Effects of Cardiorespiratory Fitness on Serum Ferritin Concentration and Incidence of Type 2 Diabetes: Evidence from the Aerobics Center Longitudinal Study (ACLS).

BACKGROUND Cardiorespiratory fitness (CRF) and physical activity (PA) are inversely related to the occurrence of type 2 diabetes (T2D). Both play an important role in reducing serum ferritin (SF) concentration. Increased SF concentration is considered a contributing factor for developing T2D. METHODS The present cohort study investigated 5,512 adult participants enrolled in the Aerobics Center Longitudinal Study (ACLS) between 1995 and 2001. The subjects completed a comprehensive medical examination and a SF evaluation, and had been followed up until either diabetes onset, death, or the cut-off date of November 2007. Three CRF levels were categorized. SF quartile levels were defined by gender and menopausal status. The incidence of T2D was calculated for 10,000 person-years, and hazard ratios (HR) were computed to predict the incidence of T2D based on SF quartiles and CRF levels. RESULTS SF concentration was significantly higher in males than in females (148.5 +/- 104.7 ng/ml vs. 52.2 +/- 45.9 ng/ml) and was inversely associated with CRF levels. In the high CRF group, 32.7% of participants had a low SF concentration whereas only 16.8% of participants had a high SF concentration level. After adjusting for potential confounders, male participants in the highest SF quartile level had a 1.7 times (HR: 1.67, 95% CI: 1.05, 2.66; p-trend = 0.027) increased risk for developing T2D compared with those in the lowest SF quartile group. CONCLUSION Lower SF concentration was associated with lower risk of developing T2D in those regularly participating in CRF. The findings from this study suggest that SF concentration could be used as a diabetic predictor. Based on these results clinicians and public health professionals should promote regular physical activity or fitness to reduce the incidence of T2D.