How Much Sodium Do Runners Need?
Sodium is one of the most critical - and most overlooked - nutrients for runners. Whether you're training for a 5K or tackling an ultramarathon, understanding how much sodium your body needs could make a significant difference to both your performance and your health.
Why Sodium Matters for Runners
Your body is made up of 50–70% water, and around a third of that exists outside your cells in fluids like blood plasma. Sodium is the primary electrolyte in these fluids, and the total volume of fluid your body holds is directly tied to how much sodium you have on board. Put simply: more sodium means more fluid; less sodium means less fluid.
Beyond fluid balance, sodium supports nutrient absorption in the gut, nerve impulse transmission, muscle contraction, and cognitive function - all things that matter enormously when you're miles into a run.
A 2015 study illustrated just how significant sodium replacement can be, finding that athletes who adequately replaced their sweat sodium losses finished a middle distance triathlon an average of 26 minutes faster than those who didn't.
How Runners Lose Sodium
Running is a sweaty business, and sweat is the primary way your body loses both fluid and sodium. What many runners don't realise is just how variable those losses are between individuals.
Sweat sodium concentration alone can range from as little as 200mg per litre to as much as 2,000mg per litre - a tenfold difference. Layer on top of that the fact that sweat rates also vary enormously, from barely anything in cool, easy conditions to several litres per hour during hard efforts in the heat, and the total sodium losses from one runner to the next can differ massively.
This is why standard government guidelines - which recommend around 2,325mg of sodium per day - need to be treated with real caution by runners. It's entirely possible to exceed that entire daily recommendation in a single hour of hard running if you sweat heavily and have a high sweat sodium concentration. Over a longer run or race, your losses could be far greater still.
What Happens When You Don't Replace Enough
As sweat losses accumulate, blood volume gradually decreases, placing increasing strain on your cardiovascular system. Your heart has to work harder to pump blood to your muscles and to your skin to regulate temperature - both of which suffer as a result.
Left uncorrected, sodium depletion can contribute to fatigue, muscle cramps, and a general decline in performance. In more severe cases, drinking large amounts of plain water without replacing sodium can dangerously dilute the blood - a condition known as hyponatremia. This can ruin a race, and in rare but tragic cases, has proven fatal.
So, How Much Sodium Do Runners Actually Need?
There's no universal answer, because sodium needs are deeply personal. However, two key factors determine your individual requirements:
How much you sweat - shaped by your sweat rate and the duration of your run
Your sweat sodium concentration - largely determined by genetics and relatively consistent over time
A sensible starting point is to categorise your losses as low, moderate, or high, and adjust your sodium intake accordingly.
Calculating your sweat rate takes a little effort but is achievable with some basic testing during training. Your sweat sodium concentration, however, can only be determined accurately through a sweat test - though the good news is you typically only need to do this once.
If you want a more precise picture, sweat testing is available at specialist centres worldwide. Alternatively, a free Fuel and Hydration Planner can help you estimate your losses and refine your approach through structured trial and error in training.
Getting your sodium strategy right won't just keep you healthy - it could be the performance edge you've been missing.