Gaining a lot of weight in adulthood appears to increase the risk of needing surgery to repair arthritic knees, new research reports.

A group of Finnish investigators found that people who were slim at age 20 but overweight for most of the next 30 years were three times more likely than people who stayed at a normal weight to need knee surgery.

People who gained their weight by age 30 and did not lose it also had around three times the risk of knee problems.

However, people who gained their weight by age 20 were twice as likely as normal weight people to require knee surgery, suggesting that being constantly overweight carries less of a risk of knee problems than adding a lot of pounds in adulthood.

“A shift from normal to overweight” in adulthood appears to be harder on the knees than being constantly overweight, say Dr. P. Manninen of the Finnish Institute of Occupational Health, and colleagues.

To protect people from knee problems, “weight control during early adulthood should be encouraged,” they add in their article in the Annals of the Rheumatic Diseases.

A propensity to knee arthritis is often inherited, but other risk factors for the condition include age, sex, previous injury and obesity.

To investigate whether it’s worse to be overweight for all or only some of adulthood, Manninen and colleagues interviewed 220 men and women between 55 and 75 years old who had undergone surgery to treat knee arthritis. They asked them what they weighed at age 20, 30, 40 and 50, and compared their responses to those of 415 similarly aged people who did not need knee surgery.

Compared to people who were never overweight in adulthood, people who started gaining weight after age 20 and were overweight during at least one other period in adulthood were significantly more likely to have undergone knee surgery.

As mentioned, people who became consistently overweight starting at age 30 had a three-fold higher risk of knee problems, and people who were overweight from age 20 on had a two-fold higher risk.

“Shifts from normal to overweight between the ages from 20 and 50 years were more closely associated with the risk of knee arthroplasty (i.e., joint repair surgery) than constant overweight,” Manninen and colleagues write.

They suggest that gaining weight may be harder on knees if the joints have problems adapting to the excess weight, or if a metabolic factor is at work.

They acknowledge that an alternative explanation may be that people with early knee problems may be unable to stay active, and gain weight as a result of their knee problems rather than vice versa.

SOURCE: Annals of the Rheumatic Diseases, November 2004.