Use of Apelin in humans has a positive effect on insulin sensitivity with a good tolerance and a good safety. Thus, a first object of the invention relates to an APJ receptor agonist for use in the treatment or the prevention of diabetes.(Eur J Pharmacol. 2015 Sep 15;763(Pt B):149-59. doi: 10.1016/j.ejphar.2015.05.017. Review.)