This paper presents a new formulation for the analysis of growth kinetics on multiple nutrients. The baseline for the theory is the concept that a noninteractive growth process occurs among perfectly substitutable nutrients if the locus of points of the substrate concentrations producing equal growth rate is linear. A deviation function is then defined with respect to this base case, and several models for this function are suggested. The underlying theory is taken by analogy with mixture thermodynamics. The proposed formulation is tested against data in the literature on growth under substitutable and complementary substrate mixtures.