The natural 13C- or 15N-abundance in plant and animal organic matter is near 1.09% and 0.37%. When expressed against an international standard, the isotope composition (δ13C and δ15N) is different from that in source material (atmospheric CO2, soil nitrates, food,…). This is simply due to isotope fractionations associated with diffusion, metabolic and biophysical reactions, which discriminate between isotopic forms. For example, usually, lipids are 13C-depleted while leaf starch is 13C-enriched because of the fractionation against 13C of the pyruvate dehydrogenase and in favour of 13C by the aldolase, respectively. Similarly, all enzymes are associated with fractionations and thus metabolites have specific isotope abundances. Enzyme fractionations also depend on commitments: obviously, a fully committed (complete) reaction consumes all the substrate and cannot fractionate. Similarly, if there is no branching point, no fractionation is possible. Thus, in addition to their ‘intrinsic’ fractionation, the effective fractionation depends on metabolic commitments. The consequence of this is simple: we can use isotope compositions in metabolites to estimate commitment and metabolic partitioning. This can be applied to plants of course, but also to studies in human health and nutrition.