Evidence suggests the focus for sodium replacement during exercise should be maintenance of plasma sodium concentration ([Na+]plasma) for any given total body water (TBW) volume. The sodium intake to achieve stable [Na+]plasma given known fluid and electrolyte intakes and losses can be mathematically estimated. Therefore the aim of this investigation was to model sodium requirements of athletes during exercise, observing the influence of sweat rate, exercise duration, body mass, baseline [Na+]plasma and sweat potassium [K+]sweat, and relevance to competition (soccer, elite marathon, and 160 km ultramarathon running). Models were constructed across a range of sweat sodium concentrations ([Na+]sweat) (20–80 mmol·L−1), sweat rates (0.5–2.5 L·h−1) and fluid replacement (10–90% of losses). In the competition-specific scenarios, fluid replacement was calculated to achieve 2% TBW losses. Sodium requirements were driven by fluid replacement (% of losses) and [Na+]sweat, with minimal or no influence of other variables. Replacing sodium was unnecessary in all realistic scenarios modelled for a soccer match and elite marathon. In contrast, the 160 km ultramarathon required ≥47% sodium replacement when [Na+]sweat was ≥40 mmol·L−1 and >80% of fluid losses were replaced. In conclusion, sodium requirements to maintain stable [Na+]plasma during exercise depend on both the proportion of fluid losses replaced, and [Na+]sweat. Only when prolonged exercise is coupled with aggressive fluid replacement (>80%) and whole body [Na+]sweat ≥40 mmol·L−1 does sweat composition testing and significant, targeted sodium replacement appear necessary. Highlights Modelling sodium intake requirements with sodium and fluid intake expressed as a proportion of losses, allows robust models to be constructed that are not influenced by sweat rate, exercise duration, body mass, and only to a minor extent by baseline plasma sodium and sweat potassium. When applied to specific sporting scenarios, targeted sodium replacement, and therefore sweat composition testing, appears unnecessary in all realistic scenarios modelled for a soccer match or elite marathon race. Athletes can therefore choose sodium intake according to taste preferences rather than physiological need. During a 160 km ultramarathon, targeted sodium replacement may be necessary because fluid replacement needs are greater as a proportion of losses. However the required quantity remains <50% of losses unless sweat rate is ≥1.5 L·hr−1, sweat sodium is ≥40 mmol·L−1, and fluid replacement is ≥90% of losses. The modelling technique described could be utilised by researchers to personalise sodium replacement in intervention studies for each participant, or by practitioners to either estimate the likely sodium needs of athletes during exercise, or to decide when sweat composition testing is or is not likely to be useful.
- plasma volume