Hydration & Electrolytes
Water gets treated as an afterthought and a religion at the same time — "just drink more" is the advice, "eight glasses a day" is the rule everyone half-remembers, and electrolytes only come up once someone gets a cramp. This guide covers what hydration actually does for training and cognition, why electrolytes (not just water) are usually the missing piece, and how much you actually need.
Why It Matters
Water is the medium almost every bodily process runs in — performance and cognition are some of the first things to slip when it's off.
🏋️ Performance drops early
As little as 2% body-mass fluid loss measurably reduces strength, endurance, and power output — long before you'd describe yourself as "thirsty."
🌡️ Thermoregulation depends on it
Sweat is the body's main cooling mechanism — under-hydrated, core temperature climbs faster during hard training or heat, raising both performance cost and heat-illness risk.
⚡ Electrolytes drive nerve and muscle signalling
Sodium and potassium are what let nerves fire and muscles contract in the first place — sweat heavily enough and it's the electrolyte loss, not just the water loss, that causes cramping and fatigue.
🧠 Cognition is sensitive too
Mild dehydration is linked to reduced concentration, slower reaction time, and worse mood — the "brain fog" a lot of people quietly fix with a glass of water without realising that's what it was.
Water vs. Electrolytes
These solve different problems — treating plain water as the whole solution is where most people's hydration strategy falls short.
Plain water
Replaces fluid volume — fine for day-to-day sipping and short, low-sweat sessions. Drinking large volumes of plain water without replacing sodium can actually dilute blood sodium levels further, which is part of why chugging water alone doesn't fix a hard cramp.
Water + electrolytes
Replaces both fluid and the sodium/potassium/magnesium lost in sweat — needed once sessions get long, hot, or sweaty, or on a low-carb diet (which flushes sodium independently of training). Look for >500 mg sodium per serving; most commercial "electrolyte" drinks are mostly sugar with a token amount of sodium.
How Much Do You Actually Need?
Less prescriptive than most advice makes it sound — the honest answer is "it depends on body size, sweat rate, climate, and diet," not a fixed number of glasses.
A simple daily check
Pale yellow urine, passed every few hours, is a reasonable day-to-day marker — pale straw, not perfectly clear. Consistently darker than that, especially with reduced frequency, is the more useful signal than any fixed water target.
Training changes the number
Sweat rate varies hugely by person, heat, and intensity — a rough method is weighing yourself before and after a session; each kilogram lost is roughly a litre of fluid (plus electrolytes) to replace.
The Hydration Stack
Ranked by impact — start at the top.
Tier S — Non-negotiable
Drink to thirst, plus a urine check
✔Thirst is a well-calibrated signal for most healthy adults in normal conditions — use pale yellow urine as a backup check rather than forcing down water on a fixed schedule.
Electrolytes on heavy-sweat days
✔Long, hot, or high-intensity sessions call for sodium replacement, not just water — this is the single most underused fix for mid-session cramping and fatigue.
Don't treat sodium as the enemy
✔Outside of specific medical conditions (hypertension, kidney/heart disease — see a doctor), an active person sweating regularly usually needs more sodium than the low-sodium public health messaging assumes, not less.
Tier A — Strong case
Pre-hydrating before training
✔Starting a session already slightly under-hydrated is common and avoidable — a glass or two of water in the hour beforehand covers most of it.
Rehydrating to match sweat loss
✔Weighing before/after a hard session and replacing roughly what's lost (water plus electrolytes) is more precise than guessing, and worth doing occasionally to calibrate your own sweat rate.
Extra sodium on low-carb/keto diets
✔Cutting carbs lowers insulin, which increases sodium excretion independent of sweat — the "keto flu" fatigue and headaches are often just an electrolyte deficit, not carb withdrawal itself.
Tier B — Situational
Dedicated electrolyte supplements
~Convenient and well-dosed, but not required if you salt food normally and aren't sweating heavily — for most casual gym sessions under an hour, food and water already cover it.
Coconut water
~Reasonable potassium content but low in sodium relative to sweat losses — fine as a pleasant option, not a substitute for a proper sodium-forward electrolyte drink after heavy sweating.
Tier C — Skip it
Forcing down a fixed daily litre count
✘The "8 glasses a day" figure isn't based on individual need — body size, climate, diet, and training volume all change the real number, and forcing a generic target ignores all of them.
Sugary sports drinks as the default
✘Most mainstream sports drinks are formulated around sugar content, with sodium as an afterthought — fine occasionally for the carbs during genuinely long endurance efforts, a poor default for a normal gym session.
Overdrinking water during endurance events
✘Drinking far more plain water than sweat losses justify — common advice at long races — can dilute blood sodium to dangerous levels (hyponatraemia); match intake to actual losses rather than "more is safer."
Myths, Debunked
Myth: "Everyone needs 8 glasses of water a day."
Reality: That figure isn't based on individual measurement — actual need varies with body size, climate, diet, and how much you're sweating from training.
Myth: "Clear urine means you're perfectly hydrated."
Reality: Completely clear urine usually just means you're overhydrated relative to need — pale yellow, not clear, is the healthier target.
Myth: "Salt is bad for you, so avoid electrolyte drinks."
Reality: Low-sodium public health advice targets sedentary populations with specific health conditions — an active person sweating regularly loses meaningful sodium and generally needs to replace it, not restrict it further.
Myth: "Cramps are always caused by dehydration."
Reality: Exercise-associated cramping correlates more consistently with electrolyte loss and neuromuscular fatigue than with fluid loss alone — which is why plain water often doesn't fix a cramp but a pinch of salt or an electrolyte drink does.