How much sodium do I need?

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).