How much water do you actually need? Separating the evidence from the ethos

Most hydration advice online sounds authoritative because it's specific. A number feels scientific in a way that "drink when you're thirsty" doesn't. But specificity isn't the same as evidence, and the history of where these numbers actually came from is a useful reminder that round figures repeated confidently enough eventually stop getting questioned. This piece goes deeper into the primary literature than our usual articles — deliberately so — because the "drink lots, it can't hurt" ethos is worth properly interrogating, not just gently caveated.

Where "8 glasses a day" actually came from

The most-quoted hydration rule in popular culture — roughly eight 8-ounce glasses, about 2 litres — traces back to a 1945 US Food and Nutrition Board recommendation. That recommendation suggested adults consume around 2.5 litres of water daily. Crucially, it explicitly included water from all sources — food, soup, fruit, metabolic water from digestion — not just what you drink. Over decades of retelling, that qualifying clause quietly disappeared, and "2.5 litres including food" became "drink 2 litres of water."

In 2002, kidney physiologist Heinz Valtin published a review in the American Journal of Physiology specifically searching for the scientific basis of the 8×8 rule. He found none — no controlled trial, no physiological study establishing that healthy people benefit from forcing fluid intake beyond thirst. His conclusion was blunt: it's hard to believe evolution left humans with a chronic water deficit that needs correcting by deliberate over-drinking. The BMJ's Christmas issue reached the same verdict independently five years later.

The most rigorous modern data point comes from a 2022 study in Science, led by Yamada and an international consortium, using doubly-labelled water — the gold-standard tracer method for measuring actual water turnover — in 5,604 people across 26 countries. The headline finding wasn't a better single number to replace 2 litres. It was that water turnover varies enormously by body size, activity level, climate, and even altitude, and that no single daily figure meaningfully applies to everyone. Official bodies reflect this: the US Institute of Medicine's Adequate Intake is roughly 3.7L total water/day for men and 2.7L for women (again, including food, which typically supplies 20–30% of that), while EFSA's figures sit a bit lower, around 2.5L and 2.0L respectively. These are not treatment targets derived from outcome trials — they're descriptive estimates of what healthy, free-living people tend to consume without ill effect.

Why forcing extra water isn't obviously beneficial — and can be harmful

The physiological argument for "just drink more" rests on the idea that thirst is an unreliable, laggy signal. For most healthy adults at rest or in mild activity, that's not well supported — the osmoregulatory system (thirst plus antidiuretic hormone) is a tightly regulated feedback loop, not a crude warning light that only fires once you're already in trouble.

There's also a real ceiling on the upside. Water intoxication — hyponatraemia from diluting blood sodium faster than the kidneys can excrete free water — is rare in sedentary contexts but not impossible, and the kidneys' capacity to clear excess water (roughly 800–1000ml/hour in a healthy adult) is finite. "More water" is not a risk-free variable you can maximise; like most physiological inputs, there's a healthy range, not a one-directional "more is better" relationship.

None of this means hydration doesn't matter — dehydration is a real risk, particularly in heat, at altitude, and in older adults whose thirst sensation blunts with age. It means the evidence supports responding to your body's signals, not overriding them with an arbitrary target.

Exercise hydration: the sweat-replacement rule that sports medicine has walked back

The "500ml per hour of training" guidance is a generic sweat-replacement heuristic descended from older sports-medicine advice (broadly ACSM-style guidelines from the 1990s–2000s), built on the idea that athletes should drink ahead of thirst to stay fully replaced. It has two problems.

First, individual sweat rates vary hugely — commonly cited ranges run from roughly 0.3L/hour to over 2L/hour depending on heat, humidity, clothing, intensity and individual physiology. A flat 500ml/hour will under-replace a heavy sweater in the heat and over-replace a light sweater in the cold. The only way to know your actual number is a sweat-rate test: weigh yourself nude before and after a representative session, accounting for fluid drunk and urine passed.

Second, and more importantly, the "drink ahead of thirst" doctrine has been directly implicated in a genuine and sometimes fatal problem: exercise-associated hyponatraemia (EAH). Cases and deaths in marathon and endurance events through the 1990s and 2000s — runners who followed "drink as much as possible" advice and finished dangerously over-hydrated rather than dehydrated — drove a major re-think. The Third International Exercise-Associated Hyponatraemia Consensus Development Conference (2015, published in Clinical Journal of Sport Medicine) concluded that EAH is primarily caused by drinking hypotonic fluid in excess of losses, combined with exercise-induced hormonal fluid retention — not primarily a sodium-deficiency problem. Their central preventive recommendation: drink according to physiologically regulated, individualised thirst. Sports drinks with a bit of added sodium don't prevent EAH if you're simply drinking too much fluid overall; volume, not sodium content, is the main driver.

It's worth being honest, though, that this isn't a fully settled debate. A 2016 point–counterpoint in Wilderness & Environmental Medicine pitted researchers who argue drinking to thirst is adequate for prolonged endurance exercise (Hoffman & Cotter) against researchers who argue it isn't always sufficient in every context (Armstrong & Bergeron) — a genuine, published scientific disagreement, not a settled consensus dressed up as one. That debate is sharpest in exactly the contexts covered next: multi-hour efforts in the cold and at altitude.

Cold and altitude: the one place "drink to thirst" breaks down

This is the important caveat to everything above. The drink-to-thirst evidence base comes overwhelmingly from warm-weather endurance and team-sport research. Cold and altitude environments introduce fluid-loss mechanisms that thirst doesn't track well at all.

Cold-induced diuresis is a well-documented phenomenon, characterised in detail by Bader and colleagues in the 1950s and reviewed extensively in the US National Academies' work on cold-weather physiology: peripheral vasoconstriction in the cold shifts blood volume centrally, which your kidneys interpret as volume overload and respond to by increasing urine output — even though you haven't actually gained fluid. It's multifactorial (influenced by exposure duration, hydration status, whether you're exercising, and several other factors), but the net effect is real fluid loss that has nothing to do with sweating or thirst.

Altitude diuresis compounds this. Acute exposure to altitude triggers increased urine output in the first 12–48 hours, mediated by hormonal changes (suppressed antidiuretic hormone, increased atrial natriuretic peptide) as the body adjusts to hypoxia. This typically settles within a week of acclimatisation, but during that window fluid losses run well above baseline. There's also a documented link between chronic dehydration and worse outcomes for acute mountain sickness, though the exact causal mechanism is still debated.

Respiratory water loss stacks on top of both. Breathing cold, dry mountain air at an elevated rate — which happens automatically at altitude and during hard exercise — carries substantial moisture out with every exhaled breath, often invisibly, since there's no sweat to see and no thirst cue driving you to replace it.

And underneath all three sits the inconvenient fact that cold itself blunts thirst — a well-replicated finding across cold-weather physiology research. So the three biggest extra fluid-loss mechanisms you'll encounter in ski touring and alpine environments (cold diuresis, altitude diuresis, respiratory loss) all arrive stacked on top of a suppressed thirst signal, which is close to the worst possible combination for a "just drink when thirsty" strategy.

This is reflected in the research on cold-weather endurance athletes: a study of a 24-hour winter mountain running race in extreme cold (average −14°C) found athletes maintained hydration status reasonably well overall, but noted that the interplay of cold-suppressed thirst and cold diuresis creates a real risk of under-hydration that isn't well studied — and, interestingly, that exercise-associated hyponatraemia appears to become less common as temperatures drop, likely because cold-suppressed thirst makes over-drinking the less common failure mode in the cold, even as heat makes it the dominant one.

Protein: the same pattern, a different number

The "2g/kg of bodyweight" protein target that's become fitness-industry gospel follows an almost identical story to the water figure — a defensible evidence range, compressed into a single memorable number that gets treated as more precise than it is.

The baseline RDA of 0.8g/kg/day comes from nitrogen balance studies in sedentary people, and both the US and Australian bodies that set it are explicit that it represents a minimum to prevent deficiency — not an optimal intake for anyone training seriously. It has limited relevance once you're doing meaningful training volume.

The evidence-supported range for people in regular training sits closer to 1.6–2.2g/kg/day. A widely cited 2018 meta-analysis in the British Journal of Sports Medicine (Morton et al., pooling ~49 studies) found that gains in muscle protein synthesis and lean mass from protein intake plateau at around 1.6g/kg/day for most people doing resistance training, with limited additional benefit above roughly 2.2g/kg. "2g/kg" is a reasonable, round figure that sits comfortably inside that evidence-supported range — not a precisely validated optimum. You wouldn't expect to notice a meaningful difference between, say, 160g and 180g a day. There's also a documented per-meal ceiling — muscle protein synthesis response appears to max out somewhere around 0.4g/kg per sitting — which is a better argument for spreading intake across 4–5 meals than for chasing a bigger single number.

Protein and the kidney myth

The idea that a high-protein diet damages otherwise healthy kidneys is one of the most persistent pieces of exercise-nutrition folklore, and it's a good example of evidence from one population being misapplied to another. The well-established finding is that protein restriction slows disease progression in people who already have chronic kidney disease — that's real and clinically important. Somewhere along the way, that got generalised into "high protein is bad for kidneys," full stop — which the evidence in people with normal kidney function doesn't support.

A 2018 systematic review and meta-analysis of RCTs (Devries et al., pooling 28 studies, 1,358 participants) found that while higher-protein diets did produce a modest rise in glomerular filtration rate (GFR) — the kidneys filtering somewhat harder — this reflects an adaptive, reversible hyperfiltration response, not damage, and the change in GFR over time didn't differ between higher- and normal-protein groups. A more recent 2026 meta-analysis of 22 RCTs reached the same conclusion using more current data: higher-protein diets increased eGFR with no significant change in serum creatinine and no consistent biochemical evidence of renal injury in adults without pre-existing kidney disease. Multiple other systematic reviews across both RCT and observational data report the same pattern — stable creatinine, stable albumin excretion, no adverse blood pressure effect — in healthy people eating well above the RDA, including athletes specifically.

The caveats worth knowing: most of the underlying trials are relatively short-duration (months, not years), and the picture is less settled in people with existing risk factors — pre-diabetes, older age, or borderline kidney function — where a small number of studies suggest that the source of protein (plant vs animal) may matter more than total quantity. None of that applies to a healthy, actively training adult without kidney disease, which is the population most people asking this question actually belong to.

Creatine: the supplement everyone worries is bad for your kidneys

Part of why the protein-and-kidneys myth persists is a genuine mix-up with a different, similarly-named substance: creatine, the supplement, versus creatinine, the blood marker doctors use to estimate kidney function. They're closely related — creatine is broken down into creatinine as a normal waste product, and supplementing with creatine mildly raises blood creatinine as a benign, expected artefact of having more substrate around to break down, not a sign the kidneys are struggling. A GP or lab unfamiliar with a patient's supplement use can misread a modestly elevated creatinine as kidney impairment when it's simply this artefact. That mix-up is a real, if minor, contributor to creatine's ongoing reputation problem.

Creatine monohydrate is, separately from that confusion, one of the most heavily studied supplements in sports science, and the evidence for its core use case is about as solid as sports supplement evidence gets. Creatine increases the muscle's stored capacity for rapid ATP regeneration — the energy system used for short, high-intensity efforts — and consistently produces measurable gains in strength, power output and lean mass when combined with resistance training, on top of what training alone achieves. The effect is well-established enough that it forms one of the few supplements the International Society of Sports Nutrition rates as having strong, consistent evidentiary support. Standard dosing is a maintenance dose of 3–5g/day taken indefinitely; an optional loading phase of around 20g/day (split across 4 doses) for 5–7 days saturates muscle stores faster, but isn't necessary — it just gets you to the same steady state a couple of weeks sooner than maintenance dosing alone.

Two groups see a larger relative response than average. Vegetarians and vegans typically start with lower baseline muscle creatine stores, since dietary creatine comes almost entirely from meat and fish, so supplementation tends to produce a bigger jump for them than for habitual meat-eaters. Older adults combining creatine with resistance training show meaningful improvements in muscle mass and strength beyond training alone, which is a large part of why it's increasingly discussed as a healthy-ageing tool rather than purely a bodybuilding one.

There's also a newer and interesting line of research into creatine's effects on the brain, not just muscle. The brain is a heavy energy consumer and appears to draw on the same creatine-phosphate energy system under metabolic stress — sleep deprivation, hypoxia, and mental fatigue among them. Several controlled trials have found that a single high dose of creatine can measurably blunt the cognitive decline seen during acute sleep deprivation, improving processing speed and short-term memory for several hours afterwards, alongside detectable changes in brain energy metabolism on imaging. This is early-stage relative to the strength and power evidence — a systematic review of creatine-and-cognition research overall found the results more equivocal across the wider literature, and most of the strongest single-dose sleep-deprivation data comes from a small number of research groups rather than being broadly replicated yet — but it's an active and promising area.

The altitude-specific version of this is worth separating into what's actually been shown versus what's still a reasonable hypothesis. On the cognitive side, there's a genuine controlled study: healthy adults given 7 days of creatine supplementation (raising brain creatine by roughly 9%) were then exposed to a hypoxic gas mixture equivalent to around 5,500m. Hypoxia predictably impaired attentional capacity in the placebo condition; in the creatine condition, that impairment was largely restored, alongside improved corticomotor excitability — a reasonable mechanistic fit, since the brain leans on the same creatine-phosphate buffer under oxygen restriction that it does under sleep deprivation. What hasn't been established yet is whether creatine meaningfully improves physical exercise performance at altitude specifically — the theoretical case is plausible (the same ATP-buffering logic that helps at sea level should, in principle, help more when oxygen-dependent energy production is constrained), but at the time of writing there isn't a completed trial testing that directly; a study designed to do exactly this was still in the trial-registration stage. Worth watching, not yet worth treating as established.

On safety: the concerns most commonly raised — kidney damage, dehydration and cramping, cancer risk — don't hold up well against the evidence in healthy adults. Kidney function markers stay stable across long-term studies (including trials running 10 months to 5 years), and a 2026 systematic review and meta-analysis of RCTs in the Journal of Renal Nutrition found the expected mild rise in serum creatinine — consistent with the creatine/creatinine artefact described above — with no corresponding change in eGFR or urea, the actual functional markers of kidney health. The water-weight gain some people notice is intracellular retention, not dehydration, and there's no evidence linking supplementation to cancer risk. The case reports behind these concerns generally involve pre-existing kidney disease or doses well above standard recommendations. As with protein, people with existing kidney disease, and those who are pregnant or on kidney-affecting medication, should get medical advice before supplementing rather than assume the general safety evidence transfers automatically.

Bringing it together

Daily hydration: the "2–3.5L target" that shows up everywhere is a population-level estimate dressed up as a personal prescription. For most healthy people, thirst plus urine colour (pale straw, not clear) is a better individual guide than a fixed number.

Exercise hydration: the generic "500ml/hour" heuristic descends from advice sports medicine has partly walked back, after that advice contributed to real, sometimes fatal cases of exercise-associated hyponatraemia. Drink-to-thirst is the better-evidenced default for most training — though genuine scientific debate continues about its limits in prolonged, extreme-context exercise.

Cold and altitude are the exception, not confirmation of the rule: cold-induced diuresis, altitude diuresis and respiratory water loss all stack on top of a thirst signal that cold itself suppresses. This is one of the few evidence-supported contexts for proactive, scheduled drinking rather than waiting to feel thirsty.

Protein at ~1.6–2.2g/kg/day is well-supported for people training seriously, spread across several meals — but the RDA of 0.8g/kg is a deficiency threshold, not an optimum, and 2g/kg is a convenient round number within a range, not a precisely validated target.

High protein doesn't damage healthy kidneys. The kidney-damage concern is a real finding from chronic kidney disease research that's been generalised well beyond the population it applies to.

Creatine is one of the best-evidenced supplements in sports science, and it doesn't damage healthy kidneys either. Its reputation problem owes a fair amount to genuine confusion with creatinine, the unrelated blood marker it happens to be metabolised into.

The common thread across all of it: single numbers travel further on social media than the nuance behind them ever does. That doesn't make the numbers useless as rough starting points — it makes them worth checking against your actual body, your actual environment, and, where it exists, the actual evidence.

Getting a hydration and nutrition strategy checked against your actual trip?

Generic figures are a reasonable starting point, but they don't account for your specific itinerary, altitude profile, or medical history. If you'd like a nutrition and hydration plan reviewed against the real conditions you're training or travelling in, that's exactly the kind of thing worth a proper conversation before you go.