The influence of sodium on growth in infancy.

Sodium (Na) is an important growth factor, stimulating cell proliferation and protein synthesis and increasing cell mass. Sodium chloride (NaCl) deprivation inhibits growth, as reflected by reduced body and brain weight, length, muscle and brain protein and RNA content and brain lipid content compared with controls. This is not due to deficiency of other nutrients since control and experimental diets were identical except for NaCl content. Subsequent NaCl supplementation restores growth velocity to control values but does not induce "catch-up" growth. In humans, salt loss causes growth failure and subsequent salt repletion improves growth. Preterm infants < 32 weeks' gestation at birth are renal salt losers in the first 2 weeks of post-natal life and are vulnerable to hyponatraemia. This can be prevented by increasing Na intake, which also produces accelerated weight gain that persists beyond the period of supplementation. Early nutrition in preterm infants can affect subsequent growth and also cognitive function: this is probably multifactorial, but NaCl intake differed substantially between study groups and is likely to be an important factor. The mechanism whereby Na promotes cell growth is not understood, but stimulation of the membrane Na+,H(+)-antiporter with alkalinization of the cell interior is a likely possibility.