Objective and study design: Our goal was to study the water balance in healthy breast-fed infants (n = 139) during their first 5 days, by cross-sectional measurements of body weight, serum sodium, serum osmolality, and hematocrit. We also investigated infants' capacity to conserve body water by increased secretion of vasopressin, the main antidiuretic hormone in human beings.
Results: The maximal body weight reduction was 5.7% +/- 1.7% (mean +/- SD) of birth weight and most infants started to gain weight when they were 3 days old. The serum sodium level at 16 +/- 4 hours (on day of birth) was 142 mmol/L; the level increased after 1 day (p < 0.01) and remained constantly high for the following 2 days (p < 0.05). The serum osmolality was increased at 1 day (p < 0.01) and 2 days (p < 0.05) compared with the value on the day of birth (296 mOsm/kg). The plasma vasopressin level was constant up to 24 hours (1 day), but decreased during the next 2 days (p < 0.01). Infants with body weight reduction exceeding 10% (n = 15) had a further elevation of the serum sodium level (p < 0.0001) and serum osmolality (p < 0.0001), and the plasma vasopressin level was twofold higher (p < 0.0001) compared with corresponding levels in infants with less weight reduction. These infants also had a reduced interval between two subsequent feedings (p < 0.001). The hematocrit remained unchanged irrespective of the degree of weight reduction.
Conclusions: When the reduction of body weight exceeds 10%, the newborn infant releases vasopressin in response to fluid hypertonicity. This state also affects feeding behavior, perhaps as an expression of thirst. It is likely that hormone release is also stimulated in parallel with a weight reduction of less than 10%, because it is also accompanied by a hyperosmotic state.