
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Researchers analyzed data from 2,481 patients previously enrolled in the Osteoarthritis Initiative, a nationwide research study sponsored by the National Institutes of Health. Participants were between 45 and 79 years old, and 202 (8 percent) had diabetes.
Subjects were evaluated in the following ways:
The KOA patients who had diabetes reported worse knee pain, as well as greater physical and mental issues, compared with people without diabetes. The findings remained consistent even when controlling for BMI, sex, age, and the severity of the disease as determined by radiographic severity. KOA is often measured by radiograph using the Kellgren and Lawrence system, which classifies severity of the disease by measuring the space between the joints as well as the presence of osteophytes, a bony outgrowth that is associated with the degeneration of cartilage at joints.
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"Our findings confirm results in a larger cohort that have been shown in previous studies in smaller patient groups," Annett Eitner, PhD, lead author and researcher at Friedrich Schiller University in Jena, Germany, wrote in an email message. “Although there is much evidence for a role of metabolic factors in the development of osteoarthritis, its relative importance is still disputed; our data show that diabetes mellitus is an independent risk factor for increased joint pain,” she noted.
The main limitation of these findings is that they’re based on observational data, says Baker. In an observational study, researchers simply “observe” the effect of a risk factor without trying to change or control behavior. These subjects in these studies can have characteristics or habits that may impact the results; researchers try to identify them and adjust accordingly, but there can be a lot of unknowns, according to the Institute for Health and Work.
“When you’re looking at observational data it’s very hard to make causal inferences. There is a correlation here between diabetes and symptoms of osteoarthritis, but what we don’t know enough to really understand the relationship,” says Baker. “We don’t really know all the differences between people who do have diabetes and who don’t have diabetes; they may be different in many different ways besides that their weight is different,” he says.
Examples of those differences could be how each person’s fat is distributed or if they have other medical conditions, says Baker. “Those kinds of factors could influence the outcomes of the study,” he adds.
It’s possible that people with diabetes mellitus may suffer from more knee pain at a comparable stage of osteoarthritis than nondiabetic patients from the increased inflammatory processes in the joint of patients with diabetes mellitus, says Dr. Eitner. The authors also suggested that the increased pain could be a product of diabetic neuropathy, which is a type of nerve damage that can occur for people with diabetes.
“Inflammation or neuropathy due to diabetes could be behind the increased pain, but there currently isn’t enough evidence to prove either of these theories,” says Baker. “People with diabetes and metabolic obesity are known to have more systemic inflammation. There’s a lot of thought that that inflammation might predispose people to more severe arthritis and even could contribute to inflammation of the joint as a result of arthritis,” he says. “Theoretically, those factors could make your symptoms worse, although that’s not a well-defined relationship and very difficult to prove,” he adds.
The other possibility is that the diabetes itself may influence the nerves, says Baker. “It’s very clear that diabetes causes neuropathy; whether diabetes has an effect on how people with diabetes experience pain in knee osteoarthritis as a result is possible, although there isn’t specific data to support that hypothesis,” says Baker.
To prove that either of those mechanisms is behind why people with diabetes experience more osteoarthritis symptoms, more research is needed, says Baker. “We need more mechanistic studies really trying to understand how the metabolic part of diabetes or how diabetes itself may specifically lead to changes in the joint or changes in the nerves that would contribute to pain or other manifestations of arthritis symptoms,” he says.
“There is currently a lot of research around trying to understand the relationship between metabolic obesity and inflammation and osteoarthritis. I think we’re learning that osteoarthritis is a probably a combination of mechanical issues in the setting of an inflammatory state,” says Baker. If that’s true, it’s likely that the severity of a person’s arthritis is related to metabolic consequences of the obesity, he adds.
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The results show that we may expect that people with diabetes will have more symptoms from their arthritis, says Baker. “This study may have public health implications, but I don’t think it changes how we currently manage osteoarthritis,” he says.
These results point out that it’s not just the extra weight that matters when it comes to symptoms of osteoarthritis, says Baker. “People can get focused on pounds and base success on whether they are making progress in terms of the scale. These results suggest that perhaps we should maybe focus more on the metabolics of obesity,” he says.
“People can do a lot of good in terms of their metabolic profile through healthy behaviors, and they should keep at it even if they’re not seeing a big loss of weight. This includes a healthy diet, less processed sugar, and more exercise,” says Baker.
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