INS and obesity due to melanocortin 4 receptor deficiency: In order to gain understanding on the mechanisms linking vitamin D deficiency to impaired metabolism, the PubMed database (https://www.ncbi.nlm.nih.gov/pubmed) was used to search for articles published between 1995 and 2021 based on the following keywords or their combinations: vitamin D; vitamin D deficiency; insulin resistance; obesity; diabetes; glucose; metabolism; insulin secretion; insulin sensitivity; inflammation; oxidative stress; or apoptosis.