Beyond Plasma: Blood Fraction-Specific Patterns of Magnesium, Calcium and Iron in Type 2 Diabetes Across Preserved and Reduced Estimated Glomerular Filtration Rate

Background: Type 2 diabetes mellitus (T2DM) is associated with disturbances in mineral element homeostasis, yet data on calcium (Ca), magnesium (Mg) and iron (Fe) across multiple blood compartments in T2DM throughout the spectrum of renal function remain limited. Objective: To assess Ca, Mg and Fe concentrations in plasma, whole blood and erythrocytes of T2DM patients with reduced or preserved estimated glomerular filtration rate (eGFR), integrating three-day dietary records to explore, rather than definitively separate, metabolic and dietary drivers of imbalance. Methods: In this single-center, cross-sectional study, 91 hospitalized adults were divided into three groups: controls (n = 32), T2DM with preserved eGFR (≥60 mL/min/1.73 m2; n = 29), and T2DM with reduced eGFR (<60 mL/min/1.73 m2; n = 30). Renal function was assessed using eGFR calculated with the Modification of Diet in Renal Disease (MDRD) equation. The eGFR-based grouping did not constitute a diagnosis or staging of chronic kidney disease. Mg, Ca, and Fe concentrations were measured in plasma and whole blood using atomic absorption spectrometry. Plasma Ca concentrations were corrected for albumin, whereas blood cell Fe and Mg concentrations were calculated using individual hematocrit values. Blood cell Ca could not be quantified. Dietary intake was assessed from three-day food diaries completed during hospitalization (hospital diet). Between-group differences in mineral concentrations were assessed using Welch’s ANOVA followed by Games–Howell post hoc comparisons. Associations between calculated blood cell Fe concentrations and hematological indices were evaluated using Spearman’s rank correlation coefficient, with Benjamini–Hochberg correction for multiple comparisons. Results: Plasma Mg (p < 0.01) and albumin-corrected plasma Ca (p < 0.05) were lower in the T2DM group with eGFR < 60 mL/min/1.73 m2 than in controls; albumin-corrected plasma Ca was also lower in this group than in the T2DM group with eGFR ≥ 60 mL/min/1.73 m2 (p < 0.01); whole-blood Mg was higher in the T2DM group with eGFR ≥ 60 mL/min/1.73 m2 than in controls (p < 0.05). Whole-blood Fe was significantly higher in both T2DM groups than in controls (p < 0.05), whereas calculated blood cell Fe did not differ significantly between groups after hematocrit-based recalculation. Blood cell Mg did not differ significantly between groups. Dietary Ca intake was lower (p < 0.01) and Mg intake higher (p < 0.01) in T2DM than in controls. Conclusions: Mineral concentration differences in T2DM varied across blood compartments. Although whole-blood Fe was elevated in both T2DM groups, no significant differences were detected in hematocrit-adjusted blood cell Fe concentrations. These findings do not establish intracellular Fe accumulation or hepcidin-mediated sequestration. Further studies incorporating direct intracellular measurements, comprehensive Fe-status markers, and adjustment for clinical confounders are needed to determine the potential diagnostic value of mineral measurements beyond plasma.