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Usual levels of sodium consumption have been estimated in dietary surveys by assessing salt intake and by measuring urinary sodium. Reported dietary intakes of sodium range from 1.8 g/day to 5 g/day in various studies, depending on the methods of assessment used (Abraham and Carroll, 1981; Dahl, 1960; Pennington et al., 1984) and on whether or not discretionary sodium use is assessed. The discretioniary intake of sodium is quite variable and can be quite large. In one 28-day study, males were found to add about 5.5 g of sodium chloride (2.2 g of sodium) to their food per day (Mickelson et al., 1977).

Because of the difficulty of assessing sodium use from dietary recall, dietary surveys probably underestimate total sodium intake, even when contributions of water and other marginal sources are included. From data on daily urinary sodium excretion over 24 hours, Dahl and Love (1957) calculated the average daily adult intake of salt to be 10 g/day (4 g of sodium per day). Dahl subsequently reported a mean sodium chloride intake of 10.3 g (range, 4 to 24 g) for 71 working men in New York. Coatney et al. (1958) reported that a 5month sodium excretion in a military population corresponded to an intake of 11 g of salt per day. Sanchez-Castillo et al. (1987a, b) found sodium chloride excretion over a 12-day period to be 10.6 ± 0.55 g in men and 7.4 ± 2.9 g in women.

Estimate of Requirements

Calculations of sodium requirements (shown in Table 11-1) are based on estimates of what is needed for growth and for replacement of obligatory losses. The amount needed to support growth depends on the rate at which extracellular fluid volume is expanded, a rate that varies with age and reproductive status.

Adults

In a temperate climate, the healthy adult can maintain sodium balance with a very low intake of sodium (Kempner, 1948). Dole et al. (1950) have estimated obligatory urinary and fecal losses by adults to be 23 mg (1 mEq)a per day. The other source of loss is sweat, which normally averages a sodium concentration of 25 mEq/ liter (Consolazio et al., 1963). Sanchez-Castillo et al. (1987a) found that sweat and fecal excretion contributed only 2 to 5% of the sodium lost by British men and women. Obligatory dermal losses have been assumed to range from 46 to 92 mg (2 to 4 mEq) per day (Fregley,

a 1 mEq of sodium is 23 mg, and 1 mmol of sodium chloride is 58.5 mg.



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