The short version
Sodium is the electrolyte that drives fluid retention and has strong evidence for rehydration; potassium and magnesium are lost in far smaller amounts and, for well-fed people, adding them hasn’t been shown to improve hydration. The popular “electrolytes cure cramps” story is also weakly supported.
The label test: read the actual sodium in milligrams, insist that every electrolyte is listed in mg (not just named), and check added sugar. A prominent potassium/magnesium/“trace mineral” pitch with low sodium is a formula built for a story, not for sweat.
What it actually is
Electrolytes are charged minerals that regulate fluid balance, nerve signaling and muscle function. In hydration products the main three are sodium, potassium and magnesium. Sodium is the dominant electrolyte in the fluid outside your cells and the primary driver of fluid retention — replacing it alongside water is what actually restores balance after heavy sweating, because sodium helps your body hold onto the fluid you drink instead of excreting it.
Potassium is the main mineral inside cells and magnesium is a cofactor in hundreds of reactions, but both are lost in sweat in much smaller amounts than sodium. For most people eating normally, food replaces them fine.
Products come as powders, tablets, concentrates and ready-to-drink bottles, ranging from high-sodium “endurance” mixes to lighter everyday ones. Sugar varies a lot: traditional sports drinks add carbohydrate (which modestly aids fluid and glucose uptake during exercise), while many newer “zero-sugar” powders leave it out.
What the research actually shows
We label each use by how strong the evidence is — well-established, mixed, or weak/preliminary — and say plainly where marketing outruns the science.
Well-established Sodium for rehydration & fluid retention. Sodium replacement is required to fully restore a fluid deficit, and sodium-containing drinks beat plain water for fluid retention — mattering most in prolonged or heavy sweating.
Well-established Preventing exercise-associated hyponatremia. Dangerously low blood sodium in endurance events is driven mainly by over-drinking plain water; adequate sodium and sensible fluid amounts are the guards. Real, but specific to long-duration exercise.
Weak / preliminary Potassium & magnesium for hydration. Adding potassium hasn’t been shown to improve fluid retention beyond sodium, and sweat magnesium loss is trace-level. In well-nourished people the hydration case for both is weak.
Weak / preliminary Electrolytes for muscle cramps. The evidence that electrolyte drinks prevent cramps is weak; cramps appear multifactorial. Don’t rely on a hydration mix as a cramp cure.
The doses studies actually used
Sweat sodium averages roughly 1,150 mg per liter of sweat but ranges enormously (about 200–1,800 mg/L) — “salty sweaters” lose far more. For exercise lasting over an hour, the American College of Sports Medicine suggests fluids contain about 500–700 mg sodium per liter, and endurance products commonly supply a few hundred to over 1,000 mg sodium per serving.
Potassium losses in sweat are low (roughly 80–310 mg/L), and magnesium is trace (single-digit mg/L), which is why the amounts of each in hydration products are small and not really driven by measured need. There is no one-number-fits-all: your need scales with sweat rate, heat and duration.
What to look for on a label
Where you’ll find it in what we’ve reviewed
Electrolytes are one of our most-reviewed categories, and the sodium spread is telling: LMNT leads with 1,000 mg, Earth Energy Daily Hydration lists 750 mg plus a broader mineral profile (common-ownership disclosed), while FlavCity and Just Ingredients sit near 100 mg — a “hydration story” more than a sweat-replacement dose.
See all seven ranked in the best electrolytes roundup, which includes a cost-per-100 mg-sodium row, or read the head-to-heads: LMNT vs Liquid I.V. and LMNT vs Earth Energy.
Frequently asked questions
Do I actually need an electrolyte drink, or is water enough?
For everyday activity and short workouts, water and a normal diet are usually enough. Electrolytes — mainly sodium — matter more during prolonged (over 1–2 hours) or heavy sweating, heat, or when replacing large fluid losses.
Which electrolyte matters most for hydration?
Sodium. It drives fluid retention and has the strong evidence. Potassium and magnesium are lost in much smaller amounts and are typically covered by food; adding them hasn’t been shown to improve hydration beyond sodium.
Will electrolytes stop my muscle cramps?
Maybe not. The evidence linking electrolyte drinks to cramp prevention is weak, and cramps appear multifactorial (fatigue, neuromuscular factors). Don’t count on a hydration product as a guaranteed fix.
Can I get too much?
Yes, in principle — very high sodium loads aren’t ideal for everyone (e.g. blood-pressure concerns), and magnesium can cause GI upset at higher doses. Over-drinking plain water during long events can also cause dangerous low blood sodium, which sensible sodium intake helps prevent.
Sources
- ACSM Position Stand — Exercise and Fluid Replacement (sodium 0.5–0.7 g/L in fluids; sweat sodium; hyponatremia guidance). pubmed.ncbi.nlm.nih.gov/17277604/
- Precision Hydration review of minor electrolytes (Baker & Wolfe sweat data; weak evidence for potassium/magnesium replacement). precisionhydration.com
- Exercise intensity effects on total sweat electrolyte losses. pmc.ncbi.nlm.nih.gov/articles/PMC6373370/
- Fluid and electrolyte supplementation for exercise heat stress (Am J Clin Nutr): sodium’s role in rehydration. sciencedirect.com/science/article/pii/S0002916523067370
This explainer describes what research has examined — it is not medical advice and does not prescribe. Talk to your clinician before starting any supplement, especially alongside medication. Medical disclaimer.