Trail Research Weekly #7: Sodium Intake for Athletes

Sodium might be the most argued-about, most purchased, and least understood product in endurance sport. The recent trend of athletes, regardless of training volume or competitive level, to use daily sodium supplementation is driven by the idea that water doesn’t hydrate on its own. A comprehensive review by McCubbin (2025) pulled together decades of research on sodium and exercise to test the common rationales athletes use for supplementing daily, as well as before, during, and after exercise. Some hold up. Most don’t.

Daily Sodium

There’s a common belief that athletes need to supplement with sodium daily because of their increased sodium requirements compared to sedentary individuals. However, the data suggests that athletes are likely already consuming considerably more sodium than required. Since sodium and caloric intake are closely correlated, athletes unknowingly increase sodium intake as their training volume increases, along with their caloric intake.

There’s also the belief that water doesn’t hydrate on its own. This is an overly literal interpretation of the failure to retain large, rapid boluses of plain water, which is excreted through diuresis (urination). That is a normal protective measure to stabilise blood osmolality. By timing water intake with food, you slow gastric emptying and allow more water to diffuse and remain in the body.

Extra sodium in your water doesn’t “take the water where it needs to go”; it gets excreted over the long term. The kidneys are very good at their job. Sodium excretion adjusts within two hours of a change in intake. So there is no evidence that athletes need a greater daily sodium intake than sedentary people, outside the specific exercise scenarios below.

Before Exercise

Rationale 1: Sodium Intake Increases Ad Libitum Fluid Intake, Improving Pre-Exercise Hydration Status

Sodium is the biggest contributor to how you perceive thirst, and subsequently seek out water. A large acute dose of sodium (60 mg/kg) increased voluntary fluid intake by 562 mL in athletes told to drink only when thirsty, in the two hours before exercise. For athletes who are typically hypohydrated before training or competition, added sodium may increase the drive to drink water and the likelihood of euhydration (optimal hydration). That said, 60 mg/kg of sodium is a substantial amount and will likely be unpalatable.

Rationale 2: Increasing Total Body Sodium in Anticipation of Losses During Exercise

This is the practice of deliberately increasing sodium intake for days, sometimes up to a week, before competition to increase total body sodium. Some athletes view sodium the same way they view glycogen, as if it gets stored somewhere in excess. It doesn’t. As mentioned before, natriuresis (the kidneys dumping excess sodium) kicks in within two hours of extra intake and ramps up further by eight hours. By contrast, an acute, same-day dose in the final hours before exercise retains far more sodium (around 77% in one study) than spreading the same total dose over three days (around 21% retention).

Rationale 3: Sodium Intake Improves Fluid Retention for Achieving Pre-Exercise Euhydration or Hyperhydration

Adding sodium to a fluid bolus prevents the drop in blood osmolality that would otherwise trigger you to urinate the excess back out, meaning more of what you drink stays in your system. For athletes who know they can’t drink enough during the event itself, typically because of heat, high intensity, or gut tolerance limits, a structured hyperhydration protocol in the 1–4 hours before the start can help them head into the race euhydrated.

The practical numbers are:

• 275–420 mg sodium per 100 mL of fluid for pre-race hyperhydration.
• 100–125 mg per 100 mL if you’re trying to nudge voluntary drinking upward without overloading on salt.
• Above ~120 mg per 100 mL, palatability drops off a cliff for most people. Capsules or salty food solve this problem better than forcing down an unpleasantly salty drink.

During Exercise

A lack of specific guidelines results in a range of approaches to sodium intake. This ranges from arbitrary mg/h recommendations on internet forums or popular opinions, to personalised targets based on sweat-sodium composition testing. Many companies claim to be able to test sweat-sodium concentration and use a variety of methods. One method uses pilocarpine iontophoresis, a resting test that chemically stimulates a small patch of skin to sweat, and is prized for being quick, standardised, and highly reproducible. Another is a new wave of wearable sweat sensors for use during activity. This topic deserves its own blog post next week, but in short, a resting test doesn’t reflect active sweat rates and concentrations, as shown by a brand-new study (Harris et al., 2026). Unfortunately, any company claiming it can do so isn’t being honest. That said, the wearable technology side is rapidly improving and becoming more reliable.

 This section explores the main rationales for sodium supplementation during exercise.

Rationale 1: A Specific Sodium Concentration Enhances Gastrointestinal Fluid and/or Carbohydrate Absorption

A common marketing claim is that a particular sodium concentration in your drink is “optimal” for the gut absorption of fluids or carbohydrates. The research doesn’t support this. Studies on beverages with varying sodium concentrations found no meaningful differences in fluid or glucose absorption. Athletes do not need to choose between drinks with different sodium concentrations for different reasons, and the best practice is still to salt to taste and drink when thirsty.

Rationale 2: Sodium Increases Thirst Drive and Ad Libitum Fluid Intake

The evidence for this during exercise is mixed. In some outdoor trials, salt capsules increased voluntary fluid intake by 30–60% compared to placebo. In other controlled lab studies over several hours, the difference in ad libitum drinking was small and not statistically significant. Even so, in longer or intermittent efforts, practical factors like aid station spacing and gut tolerance likely matter more than any osmotic thirst effect from sodium. Sodium clearly influences palatability; most people find drinks unpleasant at levels above 100 mg per 100 mL.

Rationale 3: Sodium Prevents and/or Treats Exercise-Associated Muscle Cramping

This is the most common misconception about sodium. Some observational studies link sweat sodium losses to cramping; several controlled studies find no effect of blood sodium, sweat sodium, or supplementation on cramp risk. Modern thinking treats cramping as multi-factorial: systemic fatigue, unfamiliar terrain, and neuromuscular fatigue all play a role, with sodium being only one variable. Sodium helps prevent a rapid fall in blood sodium concentration when fluid intake is high, which may reduce cramp susceptibility as a side effect rather than a direct fix. Currently, no specific sodium dose can be recommended purely for cramp prevention.

Rationale 4: Preventing a Fall in Blood Sodium Concentration, and Onset of Exercise-Associated Hyponatraemia

The dangerous drop in blood sodium during long races is driven far more by overdrinking than by under-salting. Mathematical modelling in the literature shows that for a runner with an average sweat sodium concentration, blood sodium only starts to fall once fluid replacement exceeds ~70% of losses. Once you’re replacing 100% or more, no amount of added sodium fully offsets the fall in blood sodium. Sodium during exercise is meant to complement your fluid strategy, not to fix overly aggressive drinking habits.

Rationale 5: Sodium Replacement Improves Exercise Performance

Little research exists here; only one study found a performance benefit from sodium supplementation, and the effect size is not large enough given how many other variables weren’t controlled for. In the rest of the literature, sodium appears to improve performance only when it also increases the amount of fluid athletes drink, not as an independent effect.

After Exercise

Rationale 1: Sodium Post-Exercise Increases Thirst Drive and Ad Libitum Fluid Intake

The theoretical case is similar to before exercise: added sodium should increase thirst drive and voluntary fluid intake post-race. In practice, this hasn’t been tested in isolation, and it’s complicated by the fact that you’re likely already eating and drinking a mix of recovery foods rather than a single beverage. If you finish hypohydrated, your thirst drive is probably already elevated regardless of added sodium.

Rationale 2: Sodium Post-Exercise Increases Fluid Retention and Post-Exercise Rehydration

Adding sodium to a low-calorie, low-nutrient recovery drink clearly improves how much of it you retain rather than losing through your kidneys. But the moment you add real food or a nutrient-dense drink like milk into the mix, gastric emptying slows enough on its own that added sodium makes little extra difference. Sodium retention strategies matter most when rapid rehydration with plain fluids is the priority.

Rationale 3: Sodium Post-Exercise Restores Whole Body Sodium Content for the Next Exercise Bout

Even after a large sodium deficit from a long hot effort, the kidneys clamp down and conserve sodium automatically over the following 24 hours, with no deliberate “restoring” required. One study found a sodium deficit of over 5,600 mg still triggered a 68% reduction in urinary sodium losses post-race. Complete same-day replacement isn’t necessary unless you’re backing up for another demanding session within 24 hours.

Rationale 4: Sodium Restriction Post-Exercise Reduces Restoration of TBW and Therefore Body Mass Prior to the Following Day’s Competition

Relevant in multi-day, weight-sensitive events like stage races. There’s currently no direct evidence that restricting sodium post-exercise meaningfully reduces body mass the next day, especially once you factor in normal post-race food intake. What evidence exists suggests that hypohydration is more likely to hurt next-day performance than a lower body mass is to help it, so this is a strategy to be very cautious with.

Why This Matters for Trail Runners

Trail and ultrarunning sit at the extreme end of endurance sports. We’re out for far longer than most studies have addressed, often in variable heat, with wildly inconsistent opportunities to drink depending on aid station spacing and terrain. A mismatch between sodium and fluid intake poses a real risk of exercise-induced hyponatremia. In next week’s edition, we will unpack sweat sodium testing procedures to mitigate this in the specific context.

 Outside of true ultra-distance racing in the heat, sweat testing and precision sodium dosing are unlikely to make a difference. The anxiety athletes carry about “not replacing enough salt” is, for most, unnecessary.

My Takeaway

Athletes get caught up in numbers, a specific mg/h intake that will magically solve their problems. These problems are more often than not solved with consistent, specific, and periodised training, over an extended period of time. There’s no ‘silver bullet’. The best course of action for supplementing with sodium for athletes during exercise is captured in the illustration below, which you will see is very conservative:

Reference

McCubbin, A.J. 2025. Sodium intake for athletes before, during and after exercise: review and recommendations. Performance Nutrition, 1, 11.

FAQs

Q1: Do trail runners need to eat more salt every day?

No. Most endurance athletes already consume well above recommended sodium intakes through their normal diet, and the kidneys regulate sodium efficiently within hours. There’s no evidence for a general daily sodium requirement above what a normal diet already provides.

Q2: Will salt tabs during a race stop me from cramping?

Possibly, but don’t count on it as a sole fix. Cramping is multifactorial, and sodium’s clearest role is to prevent a rapid fall in blood sodium concentration during heavy fluid intake, rather than to top up a sodium deficit directly.

Q3: Is hyponatraemia caused by too little salt or too much water?

Overdrinking is by far the bigger driver. Blood sodium tends to fall once fluid replacement climbs above roughly 70% of sweat losses, and extra sodium can’t fully compensate for excessive fluid intake.

Q4: How much sodium should I actually take during a long race?

For most sessions under four hours, don’t overthink it; a normal sports drink or salty snack is enough. In longer efforts where you can realistically drink back most of your fluid losses, matching sodium intake to your fluid plan becomes worth the effort.

Q5: Should I load up on sodium in the days before a race?

No. Multi-day sodium loading is cleared out by the kidneys well before race day. If you want a pre-race sodium strategy, keep it acute, in the final 1–4 hours before the start.