Sodium Explained for Indoor Training

With the arrival of winter, athletes are increasingly training indoors, which changes the physiological demands of their sessions. Indoors, the absence of airflow and restricted convective cooling increases thermoregulatory stress, accelerates sweating and increases fluid and electrolyte losses. This means that hydration and electrolyte strategy become key components in maintaining performance at a good level.

This article outlines why electrolyte requirements increase indoors, backed by scientific evidence, and then compares five leading electrolyte products — PureSport, Styrkr, Torq, Precision Hydration and SiS — through that lens.

Why indoor training drives higher sweat rates.

During exercise, a large percentage of the energy produced by active muscles is released in the form of heat. Outdoors, this heat is dissipated through a combination of convection, evaporation, radiation, and conduction. The airflow created by forward movement significantly improves cooling through convection and evaporation.

Indoor training, on the other hand, alters the balance of heat transfer mechanisms. This is because airflow is minimal unless athletes use fans, and even then, the airflow is uneven and often insufficient. This reduced heat dissipation leads to:

• Earlier onset of sweating

• Higher sweat rates at matched intensities

• Less efficient evaporative cooling

• Greater cardiovascular strain and higher heart rate at fixed power

Studies consistently show that body temperature and sweat rate increase more rapidly indoors than outdoors with equivalent workloads (Périard et al., 2021). As the sweat rate increases, so do electrolyte losses (Buono et al., 2008), making sodium replenishment more relevant for indoor training.

How electrolytes support hydration and performance

As mentioned above, sweating is the body's primary mechanism for dissipating heat. Sweat is mainly composed of water and electrolytes: primarily sodium, chloride, potassium, calcium, and magnesium. Of all the electrolytes, the one found in the greatest quantity and with the most physiological impact is sodium (Valentine, 2007; Shirreffs & Sawka, 2011), as it is related to:

• Fluid balance

• Nerve and muscle function

• Cognitive function

• Nutrient absorption

When sodium is not replaced in line with sweat losses, athletes risk (Veniamakis et al., 2022):

• Faster dehydration

• Muscle cramps

• Fatigue and performance loss

• Reduced plasma volume

• Hyponatremia (abnormally low sodium levels in the blood)

• Cardiovascular strain

The challenge indoors is not only that the sweat rate is higher, but also that evaporation is less effective. Sweat drips rather than evaporates, meaning that each drop that does not evaporate provides no cooling benefit but still removes fluids and electrolytes from the body.

Does taking electrolytes during exercise improve performance

Yes and no. Although the term ‘it depends’ may be the one we least like to hear when we are looking for a specific answer, it is often the most accurate when talking about hydration and electrolyte consumption.

It depends especially on the athlete and their sodium concentration in sweat. This genetic factor will directly affect losses, although it can also be influenced by the type of session, duration, intensity, environment, etc.

In conditions of high intensity, especially indoors, it is likely to be a very beneficial aid in preventing muscle cramps, headaches and improving the perception of fatigue. However, the health conditions of each athlete must always be taken into account, with particular attention to cardiovascular conditions and blood pressure. This last reason is why consuming products with sodium levels higher than your body's needs can also be harmful.

Key physiological effects of sodium during exercise:

• Increase fluid balance and retention, reducing urinary fluid losses

• Helps to stimulate thirst - encouraging athletes to drink enough

• Reduces the risk of hyponatraemia in longer indoor sessions

• Prevents disturbances in nerve transmission and muscle contraction (cramps)

• Helps maintain cardiac output

In high sweat rate conditions as indoors, sodium becomes more important, not less.

Comparing sodium concentration across five different brands

Below is a comparison of each product’s sodium content per 500ml of water, based on the recommended serving.

PureSport — Ultra Electrolytes

• 1,000 mg of sodium

• One of the highest sodium concentrations on the market

• Marketed for use 30 minutes before training or during prolonged and high-intensity sessions

Styrkr — SLT07 Hydration Tablets

• 1,000 mg or 500 mg of sodium

• Recommended for before, during and after training, depending on the goal

• Suitable for high-sweat-rate athletes and demanding indoor sessions

• Quick release and fastest absorbing formulation

Torq — TORQ Hydration

• 275 mg sodium

• Balanced, mid-range formulation

• Specialist indoor training product

• “Electrolyte Plus+”, launching soon, will likely increase sodium content

Precision Hydration — PH500, PH1000, PH1500

• 3 products: PH 500: 250 mg sodium, PH 1000: 500 mg sodium and PH 1500: 750 mg sodium

• Wide range allows matching individual sweat profiles

• The PH 1500 is often used before the session and even the night before. Also recommended during intense exercise

SiS — Hydro+ & Hydro Tablets

• Hydro+: 350 mg sodium

• Hydro tablets: 345 mg sodium

• Middle-range sodium concentration

• Appropriate for moderate sweat rates indoors
  

Summary table

Interpreting the numbers: What athletes should understand

Scientific hydration guidelines suggest:

• The average of sodium losses in athletes is between 500-1,500 mg of sodium per litre of sweat. Although these losses can exceed 2,000 mg/L or be less than 500 mg/L.

• Indoor training increases sweat volume, often significantly.

• A typical indoor session may therefore require more sodium than an outdoor session of the same duration.

What this means in practice:

• 1000mg products (PureSport, Styrkr) are well-suited to people with high sodium levels in their sweat (salty sweaters).

• 500 - 750mg products (Styrkr, Precision Fuel & Hydration) are mid-range and are widely used for pre-session hydration.

• 250-350 mg products (Torq, SiS) are suitable for lower indices in athletes with low sodium concentrations.

In other words, each product will be more or less useful depending on the athlete using it. It is true that, given that the rate of perspiration is higher indoors, athletes tend to be at the upper end of their sodium requirements compared to outdoor sessions of the same intensity. This technique is safe as long as there are no medical contraindications (blood pressure or heart problems).

Where sweat sensors fit in

Sweat rate and sodium concentration in sweat vary greatly between and within individuals (Baker, 2017), depending on the training session and the environment in which it takes place. 

Tools such as the FLOWBIO sensor help athletes understand:

• How much they actually sweat indoors.

• How sodium losses vary between sessions.

• Whether their current beverage meets their physiological needs.

This allows for personalisation beyond generic product recommendations.

Conclusion

Indoor training significantly increases thermoregulatory stress, raising the rate of sweating and fluid and electrolyte losses, even when the workload remains unchanged.

Sodium plays a fundamental role in our body, particularly in maintaining plasma volume, nerve and muscle activity, and fluid retention. As it is the electrolyte found in the highest quantity in sweat, it is also the one we need to replenish the most.

Therefore, the use of sodium-containing products is very useful. Different products vary greatly in sodium concentration, from ~250 mg to 1000 mg per 500 ml, and each product is suited to a different type of athlete and training scenario.

Knowing your sodium concentration and understanding how your sweat rate changes indoors is the key to selecting the right formula.

For athletes who want to maintain high performance during winter training blocks, adjusting electrolyte intake to indoor sweat loss is one of the most effective, yet most overlooked, performance strategies available.

References 

Anastasiou, C. A., Kavouras, S. A., Arnaoutis, G., Gioxari, A., Kollia, M., Botoula, E., & Sidossis, L. S. (2009). Sodium replacement and plasma sodium drop during exercise in the heat when fluid intake matches fluid loss. Journal of Athletic Training, 44(2), 117–123. 

Baker L. B. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports medicine (Auckland, N.Z.), 47(Suppl 1), 111–128. 

Buono, M. J., Claros, R., Deboer, T., & Wong, J. (2008). Na+ secretion rate increases proportionally more than the Na+ reabsorption rate with increases in sweat rate. Journal of Applied Physiology (Bethesda, Md.: 1985), 105(4), 1044–1048. 

Périard, J. D., Eijsvogels, T. M. H., & Daanen, H. A. M. (2021). Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews, 101(4), 1873–1979. 

Shirreffs, S. M., & Sawka, M. N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of Sports Sciences, 29 Suppl 1, S39–S46. 

Valentine V. (2007). The importance of salt in the athlete's diet. Current Sports Medicine Reports, 6(4), 237–240.

Veniamakis, E., Kaplanis, G., Voulgaris, P., & Nikolaidis, P. T. (2022). Effects of Sodium Intake on Health and Performance in Endurance and Ultra-Endurance Sports. International Journal of Environmental Research and Public Health, 19(6), 3651.

 PRODUCTS

Science in Sport Hydro+: https://www.scienceinsport.com/shop-by-need/hydration/go-hydro-hydration-tablets/hydro-plus-electrolyte-drink-mix

Science in Sport Hydro Tablets: https://www.scienceinsport.com/shop-by-need/hydration/go-hydro-hydration-tablets/sis-hydro-tablets-pack

Precision Fuel & Hydration PH 1500: https://www.precisionhydration.com/products/ph-1500-low-calorie-electrolyte-supplement/

Precision Fuel & Hydration PH 1000: https://www.precisionhydration.com/products/ph-1000-low-calorie-electrolyte-supplement/

Precision Fuel & Hydration PH 500: https://www.precisionhydration.com/products/ph-500-low-calorie-electrolyte-supplement/

TORQ Hydration: https://www.torqfitness.co.uk/product/tangerine-flavour-torq-hydration

Puresport Ultra Electrolytes: https://puresport.co/products/ultra-electrolytes-30-pack-watermelon-salt?variant=55663589589379

Styrkr SLT07 1000mg: https://styrkr.com/products/slt07-hydration-tablets-mild-citrus

Styrkr SLT07 500mg: https://styrkr.com/products/slt07-hydration-tablets-mild-berry