Abstract: The present invention relates to compositions, methods and uses, consisting of a functionalised miRNA-based nanosystem for treating obesity, weight loss and/or reduction of localised fat, which comprises miR-21 or other derived or equivalent compounds such as modified polynucleotides, synthetic mimetic and/or isomiR of miR-21; and a carrier which comprises an optimised nanoparticle for effectively binding oligonucleotides, forming an adequate nanosystem for in vivo transfection and, in particular, for the in vivo release of genes in fatty tissue.