Sweat, and specifically Sodium, is one of the three “levers” for endurance athletes when it comes to hydration and nutrition (fueling) and subsequently your performance. The other two key elements or levers are Carbohydrates and Fluids.
The most important part about sodium/sweat is that everyone has a unique sweat loss rate, and a unique sodium content ratio in their sweat. So while a more generalized approach can be defined for consuming carbohydrates, the same is not true for electrolyte loss. This is why having a sweat test is critical for athletes wanting to perform, understand and take advantage of their data to understand their personal needs and requirements.
To calculate sweat loss, it’s quite easy and straight forward, weigh yourself before exercise, track what you excrete and consume during the exercise and weigh yourself after …
To understand the amount of sodium you should replace, takes a little more effort and go from generalized (i.e. do you see what stains on your gear) to scientific, which uses medical lab equipment. An alternative is to find a sweat analysis company specializing in sports, like Precision Hydration where they will do this for you.
I have been intrigued and curious enough to do all of the above. Initially I tested a small patch from Gatorade – the results varied widely, so booked an appointment for a professionally conducted sweat test. After doing this test, I found some of the same equipment on eBay, and started helping friends and also validating the data I had. In the end what I have noticed is that my sweat concentration is close to the average – 970mg Sodium per Liter of fluid lost. Ultimately I now simply try to average around 1,000mg of Sodium for every 1L of water I consume during a longer event (+ 2 hours).