Individuals who are overweight or obese increase their risk for the development of osteoarthritis (16-18, 91, 92). The association between increased weight and the risk for development of knee osteoarthritis is stronger in women than in men (92). In a study of twin middle-aged women, it was estimated that for every kilogram increase of weight, the risk of developing osteoarthritis increases by 9 to 13 percent. The twins with knee osteoarthritis were generally 3 to 5 kg (6.6 to 11 lb) heavier than the co-twins with no disease (16). An increase in weight is significantly associated with increased pain in weight-bearing joints (175). There is no evidence that the development of osteoarthritis leads to the subsequent onset of obesity (91). A decrease in BMI of 2 units or more during a 10-year period decreased the odds for developing knee osteoarthritis by more than 50 percent; weight gain was associated with a slight increase in risk (93).
A randomized controlled trial of 6 months' duration examined the effect of weight loss on clinical improvement in patients with osteoarthritis (176). Patients taking phentermine had an average weight loss of 12.6 percent after 6 months while the control group had an average weight loss of 9.2 percent. There was improvement in pain-free range of motion and a decrease in analgesic use in association with weight loss; patients with knee disease showed a stronger association than those with hip disease. Similarly, improvement of joint pain was observed in individuals who had undergone gastric stapling, resulting in an average weight loss of 45 kg (99 lb) (94, 95).