Low versus high glycemic load (GL) diet patterns are inversely associated with obesity and chronic diseases such as cancer and cardiovascular disease. These associations persist beyond the protection afforded by increased fiber alone, representing an important gap in our understanding of the metabolic effects of GL. We conducted a randomized, controlled, crossover feeding trial of two 28-day diet periods of high and low GL. Using LC-MS, targeted metabolomics analysis of 155 metabolites was performed on plasma samples from 19 healthy adults aged 18-45 years. Fourteen metabolites differed significantly between diets (P < 0.05), with kynurenate remaining significant after Bonferroni correction (P < 4 × 10(-4)). Metabolites with the largest difference in abundance were kynurenate and trimethylamine-N-oxide (TMAO), both significantly higher after consumption of the low GL diet. Partial least squares-discriminant analysis showed clear separation between the two diets; however no specific pathway was identified in pathway analyses. We found significant differences in 14 plasma metabolites suggesting a differing metabolic response to low and high GL diets. Kynurenate is associated with reduced inflammation, and may be one mechanism through which protective effects of a low GL diet are manifested and warrants further evaluation. This trial was registered at clinicaltrials.gov as NCT00622661.