DISTRIBUTION OF A STABLE-ISOTOPE OF CHROMIUM (CR-53) IN SERUM, URINE,AND BREAST-MILK IN LACTATING WOMEN

To determine the fate and distribution of chromium during lactation, s ix lactating women (25-38 y old) were given three doses of the tracer Cr-53 (7.55 mu mol/d, or 400 mu g/d) on days 1, 2, and 3 of the study, Diet records, blood samples taken while subjects were fasting, and 24 -h composite milk and urine samples were collected from day 0 to day 6 . Fasting blood samples, morning milk samples, and 24-h urine samples were also collected on days 8, 10, 15, 30, 60, and 90. Cr-53 and natur al and total chromium concentrations in biological fluids were measure d with gas chromatography-mass spectrometry and total urinary chromium was measured with atomic-absorption spectrometry, Cr-53 was detectabl e in serum 2 h after dosing and continued to be detected from day 30 t o day 60. Changes in total serum chromium concentration in response to the oral dose suggested that chromium concentrations in blood were no t tightly regulated. Cr-53 was not detected in breast milk and no sign ificant changes in natural chromium concentration in milk were observe d in response to the oral doses, suggesting that breast-milk chromium concentrations are independent of intake. The estimated chromium intak e of exclusively breast-fed infants was 2.5 nmol/d (0.13 mu g/d), belo w the lower end of the range of estimated safe and adequate daily diet ary intakes (10-40 mu/d) for infants 0-6 mo of age. The baseline chrom ium concentration in urine and the minimum Cr-53 absorption in lactati ng women were comparable with values for nonpregnant, nonlactating sub jects. Chromium losses in breast milk do not appear to be compensated for via increased absorption or decreased excretion.