Water is the essence of life and the medium in which all essential biochemical processes for such life take place. It’s perhaps unsurprising then that humans are mostly composed of water — around 75% of total body mass as infants and 50–60% body mass by the time we’re adults.
Hydration is often reduced to a simple directive — drink when you’re thirsty — and for most situations, this is a good rule for optimal health. When we dig a little deeper into human physiology, though, we see that hydration isn’t just about water intake, but about fluid balance across bodily compartments, and this process is driven by electrolytes [1,2].
The goals of this article are to explore electrolytes and how they work so that water can be absorbed, distributed, and retained where it matters. We will also unpack the physiology of hydration and why water alone can sometimes fall short of meeting our hydration needs.
What Do We Mean by Hydration?
At the basic level, hydration refers to the maintenance of total body water and its distribution between compartments. Intracellular fluid (ICF) is the fluid located inside cells, rich in magnesium, potassium [3], and phosphate, and represents around 2/3 of total body water. Extracellular fluid (ECF) is around 1/3, comprising blood plasma and interstitial fluid, and is high in sodium, chloride, and bicarbonate.
Proper hydration isn’t just about the volume of fluid; it’s also about osmotic balance — the direction in which fluid moves. Water follows gradients, and electrolytes create these.
What Are Electrolytes and Why Are They Important?
Electrolytes are charged minerals, many of them salts, that are dissolved in body fluids. Some of the electrolytes present in body water are sodium, potassium, chloride, magnesium, and calcium.
These ions are central to maintaining fluid balance, supporting nerve transmission, acid-base regulation, and allowing muscle contraction to take place. Perhaps more importantly for hydration, electrolytes control osmotic gradients, which dictate water movement between compartments.
A vitally important concept in hydration physiology is that water follows the solute (and especially sodium). Sodium is the primary extracellular cation (positively charged ion) and is the key modulator of plasma volume, blood pressure, and osmolarity (the concentration of dissolved particles in fluid) [4,5].
When the sodium concentration rises, water is pulled into the extracellular space. When it falls, water shifts inward or is excreted from the body. This is why drinking water alone can sometimes reduce effective hydration status — it dilutes sodium levels, which is why the body will tend to excrete large volumes of fluid consumed without appropriate sodium (whether added or present in meals).
Why Water Alone Isn’t Always Enough
When you drink large volumes of plain water — especially after sweating — you dilute blood sodium concentrations. This can impair nerve signaling, muscle contraction, and cognitive function. In extreme cases, it leads to hyponatremia, a dangerous condition caused by low sodium levels. Even before this point, subtle electrolyte dilution can reduce performance and wellness [6,7].
Water doesn’t just need to enter the body — it also needs to remain there. Sodium helps retain fluid by supporting kidney reabsorption of water, reducing urine output, and maintaining plasma volume. Without adequate sodium, the body is more likely to excrete the water you drink. Practically, you can drink a high volume of water and remain functionally dehydrated purely because of the distribution of fluid in your body.
Hydration is ultimately about getting water into cells. This is the end goal of fluid consumption. The process depends on sodium gradients, potassium balance, and membrane transport systems. Electrolytes maintain the gradients required for osmosis to take place, which allows water to move into cells where it is needed. Without these gradients, water distribution becomes inefficient, even if total intake is high.
Lastly, sweat is not just water — it contains electrolytes and especially sodium. During prolonged exercise, heat exposure, or illness, the water and electrolyte losses create a compounded deficit. Replacing the lost fluid only addresses half of the problem [8].
The Sodium-Potassium Pump: The Enzyme That Powers Hydration
At the cellular level, hydration is governed by one of the most important enzymes in biology: the sodium-potassium pump Na⁺/K⁺-ATPase [9–11]. The concentration of sodium in our blood, interstitial fluids, and extracellular spaces is elevated (140 mM) compared to the concentration of sodium inside our cells (10-30 mM), thereby establishing an inward concentration gradient across the membrane of the cell.
This gradient is in contrast with the outward potassium gradient created by low extracellular potassium (3.5-5 mM) and high intracellular potassium concentrations (130-140 mM). These opposing Na+ and K+ gradients, maintained by the energy-consuming sodium-potassium pump, generate a membrane potential across the cell membrane and create electrical signals that sustain muscle contraction and nerve communication.
The sodium-potassium pump also consumes a significant portion of resting energy expenditure. Without this system, hydration at the cellular level would collapse.
Electrolytes and Water Absorption from the Gut
Water is absorbed in the gut mainly through passive osmosis in the small intestine (80–90%) and large intestine (10–20%). Importantly, the fluid follows the movement of solutes like sodium. As nutrients and electrolytes are taken up into the bloodstream, they create an osmotic gradient that helps draw in water across the intestinal lining.
In the small intestine, water is efficiently absorbed directly into the blood via the intestinal walls and is largely driven by sodium absorption and the sodium-dependent co-transport of nutrients (like glucose).
This is why oral rehydration solutions (ORS) are so effective, even when used for conditions such as severe diarrhea [12–14]. They leverage these mechanisms to a degree that plain water simply can’t. By making use of sodium-glucose co-transport to facilitate fluid absorption, these beverages ensure that water can be drawn into the bloodstream where it is needed.
The Hormonal Control of Hydration
The body tightly regulates hydration through the hormones aldosterone, antidiuretic hormone, and thirst mechanisms. Aldosterone increases sodium reuptake in the kidneys and promotes water retention, whereas antidiuretic hormone increases water reabsorption and reduces urine output. These systems are designed to maintain balance, but they rely on adequate electrolyte availability to function properly.
When Is Water Enough by Itself and When Is It Not?
Under normal circumstances, water alone is often sufficient to maintain hydration status. Extra electrolytes are not typically required when activity is low, sweat losses are minimal, the diet provides adequate minerals, and the environment is temperate. Here, the body is very good at maintaining homeostasis.
Electrolytes become critical when losses increase or the demands rise. These settings include prolonged exercise where sustained sweat loss occurs, and concentrations of sodium and potassium are depleted [15–17]. Heat exposure is another, whereby sweat rates are increased, and there is greater electrolyte turnover [18,19]. As discussed above, illnesses (especially diarrhea, for example) that provoke rapid electrolyte depletion also warrant supplementation.
Conclusion
Water is the essential medium for life, but it’s not sufficient on its own for all situations. Electrolytes are the key to making water "work" in the body by helping the fundamental biochemical processes take place. They determine whether water is absorbed from the gut, where it goes inside the body, and how long it stays prior to excretion. Understanding these concepts shifts hydration from a simplistic concept or habit towards a physiological strategy that can influence performance, cognition, and long-term health.
References
[1] M.-E. Roumelioti, R.H. Glew, Z.J. Khitan, H. Rondon-Berrios, C.P. Argyropoulos, D. Malhotra, D.S. Raj, E.I. Agaba, M. Rohrscheib, G.H. Murata, J.I. Shapiro, A.H. Tzamaloukas, Fluid balance concepts in medicine: Principles and practice, WJN 7 (2018) 1–28. https://doi.org/10.5527/wjn.
[2] D. Bagordo, G.P. Rossi, C. Delles, H. Wiig, G. Rossitto, Tangram of Sodium and Fluid Balance, Hypertension 81 (2024) 490–500. https://doi.org/10.1161/
[3] S. Mazzaferro, N. De Martini, J. Cannata-Andía, M. Cozzolino, P. Messa, S. Rotondi, L. Tartaglione, M. Pasquali, on behalf of the ERA-EDTA CKD-MBD Working Group, Focus on the Possible Role of Dietary Sodium, Potassium, Phosphate, Magnesium, and Calcium on CKD Progression, JCM 10 (2021) 958. https://doi.org/10.3390/
[4] D. Armanini, L. Bordin, G. Dona’, A. Andrisani, G. Ambrosini, C. Sabbadin, Relationship between water and salt intake, osmolality, vasopressin, and aldosterone in the regulation of blood pressure, J of Clinical Hypertension 20 (2018) 1455–1457. https://doi.org/10.1111/jch.
[5] J.J.O.N. Bosch, N.R. Hessels, F.W. Visser, J.A. Krikken, S.J.L. Bakker, I.J. Riphagen, G.J. Navis, Plasma sodium, extracellular fluid volume, and blood pressure in healthy men, Physiological Reports 9 (2021). https://doi.org/10.14814/phy2.
[6] C. Fujisawa, H. Umegaki, T. Sugimoto, S. Samizo, C.H. Huang, H. Fujisawa, Y. Sugimura, M. Kuzuya, K. Toba, T. Sakurai, Mild hyponatremia is associated with low skeletal muscle mass, physical function impairment, and depressive mood in the elderly, BMC Geriatr 21 (2021) 15. https://doi.org/10.1186/
[7] L. Armstrong, B. McDermott, S. Young, D. Casa, Exercise-Associated Hyponatremia: Serum Sodium, Symptomatology, Severity, and Sport Specificity, OAJSM Volume 16 (2025) 159–177. https://doi.org/10.2147/OAJSM.
[8] L.E. Armstrong, Rehydration during Endurance Exercise: Challenges, Research, Options, Methods, Nutrients 13 (2021) 887. https://doi.org/10.3390/
[9] K.B. Gagnon, E. Delpire, Sodium Transporters in Human Health and Disease, Front. Physiol. 11 (2021) 588664. https://doi.org/10.3389/fphys.
[10] A.G. Therien, R. Blostein, Mechanisms of sodium pump regulation, American Journal of Physiology-Cell Physiology 279 (2000) C541–C566. https://doi.org/10.1152/
[11] P.T. Nguyen, C. Deisl, M. Fine, T.S. Tippetts, E. Uchikawa, X. Bai, B. Levine, Structural basis for gating mechanism of the human sodium-potassium pump, Nat Commun 13 (2022) 5293. https://doi.org/10.1038/
[12] Z. Aghsaeifard, G. Heidari, R. Alizadeh, Understanding the use of oral rehydration therapy: A narrative review from clinical practice to main recommendations, Health Science Reports 5 (2022) e827. https://doi.org/10.1002/hsr2.
[13] S.Y. Ofei, G.J. Fuchs, Principles and Practice of Oral Rehydration, Curr Gastroenterol Rep 21 (2019) 67. https://doi.org/10.1007/
[14] M.K. Munos, C.L.F. Walker, R.E. Black, The effect of oral rehydration solution and recommended home fluids on diarrhoea mortality, International Journal of Epidemiology 39 (2010) i75–i87. https://doi.org/10.1093/ije/
[15] C.A. Anastasiou, S.A. Kavouras, G. Arnaoutis, A. Gioxari, M. Kollia, E. Botoula, L.S. Sidossis, Sodium Replacement and Plasma Sodium Drop During Exercise in the Heat When Fluid Intake Matches Fluid Loss, Journal of Athletic Training 44 (2009) 117–123. https://doi.org/10.4085/1062-
[16] Exercise and Fluid Replacement, Medicine & Science in Sports & Exercise 39 (2007) 377–390. https://doi.org/10.1249/mss.
[17] M. Fernández-Álvarez, J. Cachero-Rodríguez, C. Leirós-Díaz, S. Carrasco-Santos, R. Martín-Payo, Evaluation of Water Intake in Spanish Adolescent Soccer Players during a Competition, Journal of Human Kinetics 83 (2022) 59–66. https://doi.org/10.2478/hukin-
[18] M.L. Millard-Stafford, M.B. Brown, M.T. Wittbrodt, Perspectives on enhancing human performance in the heat: Is the solution to simply “just add water”?, Sports Medicine and Health Science 7 (2025) 317–328. https://doi.org/10.1016/j.
[19] R.A. Oliveira, A.P.R. Sierra, M. Benetti, N. Ghorayeb, C.A. Sierra, M.A.P.D.M. Kiss, M.F. Cury-Boaventura, Impact of Hot Environment on Fluid and Electrolyte Imbalance, Renal Damage, Hemolysis, and Immune Activation Postmarathon, Oxidative Medicine and Cellular Longevity 2017 (2017) 9824192. https://doi.org/10.1155/2017/
