FAO measure of undernourishment is calculated based on three main parameters: dietary energy supply (DES), minimum dietary energy requirement (MDER), and the coefficient of variation of dietary energy consumption (CV). In the current implementation of this methodology, the DES and the MDER change over time, as would be expected. The CV, however, remains constant. However, we expect the CV to change over time and be a function of income and prices. This paper discusses why the CV should change in response to changes in these variables, and suggests a practical way to estimate changes in the CV over time in the absence of survey data.