Methods 300 and systems 100 for evaluating proposed food purchases which vary from a pre-planned diet are provided. A nutrition-related scheme, the pre-planned diet, is developed on the basis of achieving a primary goal such as increasing the intake of a particular nutrient or losing a certain amount of weight. The nutrition-related scheme, which may be stored in a memory 103 of a computer 120, includes a list of approved food items. When a user proposes to substitute a food item for an approved food item a version of the program stored on a PDA 120 determines whether the substituted item negatively interacts with any of the approved diet items, for example, affecting the user's ability to metabolize a nutrient in the approved item.