Phase Angle: A Possible Biomarker to Quantify Inflammation in Subjects with Obesity and 25(OH)D Deficiency

Obesity is associated to chronic low-grade metabolic inflammation and hypovitaminosis D. Among extra-skeletal effects, an important role in inflammation has been described for vitamin D (25(OH)D). Phase angle (PhA) is a bioelectrical impedance analysis (BIA) parameter that represents an indicator of cellular health in chronic inflammatory states. However, it is still unknown whether a low 25(OH)D levels might correlate with PhA in obesity. Considering the lack of evidence correlating the 25(OH)D levels with PhA in obesity, the aim of this study was to investigate their possible relationship in a group of patients with obesity stratified according to body mass index (BMI) categories. Four hundred and fifty-five adult subjects (219 males and 236 females; 36 ± 11 years) were enrolled. Body composition, including PhA, was assessed using a BIA phase-sensitive system. Serum levels of 25(OH)D was determined by a direct competitive chemiluminescence immunoassay. Most of the participants were affected by grade III obesity (24%) and had 25(OH)D deficiency (67%). Subjects with 25(OH)D deficiency had highest BMI (p < 0.001). Stratifying the sample population according to the BMI classes, 25(OH)D levels decreased significantly along with the increase in BMI (p < 0.001), with the lowest 25(OH)D levels in the class III obesity. In addition, stratifying the sample population according to 25(OH)D categories, BMI and fat mass (FM) decreased, while PhA increased significantly along with the 25(OH)D categories (p < 0.001). The 25(OH)D levels showed significant positive associations with PhA (r = -0.59, p < 0.001), and this association remained significant also after adjusting for BMI and FM (r = 0.60, p < 0.001). The lowest values of PhA were significantly associated with the severity of obesity (OR 0.3, p < 0.001) and of 25(OH)D deficiency (OR 0.2, p < 0.001). To compare the relative predictive power of body composition parameters associated with the 25(OH)D levels, we performed a multiple linear regression analysis. The most sensitive and specific cut-off for 25(OH)D levels to predict the PhA above the median was >14 ng/mL (p < 0.001). In conclusion, we provided preliminary insights into a novel link between 25(OH)D levels and PhA in the setting of obesity. This association uncovered a new potential usefulness of PhA as expression of cell membrane integrity and predictor of inflammation in low 25(OH)D status that might help in identifying high-risk patients with obesity who could benefit from careful 25(OH)D supplementation.