The invention relates to a method for determining the metabolic capacity of at least one enzyme. It comprises the steps of the time-resolved determination of the concentration of a product in the air exhaled by an individual, wherein the product has been created by a metabolism of a substrate, previously administered to the individual, by at least one enzyme of the individual and wherein the product concentration is determined at the least until the maximum product concentration in the air exhaled by the individual is reached, the fitting of a model function to measured values of the product concentration, which were obtained by the time-resolved determination of the product concentration between a start time and an end time, and the determination of the metabolic capacity of the enzyme on the basis of parameters of the model function, which specify the model function. The method is characterized in that determining the metabolic capacity of the enzyme takes place on the basis of at least two parameters of the model function, with the proviso that the maximum value of the model function and the time constant of the model function are not selected as parameters at the same time, insofar as the model function is a mono-exponential function, and with the further proviso that the start time and/or the end time are not selected as parameters.
