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Vitamin D effects on bone and mineral metabolism are well recognized, and its anti-inflammatory actions are gaining particular interest. Delta-like 1 (DLK1) is a protein, expressed by progenitor cells of different tissues, and increases the size of progenitor cell population during the inflammatory phase of tissue regeneration. DLK1 also plays a role in energy metabolism as it antagonizes insulin signaling in bone. In this one-year randomized clinical trial of overweight elderly individuals that received either 600 or 3750 IU daily cholecalciferol we assessed the effect of vitamin D supplementation on pre-specified secondary outcomes: DLK1, leptin, adiponectin, C-Reactive Protein (CRP) and Vascular Cell Adhesion Molecule (VCAM). We also examined correlations between DLK1 and bone (BMD, bone markers), fat (adipokines, body composition), insulin sensitivity and inflammatory markers. Multivariate analyses were conducted to further explore these associations. Overall, there was a significant increase in serum DLK1 and leptin and a decrease in VCAM, but no change in CRP, after 12 months of vitamin D supplementation. DLK1 was negatively correlated with BMD and positively correlated with bone markers, associations that persisted after adjusting for age, gender and BMI. DLK1 was also positively associated with indices of insulin resistance and negatively with indices of insulin sensitivity. Correlations between DLK1 and fat parameters, such as adipokines, and DXA derived fat mass were less consistent. There were no correlations between DLK1 and inflammatory markers. In conclusion, twelve months supplementation of vitamin D3 increased serum DLK1. DLK1 was negatively associated with indices of bone health and fuel metabolism, and with 1,25(OH) 2 D levels. Similar to the role of DLK1 in animal models, our findings support the hypothesis that DLK1 can be targeted to regulate bone and energy metabolism and develop drugs to improve BMD and insulin sensitivity. However, further studies are needed to explore the role of DLK1 and its relationship to vitamin D metabolites in vivo. • DLK1 is an inhibitor of mature cells including osteoblasts and adipocytes. • DLK1 is negatively correlated with BMD, indices of insulin sensitivity, and 1,25(OH) 2 D. • DLK1 is positively correlated with bone remodeling markers and indices of insulin resistance. • The DLK1 pathway could be a target for drug development for osteoporosis and pre-diabetes. [ABSTRACT FROM AUTHOR]