PURPOSE: Lactation increases vitamin B-6 requirements because its concentration in breast milk is related to maternal intake and it is essential for infants. Exercise may also increase the requirement because it increases utilization and excretion of vitamin B-6. Therefore, the purpose of this study was to determine whether energy restriction and exercise affected vitamin B-6 status of lactating women. METHODS: Breastfeeding women with a body mass index >= 25 and <= 30 kg/m2 were randomly assigned at 4 wk postpartum to either restrict energy intake by 500 kcal/d and exercise for 45 min/d, 4 d/wk (weight loss group, WG) or maintain usual diet and not exercise (control group, CG) for 10 wk. Women were given a supplement containing 2.0 mg of vitamin B-6. Measurements included vitamin B-6 concentrations in breast milk and plasma, plasma pyridoxal 5'-phosphate, and erythrocyte alanine aminotransferase activity. RESULTS: The WG lost more weight (-4.4 +/- 0.4 versus -0.9 +/- 0.5 kg, p < 0.01) than the CG. Cardiovascular fitness increased by 12% in the WG, compared to 3% in the CG (p = 0.09). Milk vitamin B-6 concentrations increased in both groups (161 +/- 107 and 191 +/- 85 nmol/L, WG and CG, respectively, p = 0.05). There were no significant differences in other vitamin B-6 parameters. Weight and length gain (2.06 +/- 0.21 and 1.83 +/- 0.17 kg; 8.6 +/- 0.6 and 7.2 +/- 0.5 cm; WG and CG, respectively) of infants was not significantly different between groups. CONCLUSIONS: Energy restriction and exercise from 4 to 14 wk postpartum in overweight, breastfeeding women consuming adequate dietary intakes and 2.0 mg of supplemental vitamin B-6 does not adversely affect vitamin B-6 status or infant growth.

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