liter), the average total sodium requirement for the duration of pregnancy is 3 mEq (69 mg) per day in addition to the normal requirement. Since the average intake is, as has been noted, considerably above that, the sodium requirement for pregnancy is met by usual salt intake.
Lactation increases sodium requirements considerably. Since human milk contains about 7.8 mEq of sodium (180 mg) per liter (AAP, 1985), and the average milk secretion when established is about 750 ml, lactation would add about 6 mEq (135 mg) per day to the usual adult requirement. This increase is easily met by the usual dietary sodium intake.
The sodium requirement is obviously highest in infants and young children in whom extracellular fluid volume is rapidly expanding. Forbes (1952) calculated that from birth to 3 months of age, 0.5 mEq/kg (11.5 mg/kg) daily is needed for growth, or approximately 2 mEq (46 mg) per day for the reference infant. At 6 months of age, the daily requirement for growth is approximately 0.2 mEq (4.6 mg)/kg. According to calculations by Cooke et al. (1950), daily losses of sodium from the skin range from 0.4 to 0.7 mEq/kg (9 to 16 mg/kg). Because sodium losses from the kidney can be regulated precisely when intakes are not excessive, the convenient value of I mEq/kg (23 mg/kg) daily is considered more than satisfactory for the healthy infant and young child residing in a temperate climate. Human milk contains 7 mEq of sodium per liter (range, 3 to 19 mEq/liter) (Gross, 1983; Macy, 1949). Consumed at a rate of 750 ml/day, this provides the reference infant with an average of 120 mg/day, which corresponds to 1.16 mEq/kg (27 mg/kg) daily from birth through 2 months of age and 0.8 mEq/kg (18 mg/kg) daily from 3 through 5 months of age. Except for the premature infant, in whom hyponatremia can occur (Roy et al., 1976), human milk certainly provides adequate sodium for the growing infant.
Formula-fed infants consuming 750 ml/day now receive a minimum of 100 mg/day and a maximum of 300 mg/day (AAP, 1985). The American Academy of Pediatrics has estimated that there is a threefold increase in dietary sodium between 2 and 12 months of age (AAP, 1981).
Acute excessive intake of sodium chloride leads to an increase in the extracellular space as water is pulled from cells to maintain sodium concentration. The end result is edema and hypertension. Such