Goal setting is an integral component of nutritional counseling. Lower glycemic index (GI) diets can improve outcomes among adults with type 2 diabetes mellitus. Intraindividual variability in intake occurs during goal pursuit and could be captured through mobile ecological momentary assessment (EMA) to develop algorithms for personalized medical nutrition therapy. EMA assessed goal progress toward a lower GI diet among adults with type 2 diabetes mellitus using an n-of-1 design. Mean (standard deviation) intake of low GI foods increased by 1.2 (0.1) servings for all participants combined. Three participants met or exceeded their GI goal repeatedly based on EMA reports. Mobile technology through EMA holds promise for the field of personalized medical nutrition therapy.