"It's very challenging for doctors to treat this disease. Therefore prevention of this disease becomes very important," said Dr Weiya Zhang, one of the review's authors from the University of Nottingham in the UK. Weight loss, he added, is the main prevention tool that is feasible for most patients.
The analysis "is really confirming quite nicely ... how important overweight and obesity are when you're talking about osteoarthritis," said Dr Ian Janssen, who studies obesity and chronic disease at Queen's University in Ontario, Canada.
10% have knee arthritis
One in 10 people over age 55 have severe knee arthritis, and many more suffer from chronic knee pain, researchers report.
Many people with arthritis continue having symptoms despite treatment with anti-inflammatory drugs.
"We have a very long list of the treatment options, but none of them are very good," Dr. Zhang said.
Dr Zhang and his colleagues reviewed 47 studies that compared the chance of developing knee osteoarthritis in normal weight, overweight, and obese people, including a total of almost 450,000 participants.
Combining those studies, the authors found that people who were overweight were about twice as likely to get knee osteoarthritis as people of normal weight. For obese people, the risk was almost 4 times higher.
The findings are published in Arthritis Care & Research.
Obesity doesn’t cause arthritis
This review, and previous studies, cannot prove definitively that being overweight or obese causes arthritis. What is clear is that extra weight and arthritis are closely linked.
For people who are very overweight, "there's too much load on your knees," Dr David Felson, who studies osteoarthritis at Boston University, said.
Extra weight can also increase a person's chance of getting arthritis in the hip, said Dr Felson, who was not involved in the current study, but the knee "is the joint where osteoarthritis causes the most trouble, (and) it's the one where obesity has its strongest effects."
While being heavy is not the only factor that puts people at risk for arthritis –so does having a job that requires lots of heaving lifting and kneeling, but weight is the easiest to control, Dr Zhang said.
He and his colleagues calculated that if obesity were eliminated, that is, if everyone who is obese was a normal weight instead, about one-quarter of all knee osteoarthritis cases could be prevented.
That figure varied around the world because of the range of obesity rates in different countries.
On the low end, the authors found that eliminating obesity would only prevent about 8% of knee osteoarthritis cases in China - mostly because very few Chinese people are obese. In the US, the number of arthritis cases would be cut in half if no one was obese.
But hoping for obesity to be eliminated is "not a very reasonable strategy," Dr Felson said, and is not a practical solution to the problem of osteoarthritis.
Rather, Dr Janssen said, having this extra information might cause more people to realise the importance of maintaining a healthy weight, and might spur countries to aim for manageable goals in reducing obesity.