Soluble transferrin receptor levels are positively associated with insulin resistance but not with the metabolic syndrome or its individual components

The metabolic syndrome (MetS) is known to be associated with elevated serum ferritin levels. The possible association with other Fe markers has been less well studied. We aimed to investigate the cross-sectional association of soluble transferrin receptor (sTfR) and ferritin levels with the MetS components, insulin resistance and glycosylated Hb (HbA1C). The sample consisted of 725 adults, aged 19-93 years (284 men, 151 premenopausal and 290 postmenopausal women), from the Croatian island of Vis. Serum sTfR and ferritin levels were measured by immunoturbidimetry and electrochemiluminescence assays, respectively. The MetS was defined using modified international consensus criteria. Logistic and linear regression analyses were conducted to investigate the associations adjusting for age, fibrinogen, smoking status, alcohol consumption and BMI. Prevalence of the MetS was 48·7 %. Standardised values of ferritin were positively associated with all of the MetS components (except high blood pressure and waist circumference) in men (P<0·05). Ferritin was significantly associated with the MetS in men (adjusted OR 1·78 (95 % CI 1·31, 2·42)) and postmenopausal women (1·71 (95 % CI 1·12, 2·62)). Interestingly, sTfR was independently and positively associated with homoeostatic model assessment for insulin resistance in men (adjusted β=0·44 (95 % CI 0·14, 0·75), P=0·004) and postmenopausal women (adjusted β coefficient=0·34 (95 % CI 0·05, 0·63), P=0·020). However, there was no significant relationship between serum sTfR levels and the MetS or its components. Neither ferritin nor sTfR was significantly associated with HbA1C (P>0·05). sTfR levels could be spuriously elevated in subjects with insulin resistance and without association with the MetS or its components. We conclude that different markers of Fe metabolism are not consistently associated with cardiometabolic risk.