Sodium in Athletes: What Your Blood Work and Sweat Really Tell You
Table of Contents
Introduction
In the general population, sodium intake is often higher than necessary, and reducing excess salt is commonly recommended due to its association with cardiovascular risk. In athletes, however, the situation is different. During training and competition, the body loses sodium through sweat, sometimes in substantial amounts, which changes the balance between sodium and fluid in the body. Because of this, the same dietary advice does not always apply in the same way, and in some situations the concern may even begin to shift in the opposite direction.
In this article, I take a closer look at what the current evidence actually shows about sodium in athletes — how much is lost through sweat, what blood sodium truly reflects, when replacement becomes relevant, and where common beliefs begin to diverge from the data.
Sodium in Athletes: What Normal Blood Sodium Actually Means
Serum sodium is tightly regulated within a reference range of 135–145 mmol/L. Values below 135 mmol/L define hyponatraemia; values above 145 mmol/L define hypernatraemia. In healthy athletes, there is currently no evidence that they require a greater daily sodium intake than the general population, because the kidneys and sweat glands continuously regulate sodium losses to maintain balance [1].
This homeostasis creates a clinically important principle: a routine blood test showing normal serum sodium does not tell you how much sodium an athlete has lost, how salty their sweat is, or whether their hydration strategy is sound. It only reflects the ratio of sodium to total body water at the moment of sampling. This is the same interpretive challenge that arises with other markers in the athletic panel — as discussed in the guide to electrolytes in athletes, the normal reference range often obscures what is actually happening physiologically during and after training.
A common misconception is that losing large amounts of sodium through sweat will directly lead to hyponatraemia. In reality, exercise-associated hyponatraemia is primarily a dilutional problem — it occurs when fluid intake exceeds losses, lowering plasma sodium concentration, rather than simply from sodium depletion alone [10]. Sweat is hypotonic relative to plasma, meaning that more water than sodium is lost during exercise [10]. As a result, sweating alone does not typically lead to hyponatraemia — and may even increase plasma sodium concentration if fluid intake is insufficient.
How Much Sodium Do Athletes Actually Lose in Sweat?
Sweat sodium concentration varies widely between individuals. A retrospective analysis of 1,944 sweat tests from 1,304 subjects identified the key predictors of this variation: season of the year (warm months associated with lower sweat sodium), exercise mode, sex, whole body sweating rate, and body mass [2]. Because the statistical models explained only 17–23% of the variation in sweat sodium concentration, two athletes completing identical training in identical conditions can still produce substantially different sodium deficits [2].
Normative data in athletes confirm this wide inter-individual spread, with values commonly reported across a broad range depending on the population, season, and conditions [3]. Endurance athletes show some of the highest whole-body sodium loss rates across sports, with a mean whole-body sweating rate of 1.28 ± 0.57 L/h [3].
In athletes, the development of hyponatraemia is usually not determined by how much sodium is lost through sweat, but by how fluid losses are replaced. Because sweat is hypotonic, exercise can actually increase plasma sodium concentration if fluid intake is insufficient. However, when these losses are replaced primarily with plain water, without adequate sodium, plasma sodium becomes diluted — which may lead to hyponatraemia.
The Central Principle: Sodium and Water Move Together
A 2025 comprehensive review in Performance Nutrition makes an important clarification that reshapes how we should think about sodium in athletes: the primary driver of health and performance outcomes is not sodium balance in isolation, but the relationship between sodium and water [1]. This relationship determines extracellular osmolality, which in turn governs thirst drive, fluid retention, and the distribution of fluid between intracellular and extracellular compartments.
When an athlete drinks excessive amounts of plain water without adequate sodium, plasma sodium falls — not because sodium was lost, but because free water diluted what was there. Conversely, when sodium is consumed alongside fluid, plasma osmolality is maintained, thirst drive is preserved, and the kidneys retain fluid rather than excreting it.
Recommendations for sodium must therefore be made relative to changes in both sodium losses and fluid balance — not based on sodium losses alone, as many athletes and coaches have traditionally done [1].
In everyday life, sodium intake is often higher than necessary, with many individuals exceeding recommended levels through routine dietary patterns. In my clinical practice, I frequently see this reflected among my patients. For example, Finnish nutrition guidelines recommend limiting salt intake to no more than 5 grams per day (approximately 2 grams of sodium) — a level that can easily be exceeded with a single processed meal [11]. In athletes, however, the challenge is more acute and context-dependent. During prolonged exercise, particularly when there is limited opportunity to eat, sodium losses through sweat may not be matched by intake. When fluid is then replaced primarily with water, the balance between sodium and total body water can shift, leading to changes in plasma sodium concentration during or after exercise.
Exercise-Associated Hyponatraemia: The Underappreciated Risk for Sodium in Athletes
Exercise-associated hyponatraemia (EAH) — serum sodium below 135 mmol/L during or after exercise — is the clinical endpoint of a sodium-water imbalance. A narrative review of 135 articles on marathon runners found that two major studies reported EAH incidence ranging from 7 to 15% for symptomatic and asymptomatic presentations combined [4]. Risk factors identified include female sex and high ambient temperature [4], with excessive fluid consumption during the event representing an important contributor to plasma sodium dilution [1].
The primary mechanism in most EAH cases involves consuming more fluid than sweat losses require, leading to a fall in plasma sodium. Common problems relating to sodium and fluid balance generally arise because of a mismatch between sodium and fluid intake rather than sodium depletion in isolation [1]. Athletes planning their first marathon or ultramarathon should read the dedicated guide to post-marathon blood work, which covers how sodium and other markers behave in the hours and days following a race, and when results return to normal.
To reduce the risk of dilutional hyponatraemia during prolonged exercise, it is often useful for fluids to contain some sodium rather than relying on plain water alone. This can be achieved with commercially available sports drinks or simple homemade mixtures. For example, adding around 1 gram of table salt per litre of fluid provides a modest amount of sodium and can help maintain the balance between sodium and total body water during prolonged exercise.
From a practical perspective, the main downside of excessive sodium intake during exercise is not acute health risk, but tolerability. Fluids with very high sodium content often taste unpleasant and can be off-putting, which may limit how much an athlete is able or willing to drink. In my experience, this is surprisingly easy to overdo — even small increases in salt concentration can quickly make a drink taste overly salty and difficult to tolerate during longer sessions. For this reason, commercially available sports drinks or well-formulated homemade recipes can often be a more practical and reliable option.
When Does Targeted Sodium Replacement Actually Matter?
A modelling study in the European Journal of Sport Science constructed mathematical models across sweat rates (0.5–2.5 L/h), sweat sodium concentrations (20–80 mmol/L), and fluid replacement levels (10–90% of losses) for three competition scenarios: a soccer match, an elite marathon, and a 160-km ultramarathon [5].
The findings are striking. Targeted sodium replacement appeared unnecessary in all realistic scenarios modelled for a soccer match or elite marathon. Athletes completing these events could choose sodium intake according to taste preferences rather than physiological calculation [5].
The picture changes for the 160-km ultramarathon. Here, the modelling showed that sodium needs depended on the proportion of fluid losses replaced and on sweat sodium concentration. At higher fluid replacement levels and in athletes with higher sweat sodium concentrations, targeted sodium replacement became more relevant [5].
As a practical rule of thumb, I generally advise athletes to drink according to thirst rather than forcing extra fluids. In practice, exercise-associated hyponatraemia rarely develops when fluid intake matches or stays slightly below actual losses. It is primarily caused by drinking significantly more than the body needs, especially when using low-sodium fluids.
For sessions lasting longer than 1–2 hours, particularly in warm conditions or with high sweat rates, including modest amounts of sodium in fluids can help support fluid balance and reduce the small risk of dilution. Precise sodium calculations are rarely needed for typical training; a sensible amount is usually sufficient.
Sodium for Post-Exercise Rehydration in Athletes
After exercise, rehydration is not simply a matter of replacing fluid volume. Drinking plain water post-exercise triggers a fall in plasma osmolality, which suppresses thirst and stimulates diuresis — the kidneys excrete the incoming water before full rehydration is achieved.
Research indicates that beverages with sodium concentrations of at least 40 mmol/L support the restoration of sodium balance following exercise-induced dehydration, and that solutions containing 40–50 mmol/L sodium have been shown to promote greater fluid retention than beverages containing 31 mmol/L or less [6]. In a 2023 clinical trial in Nutrients, both an oral rehydration solution (45 mmol Na/L) and a standard sports drink (18 mmol Na/L) produced greater fluid retention than water placebo in 26 athletes following 2.6% body mass loss from exercise — though the two sodium-containing beverages did not differ significantly from each other [6].
For athletes with overnight recovery windows and no acute time pressure, consuming regular meals with adequate salt content alongside ad libitum fluid intake supports gradual sodium and fluid restoration. Athletes interested in how all recovery markers fit together can explore the broader picture in the guide to HRV and blood work, which discusses how fluid balance interacts with autonomic recovery markers.
During rehydration, it is important to remember that water absorption in the intestine is closely linked to the presence of sodium and glucose. The sodium–glucose co-transporter (SGLT1) facilitates the coupled uptake of both, which in turn enhances fluid absorption. This is why many rehydration solutions — including those used in clinical settings such as oral rehydration therapy — contain both sodium and glucose.
From a practical perspective, this principle also applies to recovery after exercise. Fluids that contain both sodium and a readily absorbable carbohydrate source can support more effective rehydration compared to plain water alone. In my clinical practice, I often advise patients to follow this same principle — for example, in the management of diarrhoeal illness, where patients are specifically encouraged not to rely on plain water alone, but to use solutions containing both sodium and carbohydrate to support fluid absorption.
While many commercial drinks use mixtures of carbohydrates such as sucrose or maltodextrin, these are typically broken down in the intestine before absorption. In practice, pure glucose is not something most people have readily available in their kitchen cupboard unless it is purchased specifically for this purpose, whereas salt is almost always on hand. Importantly, the difference is unlikely to be significant in most real-world situations, and regular table sugar can serve as a practical alternative. For this reason, commercially prepared drinks or well-formulated recipes can often be a more convenient and reliable option.
The Sodium-Cramping Belief in Athletes: What the Evidence Actually Shows
One of the most persistent beliefs in sport is that exercise-associated muscle cramps (EAMC) result from sodium depletion and should be treated with salt. The evidence does not clearly support this. An evidence-based review in the Journal of Athletic Training found that plasma electrolyte concentrations are often within normal limits and comparable between athletes who cramped and those who did not, with multiple studies showing no significant differences in plasma volume, electrolyte concentrations, or hydration status between groups [7].
A 2022 review of sodium in endurance and ultra-endurance sports found no documented scientific evidence supporting a direct relationship between sodium depletion and EAMC, and pointed to muscular fatigue from higher relative intensity or duration as the most commonly cited mechanism [8].
This does not mean sodium is irrelevant in every case. But athletes and coaches should be cautious about attributing all cramping to sodium deficiency and consuming large doses of sodium supplements as a universal solution.
The belief that muscle cramps are primarily caused by electrolyte imbalances remains widespread and, in my experience, is still very much alive in everyday clinical practice. I frequently see patients — both athletes and sedentary individuals — attributing various types of cramping to deficiencies in electrolytes such as magnesium or sodium. However, the evidence suggests that most exercise-associated muscle cramps are not directly caused by electrolyte disturbances, but are more likely related to neuromuscular mechanisms, particularly fatigue and altered motor control [7][8].
Despite this, magnesium supplementation remains very common for muscle cramps. In my clinical practice, I frequently see both athletes and sedentary patients using magnesium or sodium supplements, often driven more by the perception of deficiency than by clear clinical evidence. While magnesium supplementation is rarely harmful in otherwise healthy individuals, its benefits for cramping are often uncertain.
By contrast, an excessive focus on sodium loss can sometimes lead to unnecessary high sodium intake. I occasionally see athletes using aggressive salt supplementation or high-sodium products without a clear indication, which may not address the actual underlying cause of their symptoms. Over time, consistently high sodium intake is associated with adverse cardiovascular effects, particularly elevated blood pressure [12].
While electrolyte imbalances may play a role in certain individual cases, the evidence suggests they are unlikely to be the sole or dominant cause in most situations [7][8].
Conclusion
Sodium in athletes is not a simple question of “more or less,” but of context. While excessive sodium intake is a well-established concern in the general population, athletic settings introduce a different physiological landscape — one defined by fluid loss, sweat composition, and the dynamic interaction between sodium and water.
Perhaps the most important takeaway is that serum sodium does not reflect total body sodium stores, but rather the relationship between sodium and fluid at a single point in time. This distinction explains why athletes can present with normal blood sodium despite significant physiological strain — and why hyponatraemia is more often a problem of dilution than depletion.
For most athletes, precise sodium replacement is not necessary. However, during prolonged exercise, particularly in the heat, including modest amounts of sodium alongside fluid can help maintain balance and reduce the risk of dilutional hyponatraemia. At the same time, it is important to avoid overcorrecting — both in terms of fluid intake and sodium supplementation — as more is not always better.
Finally, common beliefs around sodium — particularly in relation to muscle cramps — often extend beyond what the evidence supports. Cramping is more often a reflection of neuromuscular fatigue than electrolyte deficiency, and treatment strategies should reflect this reality rather than default assumptions.
In practice, the goal is not to chase sodium numbers, but to understand the system: how much is being lost, how fluid is being replaced, and how the body responds over time. When that context is taken into account, sodium becomes less of a problem to solve — and more of a variable to manage intelligently.
References
[1] https://doi.org/10.1186/s44410-025-00011-9
[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC9942894/
[3] https://pubmed.ncbi.nlm.nih.gov/31230518/
[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC9699060/
[5] https://pubmed.ncbi.nlm.nih.gov/35616504/
[6] https://pmc.ncbi.nlm.nih.gov/articles/PMC10674530/
[7] https://pmc.ncbi.nlm.nih.gov/articles/PMC8775277/
[8] https://pmc.ncbi.nlm.nih.gov/articles/PMC8955583/
[9] https://pmc.ncbi.nlm.nih.gov/articles/PMC9811094/
[10] https://bjsm.bmj.com/content/49/22/1432
[11] https://www.ruokavirasto.fi/elintarvikkeet/terveytta-edistava-ruokavalio/ravintoaineet/suola/
