The present invention relates to the use of endogenous reference metabolites and a method for normalization of intensity data corresponding to amounts and/or concentrations of selected target metabolites in a biological sample of a mammalian subject, wherein said intensity data are obtained by a metabolomics analysis method with one or a plurality of endogenous reference metabolites, comprising carrying out at least one in vitro metabolomics analysis method of said selected target metabolites in said biological sample, simultaneously carrying out in the same sample a quantitative analysis of one or a plurality of endogenous reference metabolites or derivatives thereof, wherein said endogenous reference metabolites are such compounds in the biological sample which are present in the subject at an essentially constant level; and wherein said endogenous reference metabolites or derivatives thereof have a molecular mass less than 1500 Da.