Purpose: Obesity in middle age is associated with an increased risk of osteoarthritis of the knees in later life. We sought to determine whether body mass index in young men was a risk factor for the subsequent development of osteoarthritis of the knee and hip.
Subjects and methods: Body mass index was assessed in 1,180 male medical students at age 23 +/- 2 (mean +/- SD) years and at several times during follow-up. The incidence of knee and hip osteoarthritis was ascertained by self-report and corroborated with information on symptoms and radiographic findings.
Results: During a median follow-up of 36 years, 62 participants developed knee osteoarthritis and 27 developed hip osteoarthritis. The incidence of knee, but not hip, osteoarthritis was strongly associated with body mass index assessed at ages 20 to 29 years and 30 to 39 years (both P <0.001). For body mass index assessed at ages 20 to 29 years, the incidence of knee osteoarthritis at age 65 years was 12.8% among the heaviest subjects (range 24.7 to 37.6 kg/m2), threefold greater than the incidence of 4.0% in the leanest (15.6 to 22.8 kg/m2) category of body mass index (P = 0.0001). Thus, for a man who was 180 cm (5'11") tall, each 8 kg (18 lb) greater weight at ages 20 to 29 years was associated with an increased risk of subsequent knee osteoarthritis (relative risk = 1.7, 95% confidence interval 1.3 to 2.1), after adjustment for year of birth, physical activity, and knee injury. Body mass index at ages 20 to 29 years was more predictive of future osteoarthritis than at ages 30 to 39 or 40 to 49 years.
Conclusion: Greater body mass index in young men ages 20 to 29 years is associated with an increased risk of subsequent knee, but not hip, osteoarthritis, suggesting that cumulative exposure to greater weight during young adult life is an important cause of osteoarthritis.