GNPDA2 and obesity due to melanocortin 4 receptor deficiency: Future research in this field, mainly on GNPDA2, must go further than epidemiologic and genomic association studies and explore the mechanisms involved in the changes in sugar metabolism implied in the pathogenesis of obesity and related diseases such as diabetes mellitus type 2, and also in the development of complications derived from the glycation of blood and endothelial proteins, considering the central role of Fru6P in this process.