[Calcium and phosphorus balance in rural lactating women in Mexico]

To compare the balance of calcium (Ca) and phosphorus (P) between lactation and weaning, and to determine the Ca and P milk production in Mexican rural lactating women.

Thirty-six women aged 18-36 y, weight 49 +/- 3 kg and height 148 +/- 2 cm, were divided in six groups: four groups of lactation (1st, 3rd, 6th and 12th month) one post-weaning group and one of non pregnant non lactating women. The balance studies were performed collecting duplicate diets, 24 h urine for 3 days, 72 h feces and 24 h milk samples for 2 days. The Ca content was determined by atomic absorption spectrophotometry and P by the molybdate method.

Ca content in milk was higher in the 3rd month of lactation. The Ca balances were negative in all lactation groups (789 +/- 165 mg/d). Ca urinary excretion was lower in the lactating group (p < 0.05) suggesting a regulatory mechanism to conserve Ca during lactation. No differences were observed in the P content in milk and positive balances of P were observed in the non lactating and the post-weaning groups, whereas they were negative in the lactation groups (115 to 475 mg/d). High fecal Ca and P excretion (approximately 1300 mg/d) was observed, which contributed to the negative condition of the balance during lactation. The production of Ca and P in the milk of these rural women was similar to the one seen in rural and urban groups in Africa, Asia, Europe and the U.S.