To determine if the altered insulin-like growth factor (IGF) status in rheumatoid arthritis (RA) is due to inflammation, altered body composition, or lack of exercise.
Subjects included 73 patients with RA, 54 patients with other rheumatic diseases, both inflammatory and noninflammatory, and 28 healthy, physically active controls. Serum levels of IGF-I, IGF-II, and IGF binding protein-3 (IGFBP-3) were measured by radioimmunoassay. Body composition was estimated by bioelectrical impedance analysis, and habitual exercise level approximated by questionnaire. Statistical analysis was performed by 2 and 3 way ANOVA and moderated hierarchical regression.
Serum IGF-I (p < 0.001), IGFBP-3 (p < 0.001), and the BP-3:total IGF molar ratio (p < 0.001) were depressed in both patient groups relative to controls. In contrast, IGF-II levels were depressed only in patients with RA (p < 0.01). Differences in the IGF proteins between patients and controls could not be attributed to inflammation. Habitual exercise level, but not body composition, was shown to be a significant predictor for IGF-I, IGFBP-3, and BP-3:total IGF molar ratio (p < 0.001).
Our results indicate that the reduction in circulating IGF proteins observed in our patients is more related to their sedentary lifestyle than to the inflammatory process. This conclusion is in agreement with reports that show that highly active individuals typically exhibit higher levels of systemic IGF proteins than age matched sedentary controls.