tropical climate, acclimatized persons can survive on extremely low sodium intakes (Kempner, 1948; Oliver et al., 1975).
In nonsweating individuals living in a temperate climate who are in a steady-state of sodium and fluid balance, urinary sodium excretion is approximately equal to sodium intake (i.e., 90 to 95 percent of total intake is excreted in urine) (Holbrook et al., 1984; Pietinen, 1982). Obligatory urinary losses of sodium in adults are approximately 23 mg (1 mmol)/day (Dole et al., 1950). This estimated level of excretion is similar to those that have been actually measured in studies of the Yanomamo Indians in Brazil: in one study sodium excretion of 26 men averaged 23.5 ± 34.7 mg (1.02 ±1.51 mmol)/day (Oliver et al., 1975), and in a subsequent study (n = 195), urinary sodium excretion was 20.7 ± 52.9 mg (0.9 ± 2.3 mmol)/day (Rose et al., 1988).
Excretion of sodium in the stool is minimal. When sodium intakes ranged from 0.05 to 4.1 g/day of sodium, only about 0.01 to 0.125 g (0.4 to 5.4 mmol)/day appeared in the stool (Dahl, 1958; Dole et al., 1950; Henneman and Dempsey, 1956). In a sodium balance study with three levels of intake, 1.5, 4.0, and 8.0 g (66, 174, and 348 mmol)/day (Allsopp et al., 1998), fecal sodium excretion increased as sodium intake rose. Still, fecal excretion of sodium was less than 5 percent of intake even at the highest level of sodium intake (Table 6-2).
Daily dermal losses of sodium have been reported to average less than 0.025 g (1.1 mmol)/day (Dahl, 1958; Dahl et al., 1955). In another study, estimated obligatory dermal losses of sodium ranged from 0.046 to 0.09 g (2 to 4 mmol)/day (Fregly, 1984). Sweat sodium loss depends on a number of factors, including: (1) the sweat rate, (2) sodium intake, and (3) heat acclimation (Allsopp et al., 1998). For these reasons, the sodium concentration in sweat varies widely. Most studies that measure sodium content of sweat are short-term (Table 6-3), and report sweat sodium concentrations rather than total sodium lost in sweat. Of note, in these studies intake data on dietary sodium was frequently not given. However, in the three studies where dietary sodium information was provided, dietary intakes were high (up to 8.7 g [378 mmol]/day).