Abstract
Background: Dietary sodium intake plays an important role in the regulation of fluid and electrolyte homeostasis within the human body. Both a high sweat sodium concentration and sweat rate can induce considerable sweat sodium losses during exercise. Excessive sodium loss is an important factor involved in the aetiology of exercise associated hyponatremia (EAH); a plasma sodium concentration <135mmol.L-1 during and/or after prolonged exercise, with reported prevalence rates higher amongst females than males. Salt appetite is described as a behavioural state that arises in response to physiological sodium deficiency in humans. During periods of body sodium depletion, this regulatory mechanism drives an individual to seek dietary sources of salt to support optimal regulation of sodium homeostasis.
Objective: Limited research has investigated the relationship between habitual dietary sodium intake and sweat sodium concentration during moderate intensity exercise. Given that females are at greater risk of EAH, the aim of this study was to investigate the association between dietary sodium intakes and sweat sodium losses amongst exercising females. A secondary aim was to assess the influence of whole body sweat sodium losses on acute salt appetite.
Methods: In a cross sectional study, fourteen recreationally active females participated in two exercise trials, separated by one week. In the 24 hours preceding each trial participants’ completed a 24-hour urine collection and a 24-hour weighed food record to enable estimation of urinary sodium excretion and dietary sodium intake respectively. Participants’ cycled in temperate laboratory conditions; 21°C (±1.1°C) and 87% (±6.5%) humidity for four intervals of ten minutes cycling, separated by a five minute rest period. Blood and urine indices as well as subjective feelings of salt cravings were measured prior to and following exercise. Finally, a dietary behavior questionnaire (DBQ) regarding dietary salt intake was completed; for which a higher total score was reflective of a greater sodium intake. Sweat patches were placed prior to exercise on four regional body sites for estimation of sweat sodium concentration and whole body sweat sodium loss. Following exercise, the ad libitum addition of salt to oven-baked potato fries was measured.
Results: The mean (SD) whole body sweat sodium concentration during exercise was 46.9 (15.4) mmol.L-1. A significant positive association was found between twenty-four hour urinary sodium excretion, the gold standard measure of dietary sodium intake, and whole body sweat sodium concentration during exercise (p=0.026). Total scores from the DBQ were significantly inversely associated with whole body sweat sodium losses during exercise (p=0.012). There was no significant association between whole body sweat sodium concentration or sweat sodium losses and acute salt cravings, nor the addition of salt to food post-exercise (p>0.05).
Conclusion: A greater acute dietary sodium intake is associated with an increase in whole body sweat sodium concentration during moderate intensity cycling exercise. The concentration of sodium in sweat or volume of whole body sweat sodium losses during exercise however do not appear to influence acute salt appetite in females; a finding to support the notion that sodium appetite may be an unconscious behavioural response to an acute period of body sodium depletion.