Mineral Excretion by Low Birth Weight Infants
The preterm, low or very low birth weight (VLBW) infant has several inadequate homeostatic mechanisms, among which renal immaturity is prominent. Maximal renal concentrating ability in the VLBW infant is often less than twice the osmolality of plasma, compared to a fourfold increase in the mature infant. Equally important is the VLBW infant's limited proximal tubular reabsorption of sodium. The mature infant's response to sodium restriction results in over 99.5% of filtered sodium being reabsorbed; in the case of the VLBW infant, sodium reabsorption may be only 97% to 98% from days 4 through 14 of life.
As a result of these two important limits, the VLBW infant has a higher water requirement than the full-term infant.
