The effect of sodium balance on sweat sodium secretion and plasma aldosterone concentration

Abstract

The effect of manipulating sodium intake upon sweat sodium secretion was investigated during heat acclimation. Twenty-five male subjects were confined to an environmental chamber at a temperature of 25 degrees C for 3 days, and then acclimated to heat by a further 5 days at 40 degrees C. The subjects' daily sodium intake was controlled throughout as follows: high (HNa), 348.4 (0.8) mmol x day(-1), n=7; moderate (MNa), 174.1 (0.6) mmol x day(-1), n=9; or low (LNa), 66.3 mmol x day(-1), n=9. Sodium losses were estimated from urinary, faecal and sweat collections using a whole-body washdown method. Plasma aldosterone concentration was also measured from venous blood sampled each morning. Measurements of body temperature and heart rate during the heat exposure phase indicated a degree of heat acclimation. During this heat phase there was a reduction (P < 0.01) in sweat sodium secretion for all three conditions which was greatest for the LNa condition, although this finding was not significant (P < 0.1). In the LNa condition, plasma aldosterone concentration increased (P < 0.05) prior to heat exposure, and the secretion of aldosterone was potentiated (P < 0.01) during the heat exposure in comparison with the MNa condition. In contrast, the HNa diet produced a fall (P < 0.05) in plasma aldosterone concentration prior to heat exposure and an attenuation of aldosterone secretion thereafter. These findings are inconsistent with the hypothesis that retention of sweat sodium is dependent upon a net body sodium deficit, but demonstrate that aldosterone secretion is potentiated under such conditions.

Induction and decay of short-term heat acclimation

“…However, in the present study, an exercise-induced response of increased [  Na ] p was not evident after acclimation for either the EUH or DEH regime, in the standardised exercising heat stress test. Therefore, this is in contrast with previous findings (Brandenberger et al 1989;Francesconi et al 1993;Allsopp et al 1998 (Table 2). Blood volume expansion at rest increased after EUH (2.2%) and was elevated after the DEH regime (5.3%).…”

The Induction and Decay of Short-Term Heat Acclimation

“…Therefore, this is in contrast with previous findings (Brandenberger et al 1989;Francesconi et al 1993;Allsopp et al 1998 (Table 2). Blood volume expansion at rest increased after EUH (2.2%) and was elevated after the DEH regime (5.3%).…”

Opinion of the Scientific Panel on Dietetic products, nutrition and allergies [NDA] related to the Tolerable Upper Intake Level of Sodium

“…The ability to survive at low levels of consumption is dependent upon adaptive mechanisms which reduced losses in sweat, stool and urine. For most populations, the habitual levels of sodium consumption greatly exceed the physiological requirements, and there are few data which determine the minimal levels of sodium consumption required to maintain health in people who have adapted to low levels of sodium consumption over long periods of time (Allsopp et al, 1998). As the metabolism of sodium is closely related to that of water and other nutrients, the need for sodium can only be adequately assessed when the provision of water and all other nutrients is adequate.…”