1. The effects of dehydration on mechanisms of water balance and milk synthesis were investigated in ten lactating Gambian women who were fasting during Ramadan. Ten non-pregnant, non-lactating women acted as controls. Fasting consisted of total water abstention from 05.00 hours to 19.30 hours and was accompanied by high insensible water losses.

2. Lactating women lost 7.6% of their total body water between 07.00 hours and 19.00 hours. Control subjects lost significantly less.

3. Plasma indices of dehydration (osmolality, sodium, uric acid) showed a greater rise in the lactating women than in the control subjects over the period of fasting. However, the 19.00 hours values remained in the normal range obtained on non-Ramadan days.

4. During Ramadan the lactating women restricted their urinary output to a lesser degree than the controls, and for much of the day their urine was also less concentrated. The lactating women appeared to have adapted by superhydrating themselves overnight. This resulted in very low urine concentrations (osmolality, sodium, urea, creatinine) in morning samples. Urine concentrations approached, but did not exceed, non-Ramadan levels by late afternoon.

5. The daily water turnover of 6.4 litres in the lactating women was 2 litres greater than in the controls. This difference was much greater than that required for milk synthesis (500 ml) and may represent a further protective mechanism.

6. Fasting caused changes in milk osmolality, lactose, sodium and potassium concentrations indicative of a marked disturbance of milk synthesis. The results suggested that the normally tight junctions between the mammary secretory cells had become permeable to small ions and formed a paracellular pathway.

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