A method for determining the metabolic capacity of an enzyme includes time-resolved determination of the concentration of a product in exhaled air. The product is created by metabolism of a substrate, previously administered to an individual, by an enzyme of the individual. The product concentration is determined until the maximum product concentration in the exhaled air is reached. A model function is fitted to measured values of the product concentration, obtained by the time-resolved determination of the product concentration between start and end times. The metabolic capacity of the enzyme is determined based on parameters of the model function. Determining the metabolic capacity of the enzyme takes place based on at least two parameters of the model function, wherein the maximum value and time constant of the model function are not selected as parameters at the same time, and the start and/or end times are not selected as parameters.