In this series of blog posts we’ve discussed what sodium is, what it does in the body, how it’s lost in sweat, and how to quantify those losses. In this post, we’ll discuss how to decide if sodium needs replacing during exercise, and whether a targeted approach is necessary or not.
Why do we need to replace sodium?
As discussed in a previous post, sodium consumed during exercise may have benefits in terms of:
improving the taste of drinks
maintaining blood sodium concentration and osmolarity
increasing blood volume retention
Currently there is minimal if any evidence that sodium replacement can actually improve exercise performance, at least exercise of less than 4 hours.
Note that for the reasons described above, the first does not require sweat sodium testing, simply ‘season to taste’. The second reason is where quantifying sweat sodium losses might be potentially helpful. So when does it matter?
Predicting sodium replacement needs
We don’t currently have laboratory studies to tell us how much sodium needs replacing during exercise. However, a mathematical formula originally developed for managing water and sodium balance in hospital patients has been validated successfully during exercise. In a recently published study, this formula was used to estimate how much sodium needed to be consumed to maintain a steady blood sodium concentration (and therefore likely osmolality as well). As seen in the graph below, this modelling showed clearly the two main factors that determine the need for sodium replacement during exercise:
The extent to which sweat fluid losses are replaced (i.e. fluid intake as a % of sweat losses)
The sweat sodium concentration, which varies in the graph from very low (460 mg/L) to very high (1840 mg/L).
These two factors almost exclusively determined the % of sweat sodium losses that need replacing to maintain a stable blood sodium concentration. The more aggressively fluid is replaced, the greater need for sodium. Likewise, those with a higher sweat sodium concentration were more likely to benefit from sodium replacement.
What does this mean in the real world?
Fluid replacement during exercise is designed to prevent dehydration, which is typically defined as the loss of a certain % of body weight in fluids. But depending on sweat rate and the duration of exercise, the % of sweat losses that need replacing to achieve this will vary. Higher sweat rates and longer durations of exercise increase the % of fluid losses that need replacing to prevent a certain level of fluid loss. And this helps us determine when targeted sodium replacement might be helpful, or when the sweat testing process is unnecessary.
In exercise of less than 2 hours, the amount of fluid that needs replacing to prevent dehydration is likely to be low, unless the sweat rate is extremely high. In the example above if someone sweats 0.9 L/h for 2 hours, they will need to replace approximately 20% of their fluid losses (a very small amount of fluid). There may be other reasons as well why athletes cannot consume a large percentage of their losses. There is a limit to how much fluid can be practically consumed and tolerated during exercise. There may be limited opportunities to drink, and it’s harder to tolerate large quantities of fluid. So even when athletes have a very high sweat rate, it’s unlikely they will be able to replace much more than 50% of their fluid losses.
This means that in these situations sodium replacement is unnecessary to maintain a stable blood sodium concentration. Sweat sodium testing is therefore unlikely to be of value, and instead athletes in these sports can simply choose their sodium based on taste preferences.
The situation is very different for ultra-endurance sports, however. A relatively lower sweat rate, but with a long time for fluid losses to accumulate and greater opportunities to drink and tolerate fluids, means that replacing a greater % of fluid losses is both necessary and possible. It is in these scenarios that sweat testing and targeted sodium replacement may be more helpful. Keep in mind though from the previous blog post that sweat sodium concentration varies from day-to-day, and so very precise sodium replacement is likely to be neither necessary or possible.
Currently, the only defined purpose for providing a targeted amount of sodium during exercise appears to be to maintain a stable blood sodium concentration and therefore osmolality, for any given fluid intake. Due to the nature of sweat losses, blood sodium concentration will only fall when athletes are required (and do) replace the majority (>70%) of fluid losses to prevent dehydration, and whole body sweat sodium concentration is above average (>40 mmol/L or 1g/L). Only in ultra-endurance exercise is this likely to realistically occur – in all other cases athletes can simply season to taste when it comes to sodium intake during exercise. Given that the degree of fluid replacement is one of the main determinants of sodium replacement needs, athletes should first estimate their sweat fluid losses and replacement needs, before deciding if sweat sodium testing and targeted replacement is actually necessary.
Baker LB et al. Quantitative analysis of serum sodium concentration after prolonged running in the heat. J Apply Physiol. 2008; 105(1):91-99.
McCubbin AJ. Modelling sodium requirements of athletes across a variety of exercise scenarios – Identifying when to test and target, or season to taste. Eur J Sport Sci. 2022; DOI: 10.1080/17461391.2022.2083526.