Hydration science decoded: why water alone isn't enough (and how electrolytes transform your training)

What's actually happening in your cells when you're dehydrated, and why sodium, potassium and magnesium matter more than the number on your drink bottle.

You've done everything right. Litre bottle by your desk, refilled on repeat, eight-plus glasses ticked off before lunch. And yet by the time you're halfway through a hard session, your legs feel heavy, your head's foggy, and that familiar cramp in your calf arrives right on cue. If more water hasn't fixed how you feel, the problem probably isn't how much you're drinking. It's what's dissolved in it.

This is the gap that hydration science keeps circling back to: water moves where sodium tells it to go. Without the right electrolyte minerals in the mix, a good portion of the water you drink passes straight through you rather than reaching the cells that actually need it. Here's what's really happening inside your body during dehydration, why sodium, potassium and magnesium matter for exercise, and how a formula like Hydrate⁴ is built around that science.

Cellular hydration vs. water volume: two very different things

Most of us think about hydration as a volume problem: drink X litres, tick the box, done. But your body doesn't experience hydration as a total. It experiences it cell by cell.

Roughly two-thirds of your body's water sits inside your cells (intracellular fluid), with the rest in your blood plasma and the fluid between cells (extracellular fluid). Water moves freely between these compartments, but not at random. It follows osmotic balance: sodium and other electrolytes create a concentration gradient, and water is drawn across cell membranes toward the side with more dissolved particles in it.

This is why "just drink more water" can fall short. Drink plain water without replacing the sodium you've lost through sweat, and you dilute the sodium concentration in your blood. Your kidneys respond by increasing urine output to correct that dilution, so a chunk of what you just drank is filtered straight back out before it can properly rehydrate your cells. Keep electrolytes in the mix and the osmotic gradient holds, so water is retained for longer and distributed more effectively into intracellular fluid, where it's actually needed for muscle contraction, nutrient transport and temperature regulation.

A study comparing plain water with an electrolyte drink after exhaustive exercise found exactly this pattern: total body water, intracellular water and extracellular water were all better restored with the electrolyte drink than with water alone, alongside faster clearance of exercise-related fatigue markers (Choi et al., 2021). It's a small study of ten men, so it shouldn't be read as the final word, but it illustrates the mechanism clearly: rehydration is about composition, not just volume.

The electrolyte minerals doing the actual work

“Electrolytes” gets used as a marketing buzzword, but the term refers to specific minerals that carry an electrical charge and each play a distinct role in the body.

Sodium is the primary regulator of fluid balance. It drives the osmotic gradient described above, and it's essential for nerve signalling and muscle contraction, including the muscle that makes up your heart. Sodium is also, by a considerable margin, the electrolyte you lose in the greatest quantity through sweat.

Potassium works largely inside your cells (it's the dominant intracellular electrolyte, where sodium dominates outside them), supporting the nerve impulses and muscle contractions, including your heartbeat, that keep everything firing correctly.

Magnesium supports muscle and nerve function too, alongside a role in energy production, and it's easy to overlook because you lose far less of it in sweat than sodium. But the loss is real and well documented in humans: a 2023 systematic review and meta-analysis pooling data from 855 athletes and 521 non-athletes found that athletes consistently have lower blood magnesium levels and excrete significantly more magnesium in urine than untrained people, despite eating more of it, evidence that training itself raises the body's magnesium requirement (Zhang et al., 2023).

What's less clear is whether topping magnesium back up directly boosts performance in athletes who aren't deficient: a 2017 meta-analysis of 14 human trials found no significant strength or power gains from magnesium supplementation in physically active adults, with benefits appearing mainly in older or magnesium-deficient groups (Wang et al., 2017).

Where a magnesium-inclusive electrolyte formula has shown a more direct benefit is muscle cramping: in a single study of 184 half-marathon runners, those given a magnesium-rich electrolyte drink reported roughly half the rate of muscle cramps (21%) compared with those drinking water alone (46%) (Kharait, 2022), though it's one study and would benefit from independent replication.

Calcium, while more commonly associated with bone health, also supports nerve transmission and muscle contraction, which is why it's often included alongside the “big three” electrolytes.

What's in your sweat (and why it's different from what's in mine)

Sweat isn't just water. It's a dilute solution of the same electrolytes carried in your blood, principally sodium, with smaller amounts of potassium, magnesium and calcium. But the concentration of sodium in sweat varies enormously between individuals, and that variation matters more than most generic hydration advice accounts for.

A comprehensive review of sweat testing in athletes found considerable variability in sweat sodium concentration, both between people and within the same person across different sessions, influenced by factors including aerobic fitness, heat acclimatisation, body size, sex, age and how hard you're actually sweating. Sodium reabsorption along the sweat duct is flow-dependent, so the harder and faster you sweat, the higher the sodium concentration tends to be in that sweat (Baker, 2017).

In practice, this means two people doing an identical session in identical conditions can lose very different amounts of sodium. It's part of why “salty sweaters” (the people who finish training with visible white marks on dark kit) often need more electrolyte replacement than blanket hydration guidelines suggest.

Electrolyte imbalance symptoms: what your body is telling you

Because electrolytes are involved in nerve signalling, muscle contraction and fluid balance all at once, an imbalance tends to show up across more than one system simultaneously. Commonly reported electrolyte imbalance symptoms include fatigue, headaches, muscle cramps, spasms or weakness, an irregular or unusually fast heartbeat, nausea, confusion or irritability, and numbness or tingling in the hands and feet (Cleveland Clinic).

Muscle cramps deserve a specific mention, because they're the symptom most people associate with electrolyte loss, and the research here is more nuanced than the popular narrative suggests. A review of exercise-associated muscle cramps found that while older evidence links salt depletion to cramping, modern athlete studies haven't consistently found a correlation between blood electrolyte levels and who cramps and who doesn't. An alternative theory points to altered neuromuscular control from fatigue rather than electrolytes alone (Maughan & Shirreffs, 2019). The honest summary: cramping is likely caused by more than one mechanism, and staying well hydrated with adequate electrolytes is still sound practice, just not a guaranteed fix for every cramp.

If you notice several of these symptoms together, especially alongside heavy sweating, illness or reduced fluid intake, it's worth checking in with your GP rather than self-diagnosing.

Matching your hydration strategy to how hard you're training

Not every day calls for the same approach.

Everyday activity and light movement: For general daily hydration, especially through New Zealand's warmer months, water covers most people's needs. A daily electrolyte top-up can still help if you're not getting much sodium from food, you're following a lower-sodium diet, or you simply find it hard to get through plain water.

Moderate training (roughly 45–60 minutes): Once a session runs past about 45 minutes to an hour, or involves noticeable sweating, fluid losses start to matter more. This is the range where the American College of Sports Medicine's long-standing position stand on exercise and fluid replacement recommends including sodium in fluid replacement, both to help maintain the drive to drink and to support fluid retention. This guidance dates from 2007, but it remains ACSM's current, most-cited position on the topic.

Intense, long or hot-weather sessions: The longer, hotter or harder the session, the greater the sweat and electrolyte losses, and the more a plain-water strategy risks under-replacing sodium specifically. This is also where the cognitive cost of under-hydrating becomes measurable. In one study, men who lost just 1.59% of body mass through mild dehydration, with no rise in body temperature, showed slower working memory, more errors on vigilance tasks, and increased fatigue, tension and anxiety compared with when they were properly hydrated (Ganio et al., 2011). That's a genuinely small fluid deficit with a measurable effect on how you think and feel, often before you'd notice thirst.

Recovery timing: the post-workout window matters too

Hydration doesn't end when the session does. What (and when) you drink afterwards determines how much of that fluid you actually retain.

A study comparing rehydration drinks after exercise-induced dehydration of around 2.6% body mass found that beverages containing sodium (an oral rehydration solution and a standard sports drink) produced significantly better fluid retention over the following 3.5 hours than plain water, with roughly three-quarters of fluid retained on the sodium-containing drinks compared with just over half on water alone (Ly et al., 2023).

The practical takeaway for recovery: reaching for an electrolyte drink in the hour or so after training, rather than relying on water alone, gives your body the sodium it needs to actually hold on to the fluid you're replacing, instead of losing much of it straight back out as urine.

How Hydrate⁴ is formulated around this science

Hydrate⁴ exists because of the gap covered above, between drinking water and actually rehydrating. Each sachet delivers sodium (from natural sea salt), potassium, calcium and magnesium in one dissolvable dose, designed to be mixed into 600–750ml of water depending on your flavour preference.

• Sodium (natural sea salt): 1,000mg
• Potassium (citrate): 300mg
• Calcium (citrate): 175mg
• Magnesium (citrate): 100mg

That combination is designed to support the osmotic balance that draws water into your cells, along with the nerve and muscle function that sodium, potassium and magnesium each contribute to. It's formulated to support cellular fluid balance and help reduce tiredness and fatigue, rather than simply adding flavour to your water bottle.

“So much hydration advice still stops at 'just drink more water'. That never sat right with me, because water on its own doesn't tell your cells what to do with it. Hydrate⁴ was built around the actual science of cellular hydration, not just another number to hit on a bottle.” - Julia Matthews, Founder, Two Islands

When Creatine Hydration is the better fit

If you're training regularly and want hydration support alongside strength and recovery benefits, Creatine Hydration is worth a look. Alongside a 5g dose of creatine monohydrate, it includes a full electrolyte blend of sodium, potassium and magnesium, plus glycine, taurine and L-theanine, so you get daily hydration support and creatine's well-established muscle and cognitive benefits in a single scoop.

It's designed to be taken daily, rather than dosed around specific sessions, which makes it a good fit for people who want consistent electrolyte and creatine support built into their everyday routine. Hydrate4 remains the go-to for topping up electrolytes specifically around harder or longer training sessions, or on hot days when your sweat losses spike.

The bottom line

Water is essential, but it's only half the hydration equation. The other half is the electrolyte minerals, sodium, potassium and magnesium chief among them, that determine whether the water you drink actually reaches your cells or passes straight through. Matching your electrolyte intake to how much you sweat, how long you're training and the conditions you're training in is often the difference between feeling flat by the second half of a session and finishing strong.

Check out Hydrate⁴ and shop online.

Read more about Creatine Hydration and shop online.

Frequently Asked Questions

How do I know if I need electrolytes or just more water?

If you're only doing light daily activity, plain water is usually enough. If you're training for more than 45–60 minutes, sweating heavily, training in the heat, or noticing symptoms like fatigue, headaches or muscle cramps, that's a sign your electrolyte losses, not just your fluid losses, need replacing.

Can you get enough electrolytes from food alone?

For many people eating a varied diet, yes, particularly for sodium. But if you're training hard, sweating heavily, or following a lower-sodium diet, a dedicated electrolyte formula makes it easier to replace what you're losing in one measured dose, rather than guessing.

Do electrolytes stop muscle cramps?

Not definitively. The research on what actually causes exercise-related cramps is mixed, and electrolyte depletion is only one of several proposed mechanisms. Staying well hydrated with adequate electrolytes is still good practice, but it isn't a guaranteed cramp-prevention strategy for everyone.

When's the best time to take Hydrate⁴?

Any time you need hydration support: before, during or after training, in hot weather, or as part of your everyday routine. Mix one sachet with 600–750ml of water, depending on your flavour preference.

References

Baker, L. B. (2017). Sweating rate and sweat sodium concentration in athletes: A review of methodology and intra/interindividual variability. Sports Medicine, 47(Suppl 1), 111–128. https://link.springer.com/article/10.1007/s40279-017-0691-5

Choi, D-H., Cho, J-Y., Koo, J-H., & Kim, T-K. (2021). Effects of electrolyte supplements on body water homeostasis and exercise performance during exhaustive exercise. Applied Sciences, 11(19), 9093. https://www.mdpi.com/2076-3417/11/19/9093

Cleveland Clinic. (2024). Electrolyte imbalance: Symptoms, causes and treatment. https://my.clevelandclinic.org/health/symptoms/24019-electrolyte-imbalance

Ganio, M. S., Armstrong, L. E., Casa, D. J., McDermott, B. P., Lee, E. C., Yamamoto, L. M., Marzano, S., Lopez, R. M., Jimenez, L., Le Bellego, L., Chevillotte, E., & Lieberman, H. R. (2011). Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition, 106(10), 1535–1543. https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/mild-dehydration-impairs-cognitive-performance-and-mood-of-men/3388AB36B8DF73E844C9AD19271A75BF

Kharait, S. (2022). A magnesium-rich electrolyte hydration mix reduces exercise associated muscle cramps in half-marathon runners: Direct original research. Journal of Exercise and Nutrition, 5(3). https://www.journalofexerciseandnutrition.com/index.php/JEN/article/view/126

Ly, N. Q., Hamstra-Wright, K. L., & Horswill, C. A. (2023). Post-exercise rehydration in athletes: Effects of sodium and carbohydrate in commercial hydration beverages. Nutrients, 15(22), 4759. https://www.mdpi.com/2072-6643/15/22/4759

Maughan, R. J., & Shirreffs, S. M. (2019). Muscle cramping during exercise: Causes, solutions, and questions remaining. Sports Medicine, 49(2), 115–124. https://link.springer.com/article/10.1007/s40279-019-01162-1

Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377–390. https://pubmed.ncbi.nlm.nih.gov/17277604/

Wang, R., et al. (2017). The effect of magnesium supplementation on muscle fitness: A meta-analysis and systematic review. Magnesium Research, 30(4), 120–132. https://pubmed.ncbi.nlm.nih.gov/29637897/

Zhang, H., Wang, R., Guo, S., Tian, Q., Zhang, S., Guo, L., & Liu, T. (2023). Lower serum magnesium concentration and higher 24-h urinary magnesium excretion despite higher dietary magnesium intake in athletes: A systematic review and meta-analysis. Food Science and Human Wellness, 12(5), 1471–1480. https://www.sciencedirect.com/science/article/pii/S2213453023000150

This article is for educational purposes only and does not constitute medical advice. Always consult your GP or a registered health professional before starting any new supplement, particularly if you are pregnant, breastfeeding, or managing an existing health condition.