Background: C4 plants (e.g. corn and sugar cane) have greater <sup>13</sup>C enrichment than C3 plants (e.g. wheat and sugar beet). Objective: To assess whether <sup>13</sup>C enrichment of CO<sub>2</sub> in the breath and breast milk of women on diets based on C3 and C4 foods changes from one diet to the other. Materials and Methods: Six breast-feeding women were studied at 5-6 months postpartum. They ate a controlled C4 diet on days 1 and 2 followed by a C3 diet on days 3 and 4. Diet duplicates, breast milk on days 2 and 4 and hourly breath samples were collected over 4 days. <sup>13</sup>C enrichment was measured by isotope-ratio mass spectrometry. Values of δ<sup>13</sup>C were calculated from the international PDBV standard (δ<sup>13</sup>C<sub>PDBV</sub>). Differences between means were compared by paired t test or t test for repeated measurements. Results: δ<sup>13</sup>C<sub>PDBV</sub> values were significantly higher in the C4 diet than in the C3 diet composites (p < 0.01). In breath CO<sub>2</sub>, the δ<sup>13</sup>C<sub>PDBV</sub> value was greater on days 1 and 2 (range -15.4 to -13.2, respectively) and declined on days 3 and 4 (range -20.0 to -21.8, respectively, p < 0.01). The lipid and milk serum fractions of breast milk had significantly higher δ<sup>13</sup>C<sub>PDBV</sub> on the C3 diet than on the C4 diet (p < 0.01). Conclusions: Subjects eating a C4 diet have a higher δ<sup>13</sup>C<sub>PDBV</sub> value in the breath and breast milk fractions, which diminish rapidly on a C3 diet. Further studies focusing on individual nutrients are warranted.