Hydration and electrolytes for training and sport

An evidence-based guide to staying hydrated without overthinking it — or overdoing it. Hydration is one of the most personal parts of sport, because sweat losses differ enormously from one body to the next. This covers how much fluid to drink and when, how to measure your own sweat rate, the widely used keep-loss-under-2% guideline, when sodium and other electrolytes actually matter, the drink-to-thirst versus planned-drinking debate, the real and occasionally serious danger of over-drinking, and how heat changes the stakes. Practical, honest, and grounded in established sports-medicine consensus.

Here's the short answer: there's no universal number of millilitres per hour, because sweat rates vary hugely — so the goal is to replace most (not all, and not more) of what you lose, aiming to keep body-mass loss under about 2% on long efforts, drinking largely to thirst, and adding sodium when sessions get long, hot or salty. Everything below is the detail behind that sentence: where the numbers come from, how to find your sweat rate, and why drinking as much as possible is the wrong instinct.

A framing note before the numbers. Hydration science is well studied, but people differ enormously — body size, fitness, how much you sweat, how salty that sweat is, the weather, the intensity and your health status all shift what's right on the day. So this is written as can, tends to and around, never as a guarantee, and every specific figure is a population-level guide drawn from published consensus, not a personal prescription. It's general education, not medical or dietary advice, and it isn't a personalised plan.

Why hydration matters (and why it's personal)

When you train, your working muscles produce heat, and your main way of shedding it is sweating — evaporating fluid off the skin. That cooling is essential, but it comes at a cost: you're steadily losing water, and with it some salts. If you lose enough, blood volume falls, your heart has to work harder to move blood to both muscle and skin, core temperature climbs faster, and effort starts to feel higher for the same pace. Significant dehydration — commonly defined as losing more than about 2% of body mass as sweat — is associated with rising heart rate and perceived effort and, particularly in the heat, with a measurable dip in endurance performance.

The catch that makes hydration advice so slippery is that sweat losses are wildly individual. Depending on the person, the intensity and the environment, sweat rates run from well under half a litre to more than two litres an hour; most endurance athletes sit somewhere around 0.5–1.5 litres per hour. That's a threefold-plus spread, which is exactly why a single "drink X ml every 15 minutes" rule can leave one person parched and cause another to dangerously over-drink. The whole point of a good hydration strategy is to make it yours.

How much to drink, and when

It helps to split hydration into three windows — before, during and after — because the job is different in each. The figures below draw on the American College of Sports Medicine's position stand on exercise and fluid replacement; treat them as starting points to personalise, not exact doses.

A few plain-English notes on that table. Before, the aim is simply to start in good shape: roughly 5–7 ml of fluid per kilogram of body weight at least four hours out gives time to absorb it and pass any surplus (for a 70 kg person that's very roughly 350–500 ml). If your urine is still dark two hours before, a smaller top-up of 3–5 ml/kg helps. There's no need to "pre-load" litres beyond that — you can't bank meaningful extra water, and trying just sends you to the toilet.

During, short sessions rarely need anything; for efforts past about an hour you're trying to keep pace with your losses without overshooting, which is where knowing your sweat rate pays off. If you're taking a carbohydrate sports drink for fuel, it usually carries sodium too, so hydrating and fuelling overlap — the ACSM notes drinks used in exercise longer than an hour often contain around 0.5–0.7 g of sodium per litre. After, because you keep losing some fluid in urine as you rehydrate, replacing around 1.5 litres for every kilogram lost over the following hours (with sodium, from a drink or salty food) restores balance more reliably than matching losses litre-for-litre.

Measuring your own sweat rate

Because sweat losses are so individual, the single most useful thing you can do is measure your own. It takes one session and a set of scales, and it turns "am I drinking enough?" from a guess into a number. The physics that makes it simple: one kilogram of body-mass change is roughly one litre of fluid, so the scale is a good-enough sweat gauge for a single session.

The method, step by step:

• Weigh yourself before a representative session — ideally naked or in dry, minimal kit, after emptying your bladder.
• Train for about an hour at a realistic intensity and in realistic conditions, noting how much you drink (in litres).
• Weigh yourself again after, towelled dry and in the same state as before.
• Do the sum: sweat loss = (weight before − weight after, in kg ≈ litres) + fluid you drank. Divide by the hours to get your sweat rate per hour.

For example, if you're 1 kg lighter after an hour and drank 0.5 L during it, you sweated roughly 1.5 L that hour. Repeat the test in different conditions — a cool easy session and a hot hard one will give very different numbers, and heat can push losses up dramatically — so you end up with a small mental table rather than one figure you wrongly apply everywhere. Knowing you lose, say, closer to two litres an hour in summer heat is precisely the information that lets you plan sensibly for a long, hot event instead of guessing.

The under-2% body-mass guideline

The most widely used hydration target isn't a volume — it's a limit on how much you let yourself shrink. The long-standing guidance from the ACSM is to drink enough to keep body-mass loss under about 2% over a prolonged effort, rather than following a rigid ml-per-hour schedule. Beyond roughly that 2% mark, endurance performance tends to suffer, especially in the heat, as cardiovascular and thermal strain climb.

Two honest caveats keep this from becoming dogma. First, the 2% figure comes largely from controlled and hot-weather settings; in cooler conditions, and among athletes drinking to thirst, some studies show people finishing long events several percent down on body mass with no obvious performance penalty and no harm — so it's a sensible ceiling to respect on hard, hot, performance days, not a red line that ruins a cool morning jog. Second, "body-mass loss" during a very long event isn't purely water; you also burn fuel, so the scale slightly overstates true dehydration. The practical takeaway holds either way: on long or hot efforts, don't let yourself drift far past about 2% down — but chasing 0% loss by over-drinking is its own, worse mistake (see below).

Sodium and electrolytes — when they matter

Sweat isn't just water — it carries salts, chiefly sodium (plus smaller amounts of potassium, chloride, calcium and magnesium). Sodium is the electrolyte that matters most for athletes, because it's lost in the largest amounts and it's central to fluid balance. The important, under-appreciated fact is that how salty your sweat is varies enormously between people. In one study of marathon runners, sweat sodium averaged around 43 mmol per litre but ranged from about 7 to 95 mmol per litre — more than a tenfold spread. Anyone above roughly 60 mmol/L is sometimes described as a "salty sweater" in practical guidance.

You can spot a salty sweater without a lab: white, gritty salt marks caked on skin, cap or dark kit after a hard session, stinging eyes from sweat, or a persistently salty taste are all clues that you're losing sodium at the higher end. That matters because two people doing the same event can have very different sodium needs.

So when does sodium actually earn a place in your bottle rather than just your meals? The honest answer is: later than most marketing implies, but genuinely for the right sessions. Broadly:

• Short or easy sessions (under ~1 hour): water is fine for most people; you'll replace the modest sodium losses at your next normal, lightly-salted meal.
• Longer efforts (beyond ~1 hour), heat, or salty sweaters: replacing some sodium alongside fluid becomes worthwhile. The ACSM notes that including sodium in fluids during exercise longer than an hour can improve palatability (you drink a bit more willingly) and help you retain the fluid rather than pee it straight out.
• Very long events (beyond a few hours), especially ultra-endurance: this is where blood sodium is genuinely more likely to drift and where sodium replacement matters most — both for performance and, in combination with sensible fluid intake, for safety.

Convenient overlap: most carbohydrate sports drinks already contain sodium (commonly in the region of 0.5–0.7 g per litre), so for many people the same drink handles fuelling and electrolytes together. Afterwards, sodium in a recovery drink or simply salting your food helps you hold onto the fluid you take in, which is why plain water alone can be a slightly less effective way to rehydrate after big sweat losses.

Drink to thirst vs planned drinking

There's a long-running debate in sports science between two philosophies, and the useful news is that they mostly agree for ordinary athletes. Drinking to thirst means letting your body's own thirst signal set the pace — simple, self-limiting, and hard to get badly wrong. Planned (programmed) drinking means following a schedule based on your measured sweat rate, to match losses more precisely.

For most people in most conditions, drinking to thirst is a sound default. Field studies consistently find that athletes who drink to thirst stay adequately hydrated, perform well, and — crucially — largely avoid the over-drinking that causes the dangerous problem in the next section; some ultra-endurance studies have seen thirst-guided runners finish safely even several percent down on body mass. Thirst isn't a perfect real-time gauge, but it's a good one, and it rarely leads you to drink too much.

Planned drinking has its place, though, precisely where thirst can lag behind losses: prolonged efforts (beyond about 90 minutes) in real heat (roughly above 30 °C), or an experienced athlete doing multiple hard sessions in a day, where starting each one already down on fluid stacks up. In those settings, using your sweat-rate knowledge to drink a little proactively can reduce strain. The best of both worlds for most: thirst as your default, informed by knowing your sweat rate, so you can top up sensibly when the conditions are genuinely demanding — without ever forcing fluid past comfort.

The real risk of over-drinking

For decades the only hydration message athletes heard was "drink more." The correction that sports medicine has spent years trying to spread is that you can drink too much, and it can be far more dangerous than being a little dehydrated. Drinking more fluid than you sweat dilutes the sodium in your blood, and if blood sodium falls far enough you develop exercise-associated hyponatraemia (EAH) — a condition that ranges from nausea, bloating, headache and confusion to, in severe cases, seizures, fluid on the brain and lungs, and death.

The key facts, from the international consensus on this condition:

• Over-drinking is the primary cause. The 2015 consensus statement identifies overconsumption of hypotonic fluids (water and typical sports drinks) as the main driver of EAH.
• Sodium doesn't rescue over-drinking. Because sports drinks are more dilute than blood, taking extra sodium or switching to an electrolyte drink does not prevent EAH if you're still drinking too much overall — it's the total volume of fluid that sets the outcome. Electrolytes matter for other reasons, but they are not a licence to over-drink.
• It hits slower, over-drinking finishers hardest. The classic cases are slower marathon and ultra participants who drink at every aid station "to be safe," ending up heavier at the finish than the start — the opposite of dehydration.

The consensus advice is refreshingly simple: drink according to thirst — no more, no less. Because the same thirst-led approach that guards against harmful dehydration also guards against over-drinking, it's the safest general rule for most athletes, with planned drinking reserved for the demanding cases above.

Heat — when the stakes rise

Everything above matters more in the heat, because heat and dehydration compound each other. When it's hot you sweat more (so losses rise), and dehydration in turn reduces your ability to sweat and shift blood to the skin — so you cool less effectively just as you need to cool more. The result is a faster climb in core temperature and heart rate, and a steeper fall in performance.

The physiology is striking even at small losses. Research shows that a body-mass deficit of only around 1% can already elevate core temperature during exercise, and that being both hypohydrated and hyperthermic can reduce maximal aerobic power by roughly 6% and cut time to exhaustion meaningfully compared with being well hydrated and cool. Beyond performance, the serious end of the spectrum is heat illness — heat exhaustion and, dangerously, heat stroke — which is a medical emergency and is about far more than hydration alone.

Sensible heat practices, none of them exotic:

• Start well hydrated and know your (higher) hot-weather sweat rate from testing.
• Take fluids with some sodium on long or hard sessions, and don't wait until you're parched.
• Slow down, seek shade, and use cooling (cold drinks, dousing) — pace and heat management matter as much as fluid.
• Acclimatise gradually. Your body adapts to heat over roughly one to two weeks of exposure, sweating earlier and losing less salt — don't attempt a hard hot session cold.
• Know the red flags: dizziness, disorientation, stopping sweating, nausea or a pounding headache mean stop and cool down, and seek help. These can signal heat illness or, from over-drinking, hyponatraemia — both need attention, not "push through."

Common questions

How much should I drink during exercise?

There is no single number, because sweat rates vary enormously — from well under half a litre to more than two litres an hour depending on the person, the intensity and the heat. Rather than a fixed schedule, the long-standing sports-medicine guidance from the American College of Sports Medicine is to drink enough to keep body-mass loss under about 2% over a long effort. In practice that means learning your own sweat rate (below) and drinking to thirst plus that knowledge. For most everyday training under an hour, plain water and thirst are plenty; you don't need to force fluids.

How do I measure my sweat rate?

Weigh yourself (ideally naked, or in dry kit) immediately before and after a representative session of around an hour, and note anything you drank. Each kilogram of body mass lost is roughly one litre of sweat. Add back any fluid you drank during the session, then divide by the hours to get your sweat rate per hour. Repeat in different conditions — heat and hard efforts push the number up a lot — so you have a realistic sense of your losses rather than one figure for all situations.

Do I need electrolytes and sports drinks, or is water fine?

For most sessions under about an hour, water and normal meals cover your electrolyte needs. Sodium starts to matter for longer efforts (beyond roughly an hour), in the heat, and for heavy or salty sweaters, where the American College of Sports Medicine notes that including sodium in fluids during exercise longer than an hour can aid palatability and fluid retention. Sweat sodium varies hugely between people — from about 7 to 95 mmol per litre in one study of marathoners — so a salty sweater who cakes white marks on their kit needs sodium sooner than someone who barely loses any. Sodium becomes most important in very long events beyond a few hours.

Can you drink too much water during exercise?

Yes, and it can be dangerous. Drinking far more than you sweat can dilute blood sodium and cause exercise-associated hyponatraemia. The 2015 international consensus statement identifies over-drinking of fluids as the primary cause, and stresses that taking extra sodium or a sports drink does not prevent it if you are still over-drinking — the driver is total fluid volume. Their headline advice is to drink according to thirst, no more and no less. More fluid is not automatically safer; the aim is to replace most of what you lose, not to over-fill.

Should I drink to thirst or to a set schedule?

For most people and most conditions, drinking to thirst works well and reduces the risk of over-drinking; field studies find thirst-guided athletes stay adequately hydrated and perform well. A planned drinking approach — informed by your measured sweat rate — can help in specific cases: prolonged efforts (beyond about 90 minutes) in real heat (roughly above 30°C), or an experienced athlete training multiple hard sessions a day, where thirst can lag behind losses. The sensible middle ground is thirst as your default, backed by knowing your own sweat rate so you can top up sensibly when conditions are extreme.

Does dehydration really hurt performance, especially in the heat?

Meaningful dehydration tends to raise heart rate and perceived effort, and in hot conditions it worsens performance — losing more than about 2% of body mass is the commonly cited threshold where endurance is affected. Heat and dehydration compound each other: even around a 1% loss can nudge core temperature up during exercise, and studies show that being both hypohydrated and overheated can reduce maximal aerobic power by around 6% and shorten time to exhaustion. That's why hot-weather sessions deserve more attention to fluid — and, for longer efforts, some sodium — than a cool easy run.

Takeaways

• There's no universal ml-per-hour. Sweat rates vary hugely (roughly 0.5–1.5+ L/h), so hydration has to be personalised — replace most, not all and not more, of what you lose.
• Aim to keep body-mass loss under ~2% on long or hot efforts, rather than following a rigid drinking schedule.
• Measure your own sweat rate. Weigh before and after a session (1 kg lost ≈ 1 L), add what you drank, and divide by the hours — then repeat in different conditions.
• Water is fine for short sessions. Sodium matters for efforts beyond about an hour, in the heat, for salty sweaters, and most of all in very long events.
• Sodium losses are individual. Salty sweaters (white marks on kit) need electrolytes sooner; sports drinks conveniently carry sodium with the carbs.
• Drink to thirst as your default. Planned drinking, guided by your sweat rate, helps mainly for long efforts in real heat or repeated hard sessions.
• Don't over-drink. Drinking far more than you sweat causes hyponatraemia, which is more dangerous than mild dehydration — and extra sodium doesn't make over-drinking safe.
• Respect the heat. It compounds dehydration; start hydrated, add sodium on long sessions, cool and pace yourself, acclimatise gradually, and treat confusion or collapse as an emergency.

If you take one thing away, make it this: good hydration isn't about drinking as much as possible — it's about knowing roughly how much you lose and replacing most of it, leaning on thirst, and adding sodium when sessions get long, hot or salty. Strathlon's role is to keep the context honest — logging the session so a long or hot day is treated differently, and putting the coach a tap away when you're unsure whether this is a water day or an electrolyte one.

This is general educational information, not medical, dietary or coaching advice. The hydration figures here are drawn from established sports-medicine consensus and are framed as population-level guides — individual needs vary widely with body size, sweat rate and sweat sodium, fitness, conditions and health, and are best personalised with a qualified professional. Exercise-associated hyponatraemia and heat illness are medical emergencies; anyone experiencing confusion, collapse, persistent vomiting or a severe headache during or after exercise should stop and seek urgent medical care. Anyone with a health condition (including kidney, heart or blood-pressure conditions), who is pregnant or postpartum, or who is unsure about fluid or sodium needs, should consult a qualified professional before making changes. See our Terms for more.

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