Abstract
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.
Highlights
Vitamin D is a fat-soluble vitamin which can be introduced with diet, but is mainly produced endogenously in the skin by conversion from 7-dehydrocholesterol upon ultraviolet-B radiation [1].The major circulating form of vitamin D is the 25(OH)D and its serum concentration is considered a gold standard biomarker to assess vitamin D status in humans [2]
Obesity, which is characterized by hypertrophy or/and hyperplasia of adipose tissue, contributes to "meta-inflammation" [18], a phenomenon characterized by chronic low-grade metabolic inflammation [19] that acts as a key underlying mechanism for the development of obesity-related diseases [20]
The stratification of the study population according to the body mass index (BMI) classes allowed us to confirm the negative relationship between 25(OH)D levels and body weight, being the lowest 25(OH)D levels present in study participants with class III BMI
Summary
Vitamin D is a fat-soluble vitamin which can be introduced with diet, but is mainly produced endogenously in the skin by conversion from 7-dehydrocholesterol upon ultraviolet-B radiation [1].The major circulating form of vitamin D is the 25(OH)D and its serum concentration is considered a gold standard biomarker to assess vitamin D status in humans [2]. The meta-inflammation is characterized by increased levels of inflammatory mediators, which may induce cellular damage and cell death through apoptosis or necrosis [21]. Monitoring these markers in subjects with obesity might require expensive biochemical analysis, with limited use in clinical practice. The expression of vitamin D receptors on inflammatory cells, including adipose tissue resident immune cells, suggested that there is a potential role for vitamin D in the regulation of cytokine release and adipose tissue inflammation mediated by the inhibition of NF-κB signalling [24]
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