Sweat & The Role of Sodium for Endurance Runners
By Doug Stewart, Performance Coach, & Rebecca Dent, High Performance Dietitian
In a previous post we have discussed the impact that heat has on endurance performance, and you can find additional information here on hydration, and heat mitigation strategies when racing in the heat here.
In this blog, we discuss the role of sodium for endurance runners. Through sweating, athletes lose water and electrolytes. The rate of sweat loss and concentration of electrolytes within the sweat varies greatly amongst individuals (Baker, 2017). Why are electrolytes, and in particular sodium, of potential importance to endurance runners?
“Electrolytes are essential for basic life functioning, such as maintaining electrical neutrality in cells, generating and conducting action potentials in the nerves and muscles. Sodium, potassium, and chloride are the significant electrolytes along with magnesium, calcium, phosphate, and bicarbonates. Electrolytes come from our food and fluids. These electrolytes can have an imbalance, leading to either high or low levels. High or low levels of electrolytes disrupt normal bodily functions and can lead to even life-threatening complications.” Shrimanker and Bhattarai (2022)
If you lose electrolytes and water through sweating, then should we be ingesting electrolytes with water?
A 2018 review of the existing research on sodium ingestion during exercise and its subsequent impact on performance showed that, of the 5 studies that met the inclusion criteria, only one pointed to any significant benefit on endurance performance when taking sodium (McCubbin and Costa, 2018). The authors identified a number of limitations of the existing studies and stated that more high quality research is required, but it appears the majority of the research suggests additional sodium is not beneficial to performance. However, research with 344 endurance athletes revealed that they deliberately increase the amount of sodium they have prior to and during races, with the belief that this is needed to perform well (and informed by non-scientific sources) (McCubbin et al., 2019).
For trail and ultra runners, one of the main considerations is the duration of the exercise / event. When thinking about ultra-endurance performance Hoffman et al. (2018) suggest avoiding excessive sodium intake and using food cravings as a guide – not the white salt marks on clothing. More recent research has seen 9 runners run for 5 hours on a treadmill in hot conditions – completing this twice, once with sodium, once with a placebo in a random order. The sodium and the placebo trial each lasted for 5 days with the protocol detailed below:
Source: McCubbin and Costa (2023)
The amount of sodium taken during the 5-hour treadmill run was determined by sweat sodium testing during the trial. However, some participants displayed large variances between their sweat sodium levels between the familiarisation and the trial. Overall, the results suggest that personalised sodium replacement over the 5 hours of running had minimal effect on how thirsty the participants felt or the amount of water they drank. Salt cravings increased throughout the 5 hours of running, but there was no difference between the placebo and sodium groups. Similar to salt cravings, thirst rating increased over the 5 hours, but again there was no difference between the two groups or the amount drunk during the trial. However, after the 5 hours, the sodium group did drink significantly more (McCubbin and Costa, 2023).
There is a lack of quantifiable research and consensus when it comes to guidelines for sodium replacement during exercise (McCubbin, 2021). To try and rectify this, McCubbin (2023) used mathematical modelling to try and determine if sodium replacement is needed for athletes competing in various sports to ensure that sodium plasma volume remains stable. Through his modelling, he then provided examples of sodium replacement required based on athlete sweat rates and sodium levels within their sweat. Two examples are provided in the table below, illustrating runners with a relatively low, medium or higher sweat rate, and then also for each sweat rate 4 different levels of sodium concentration in the sweat.
Source: McCubbin, 2023. Calculated sodium intake requirements, expressed as the proportion of sodium losses and as an hourly intake rate.
ETT athlete Gian during a sweat test at Loughborough University
The marathon runner example displays a 2% loss in bodyweight over the course of the race, whilst the ultramarathon runner is losing 4% bodyweight. Whilst, generally, the marathon runner doesn’t require sodium, the ultramarathon runner does, in most scenarios, require sodium to maintain sodium plasma at starting levels. For the ultrarunner, unless their sodium in the sweat is in the higher brackets, less than 50% of the sodium losses require replacing. For shorter durations or lower sweat rates and sodium concentrations, it appears that sodium can be consumed based on taste preference alone (McCubbin, 2023).
Overall, more research is needed to fully understand electrolytes and endurance performance. However, the work of researchers like McCubbin suggests that their role is less important for performance and maintenance of sodium blood plasma volume, unless the athlete has a higher sweat rate, high sweat sodium levels, or is doing very long endurance events. Another factor to consider if thinking about supplementing with electrolytes in your drink or using salt tablets, is the amount you get through your other sports nutrition, such as gels, chews and bars. Often, these will also contain sodium, which may be enough to achieve the levels suggested by McCubbin (2023) in his mathematical modelling.
References:
Baker, L. B. (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 47, 111-128.
Hoffman, M. D., Stellingwerff, T., & Costa, R. J. (2019). Considerations for ultra-endurance activities: Part 2–hydration. Research in sports medicine, 27(2), 182-194.
McCubbin, A. J. (2021). Exertional heat stress and sodium balance: Leaders, followers, and adaptations. Autonomic Neuroscience, 235, 102863.
McCubbin, A. J. (2023). Modelling sodium requirements of athletes across a variety of exercise scenarios–Identifying when to test and target, or season to taste. European Journal of Sport Science, 23(6), 992-1000.
McCubbin, A. J., & Costa, R. J. (2018). The impact of dietary sodium intake on sweat sodium concentration in response to endurance exercise: a systematic review. Int J Sports Sci, 8(1), 25-37.
McCubbin, A. J., Cox, G. R., & Costa, R. J. (2019). Sodium intake beliefs, information sources, and intended practices of endurance athletes before and during exercise. International journal of sport nutrition and exercise metabolism, 29(4), 371-381.
Shrimanker I, Bhattarai S. Electrolytes. In: StatPearls. StatPearls Publishing, Treasure Island (FL); 2022.