• Paul Jarle Mork, Andreas Holtermann, and Tom Ivar Lund Nilsen.
• Department of Human Movement Science, Norwegian University of Science and Technology, Dragvoll Idrettssenter, N-7491 Trondheim, Norway. paul.mork@svt.ntnu.no
• J Epidemiol Community Health. 2012 Aug 1;66(8):678-83.

BackgroundMechanical joint stress imposed by high body mass index (BMI) is associated with increased risk of knee and hip osteoarthritis. This prospective study investigated the independent and joint association of BMI and physical exercise on risk of knee and hip osteoarthritis.MethodsThe study includes 15,191 women and 14,766 men in the Norwegian HUNT Study without pain or physical impairment at baseline. Occurrence of self-reported physician-diagnosed osteoarthritis was assessed at 11 years of follow-up.ResultsBMI was positively related to risk of knee osteoarthritis (P(trend)<0.001), with an RR of 4.37 (95% CI 3.01 to 6.33) in women and 2.78 (95% CI 1.59 to 4.84) in men, comparing obese and normal weight persons. No clear association was observed for hip osteoarthritis. Obesity increased the risk of severe activity-limiting osteoarthritis, with an RR of 2.30 (95% CI 1.68 to 3.15) and 2.50 (95% CI 1.56 to 4.03) in women and men, respectively. Physical exercise did not modify the above associations (P(interaction)>0.34). Exercise intensity was not associated with risk of osteoarthritis in any BMI category; that is, obese persons reporting high-intensity exercise had an RR of 1.28 (95% CI 0.59 to 2.79) for severe osteoarthritis compared with inactive persons.ConclusionHigh BMI increases the risk of knee osteoarthritis and severe osteoarthritis. Physical exercise does not increase the risk of osteoarthritis at any level of BMI, suggesting that exercise could be encouraged also among individuals with excessive body mass, without concern for an increased risk of osteoarthritis.

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