Crosstalk Between Bone and Fat Tissue: Associations Between Vitamin D, Osteocalcin, Adipokines, and Markers of Glucose Metabolism Among Adolescents

Abstract

Objectives: This study aimed to investigate the relationship between 25-hydroxyvitamin D (25(OH)D), osteocalcin, markers of glucose metabolism, and obesity-related parameters among adolescents.

Methods: This was a cross-sectional study with 198 adolescents age 14-18 years. Weight, height, and waist and hip circumferences were measured, as well as the following biochemical parameters: serum 25(OH)D, parathyroid hormone (PTH), total (tOC) and undercarboxylated (ucOC) osteocalcin, adiponectin, leptin, glucose, and insulin. The homeostasis model of assessment estimate of insulin resistance (HOMA-IR) and β-cell function (HOMA-β) and quantitative insulin sensitivity check index (QUICKI) were calculated. Student's t test, analysis of variance (ANOVA), Pearson's correlation, and linear regression models were performed.

Results: Overweight was observed in 42.6% of the sample. This group presented significantly higher PTH, leptin, insulin, HOMA-IR, and HOMA-β and lower 25(OH)D, adiponectin, tOC, ucOC, and QUICKI than normal-weight subjects. 25(OH)D was positively correlated with ucOC and adiponectin and negatively associated with body mass index (BMI), weight, and waist circumference (p < 0.05 for all). The association between 25(OH)D and ucOC was also observed in the multiple regression analysis, adjusted for age, BMI, and season of the year (partial r² = 0.071, p < 0.0001). 25(OH)D and ucOC were not associated with markers of glucose metabolism. However, leptin was strongly correlated with insulin, HOMA-IR, HOMA-β, and QUICKI (p < 0.0001 for all).

Conclusion: The present study demonstrated that undercarboxylated osteocalcin is related to 25(OH)D and adiponectin concentrations. Both ucOC and 25(OH)D were lower in overweight and obese adolescents, reinforcing the importance of fighting obesity. Although a relationship of ucOC and 25(OH)D with markers of glucose metabolism was not observed, leptin has shown to be the hormone most related to energy homeostasis.

Keywords: Vitamin D; bone; glucose; insulin resistance; leptin; obesity; osteocalcin.