The Science Behind Hydration & Electrolyte Supplements

Therapeutic dose: 300–700 mg sodium/hour during prolonged (>90 min) or hot-weather exercise, paired with drinking to thirst. Not justified for general/office "hydration" use.

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• Mechanistically, sodium is the principal extracellular cation driving plasma osmolality and fluid distribution; sweat sodium losses are substantial during heavy exertion (typically 20–80 mmol/L), so replacement is justified for athletes losing several liters of sweat over hours. This is a mechanism/physiology rationale, not a supplementation efficacy result.
• A consensus systematic review of exercise-associated hyponatremia (Rosner & Kirven, 2007, Clin J Sport Med) found most cases in endurance athletes trace to overhydration with hypotonic fluids, not sodium loss alone; sodium intake during prolonged exercise can help prevent symptomatic hyponatremia, but only when paired with appropriate (not excessive) fluid intake. Result graded mixed.
• In marathon and ultramarathon athletes (Hew-Butler et al., 2008 IMMDA hydration consensus), sodium supplementation during events did NOT consistently outperform drinking-to-thirst with normal salt intake before and after — the explicit null/mixed result in our source, showing the supplementation case is narrow even in its strongest population.
• The category caveat cuts against routine use: mean US sodium intake (~3,400 mg/day) already exceeds the 2,300 mg/day recommended limit, so 'hydration' electrolyte powders marketed to office workers and casual exercisers address a deficit most consumers do not have.

Therapeutic dose: 200–600 mg/day within an electrolyte mix (heavy sweat/GI-loss contexts); OTC pills capped at 99 mg per dose

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• Mechanism is well-understood and not in dispute: potassium is the dominant intracellular cation, and the sodium-potassium balance governs cellular hydration, osmotic regulation, and nerve conduction. This is why potassium is a standard component of oral rehydration solutions for fluid loss from vomiting and diarrhea. Note the inclusion rests on physiological rationale and clinical consensus, not head-to-head trials isolating potassium's rehydration effect.
• Potassium losses through heavy sweat, vomiting, or diarrhea are real and can be meaningful over multi-hour endurance sessions in heat, so restoring intake is part of standard rehydration. However, there are no controlled trials showing that supplemental potassium improves rehydration or fluid-balance outcomes in healthy athletes — the evidence for this specific outcome is absent, not positive.
• The strong potassium evidence base — the DASH-Sodium feeding trial, the WHO-commissioned Aburto 2013 meta-analysis, and the 20,995-person Neal 2021 salt-substitute RCT — measures blood pressure and stroke, not rehydration or electrolyte-replacement endpoints. Those cardiovascular results should not be borrowed to claim a hydration benefit.
• Dose matters for safety here, not just efficacy: modest amounts (roughly 200–600 mg) inside an electrolyte mix are reasonable for high-sweat-loss athletes, but large supplemental doses are the wrong tool for everyday hydration. FDA caps OTC potassium at 99 mg per pill because acute high doses have caused fatal arrhythmia, especially with kidney impairment or potassium-sparing medications.

Therapeutic dose: 200–400 mg/day elemental (glycinate for GI tolerance); replaces documented sweat losses, not a rehydration dose

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• The strongest category-adjacent data is null: the 2020 Cochrane systematic review (Garrison et al., 11 RCTs, n=735) found magnesium no better than placebo for idiopathic muscle cramps in older adults across frequency, intensity, and duration. The review explicitly notes that exercise-induced cramps in heavy sweat-loss athletes were not well represented, so it neither supports nor tests the hydration/heat use case.PubMed ↗
• Magnesium is a genuine electrolyte lost in sweat, and endurance athletes in hot or humid conditions are a documented at-risk group for depletion — a physiological rationale for replacing losses, not evidence that supplementation improves rehydration or fluid balance.
• No trial in the evidence base tests magnesium for rehydration, plasma volume, fluid balance, or electrolyte-replacement outcomes. Its RCT support sits in unrelated areas (blood pressure, migraine prevention, glycemic control), so for this category the honest read is deficiency-correction and mechanism only, with no category-specific human efficacy data.

Therapeutic dose: No rehydration-specific dose exists; general repletion RDA is 1,000–1,200 mg/day total intake (supplement gap usually only 200–500 mg/day)

• No human trials test calcium for fluid balance, rehydration, or exercise/heat electrolyte replacement. Standard oral rehydration solutions (WHO ORS) are built on sodium, potassium, glucose, and citrate — calcium is not a component. Calcium is an electrolyte anatomically, but it has no established role in acute rehydration formulas.
• The landmark calcium trials measured skeletal, not hydration, endpoints: the Tang 2007 Lancet meta-analysis (29 trials, 63,897 adults 50+) reported a 12% fracture reduction, and the WHI RCT (36,282 postmenopausal women, 1,000 mg calcium carbonate + 400 IU vitamin D) tracked hip fracture and bone density. Neither assessed fluid balance or electrolyte replacement — these are off-target for the hydration category.PubMed ↗
• Calcium's electrolyte role is visible only in severe acute hypocalcemia, where deficiency can cause muscle cramps, tetany, and cardiac rhythm abnormalities. This is deficiency correction, not a hydration benefit — replete individuals gain nothing for fluid status from supplementation.
• A hydration-relevant caution rather than a benefit: supplemental calcium raises urinary calcium and kidney-stone risk (the WHI saw a 17% increase), an effect made worse by inadequate fluid intake. Dietary calcium shows the opposite, protective association.

Therapeutic dose: 1–3 g/day (studied pre-exercise for endurance); no hydration- or electrolyte-specific dose established

• A 2018 meta-analysis (Waldron et al., Sports Medicine; 10 studies, ~86 trained/recreationally active adults) found acute taurine 1–3 g taken 1–3 hours pre-exercise modestly improved time-to-exhaustion — but this is an endurance-capacity signal, not a hydration or electrolyte-balance outcome, and several included trials stacked taurine with caffeine, making isolated attribution difficult.
• Mechanism only: taurine is one of the most abundant intracellular free amino acids / organic osmolytes in skeletal muscle and cardiac tissue and participates in cell-volume regulation and osmoregulation in contracting fibers — a plausible link to fluid handling that has never been tested as a hydration intervention in humans.
• No human RCT has evaluated taurine for hydration status, rehydration, or sweat electrolyte loss from exercise, heat, or illness; relatedly, a 2021 systematic review (Kurtz et al.) found no benefit for strength or anaerobic power, showing the endurance signal does not generalize. Most 'taurine works' impressions come from energy drinks, where the acute effect is dominated by caffeine and sugar, not taurine.

Therapeutic dose: No validated hydration dose; trials in adjacent contexts (gut barrier, exercise) used 5–15 g/day oral L-glutamine. Peptide forms (L-alanyl-L-glutamine) in electrolyte formulas typically appear at ~1–2 g.

• No RCT has tested L-glutamine for fluid balance, rehydration, or electrolyte replacement — the endpoints that define this category. Its human evidence base sits in gut barrier, sickle cell disease, and exercise recovery/immunity, none of which measured hydration status, sodium/potassium retention, or plasma volume.
• The only heat/exercise-adjacent data: a small crossover RCT (Pugh 2017, n=10 recreationally active males running 60 min at 70% VO2max in 30°C heat) found acute glutamine (0.25–0.9 g/kg fat-free mass) dose-dependently reduced a gut-permeability marker (lactulose:rhamnose). But GI symptoms were low and unchanged, and it measured barrier integrity, not hydration or electrolyte balance.PubMed ↗
• Glutamine peptides (L-alanyl-L-glutamine, e.g. Sustamine) appear in hydration and intra-workout formulas for their solubility and heat-stability, but for oral consumer use the absorption advantage rarely translates to a meaningful outcome difference — this is a formulation/marketing choice, not a demonstrated rehydration benefit.
• Mechanistically, glutamine is conditionally essential (Lacey & Wilmore, 1990): a healthy, well-fed person synthesizes ample glutamine and has no deficit to correct, and nothing in its physiology addresses the sodium/potassium/water replacement that hydration products exist to deliver.PubMed ↗

Therapeutic dose: No rehydration-specific dose exists. RDA 8–11 mg/day (bisglycinate or picolinate for repletion); do not exceed 40 mg/day elemental zinc long-term.

• No hydration, electrolyte-replacement, or rehydration trials exist for zinc in this record. Every study on file targets the common cold, infections, acne, wound healing, COVID, or cancer risk — none measures fluid balance, sweat/heat losses, or rehydration endpoints. For this category the honest verdict is: no category-specific human evidence.
• Zinc is a trace mineral (serum measured in µg/dL, deficiency threshold <70 µg/dL), not a bulk electrolyte like sodium or potassium, so it plays no measurable role in acute fluid balance or oral rehydration. The record does note zinc is lost through diarrhea and via GI disease and heavy alcohol use, but lists diarrhea only as a deficiency symptom — not as a rehydration indication supported by any trial.
• Even in acute illness where fluid status matters, systemic zinc failed a properly powered test: the Ben Abdallah 2023 RCT (JAMA Network Open, 470 hospitalized COVID-19 patients) found oral zinc did not reduce 30-day mortality or ICU admission versus placebo. It is cited here to show zinc does not move acute-illness outcomes, not as a rehydration result.
• There is no rehydration rationale to take high-dose zinc, and doing so carries real harm: chronic intake above 40 mg/day induces copper deficiency (anemia, potentially irreversible myeloneuropathy), and the Leitzmann 2003 cohort (46,974 men, 14-year follow-up) linked intake above 100 mg/day to a 2.29-fold higher risk of advanced prostate cancer — 2.37-fold in men supplementing at high doses for 10+ years.PubMed ↗

Therapeutic dose: No hydration-specific dose exists; general adequacy is 75–90 mg/day (RDA), with plasma saturating around 200 mg/day (Levine inpatient pharmacokinetic data)

• No RCTs test vitamin C for fluid balance, electrolyte replacement, or rehydration. The entire ~11-trial evidence base covers colds, scurvy, cancer, cardiovascular disease, and sepsis — none measured a hydration or electrolyte endpoint, so there is no category-specific human evidence to grade.
• Mechanistically vitamin C is a water-soluble antioxidant and a cofactor for collagen-crosslinking enzymes (prolyl/lysyl hydroxylase). It is not an electrolyte and contributes nothing to osmotic sodium/potassium/water retention, which is what rehydration formulas rely on. Its presence in electrolyte powders is an antioxidant add-on, not a fluid-balance active.
• Levine's inpatient pharmacokinetic work showed plasma vitamin C saturates near 200 mg/day, with excess excreted in urine. In a sweat-loss or heat-stress context this makes supplemental vitamin C a passenger rather than an active rehydration agent — above threshold it is simply cleared.
• The one exercise-adjacent signal is not a hydration finding: in heavy-physical-stress groups (marathoners, skiers, soldiers), 600 mg–1 g/day cut cold incidence roughly 50% (Hemilä subgroup, risk ratio 0.48). That is an infection-under-exertion outcome, not electrolyte balance or rehydration — do not conflate 'athlete-relevant' with 'hydration benefit.'