['C4 plants (e.g. corn and sugar cane) have greater (13)C enrichment than C3 plants (e.g. wheat and sugar beet).', 'To assess whether (13)C enrichment of CO2 in the breath and breast milk of women on diets based on C3 and C4 foods changes from one diet to the other.', '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. (13)C enrichment was measured by isotope-ratio mass spectrometry. Values of δ(13)C were calculated from the international PDBV standard (δ(13)CPDBV). Differences between means were compared by paired t test or t test for repeated measurements.', 'δ(13)CPDBV values were significantly higher in the C4 diet than in the C3 diet composites (p < 0.01). In breath CO2, the δ(13)CPDBV 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(13)CPDBV on the C3 diet than on the C4 diet (p < 0.01).'] Is ( 13 ) C enrichment of the CO2 in breast milk and in the breath rapidly modified by changes in the ( 13 ) C content of the diet?