The runners who drank too much

In the spring of 2002, medical staff at the Boston Marathon finish line expected to treat dehydrated runners. They were prepared for it. What they found was something different.

A team of researchers led by Christopher Almond at Boston Children’s Hospital collected blood samples from 488 runners at the finish line and measured their sodium levels. Thirteen percent had hyponatremia, meaning abnormally low blood sodium. Nearly one in a hundred runners had critical hyponatremia, with sodium levels low enough to cause brain swelling, seizures, or cardiac arrest. These were not runners who had collapsed from the heat. They were runners who had drunk carefully, conscientiously, all the way to the finish.

The data was published in the New England Journal of Medicine in 2005. The finding that stopped the sports medicine world: the single strongest predictor of hyponatremia was not heat, not pace, not duration. It was weight gain during the race. The runners with the lowest sodium had drunk more fluid than they had sweated out.^[1]

The medics expected the problem to be too little water. The problem was too much, and the wrong kind.

This is the central paradox of hydration. You can drink yourself into a state that mimics the symptoms of dehydration, or worse, by drinking water alone, and in severe cases that state is more dangerous than dehydration would have been. Understanding why requires understanding what dehydration actually is.

One important note before going further: exercise-associated hyponatremia is most common in slower runners. A four-hour marathoner spends twice as long on the course as a two-hour runner, drinks more total fluid, sweats less per minute, and has less ability to clear excess fluid through normal kidney function. The risk profile is not uniform. But the mechanism, and what it reveals about hydration, is universal.

What dehydration actually is

Most people think of dehydration as running low on water. That is not quite right, and the imprecision matters.

Dehydration is the loss of water and electrolytes. Your body is roughly 60 percent water by weight, but that water is not pure. It is a solution. It contains dissolved minerals, principally sodium, potassium, chloride, magnesium, and bicarbonate, that govern electrical signalling, muscle contraction, nerve transmission, and the movement of fluid between compartments inside and outside your cells.

The key concept is osmolarity: the concentration of dissolved particles in a fluid, measured in milliosmoles per kilogram (mOsm/kg). Blood plasma runs at roughly 285 to 295 mOsm/kg. Your kidneys, your adrenal glands, and a hormone called antidiuretic hormone (ADH) work constantly to keep this concentration within that window. Drift outside it and serious consequences follow: too concentrated, and cells begin to shrink; too dilute, and cells swell. In the brain, where there is no room to expand inside the skull, swelling is catastrophically dangerous.

Dehydration, precisely defined, is not simply running low on fluid. It is running low in a way that pulls the concentration of your blood out of the range the body defends. The fix is not simply to drink more water. The fix is to restore the fluid and the concentration simultaneously.

Sodium, osmosis, and the marathon maths

Sodium is the dominant electrolyte in the fluid outside your cells, sitting at approximately 140 milliequivalents per litre (mEq/L) in blood plasma. It is the primary determinant of plasma osmolarity. When sodium moves, water follows: that is the principle of osmosis. Water crosses cell membranes from regions of lower solute concentration to regions of higher concentration, trying to equalise the balance. Sodium sets those gradients.

Here is the biochemistry that makes the marathon data make sense. Sweat is hypotonic relative to blood. It contains roughly 20 to 80 mEq/L of sodium, a fraction of the 140 mEq/L in plasma.^[2] This means that when you sweat, you lose proportionally more water than sodium. As you exercise, your blood plasma actually becomes slightly more concentrated, not less. Blood sodium tends to drift upward during prolonged effort, not downward. Your kidneys and ADH manage this in normal circumstances.

The problem emerges when you aggressively replace sweat losses with plain water. Plain water contains essentially no sodium. A large bolus of plain water enters the bloodstream and dilutes it, pushing plasma sodium down. In ordinary resting conditions, your kidneys would excrete the excess fluid and restore balance within an hour or two. During exercise, this compensatory mechanism is suppressed: ADH levels are elevated, which tells the kidneys to retain fluid rather than excrete it. You cannot clear the excess water fast enough.

Sodium falls. The osmolarity of blood plasma drops below the range the brain defends. Water moves into cells, including brain cells, by osmosis. In mild cases this produces nausea, headache, and confusion. In severe cases: seizures, loss of consciousness, brain herniation, death.^[1,3]

The runners at the Boston finish line who were in the most danger had, by every instinct and every piece of conventional advice, done the right thing. They had kept drinking throughout the race. The advice was simply wrong for what they were actually doing.

Isotonic, hypotonic, hypertonic

These three words describe where a fluid sits relative to blood plasma osmolarity (~285 to 295 mOsm/kg).

An isotonic fluid has roughly the same concentration as blood plasma. When you drink it, there is no concentration gradient to drive water across gut or cell membranes: fluid moves freely and is absorbed directly into the bloodstream. Net effect: you replenish both fluid and electrolytes at similar rates, and plasma osmolarity stays stable.

A hypotonic fluid has a lower concentration than blood. Water moves out of the gut and into the bloodstream quickly, because it is moving down a concentration gradient. This means fast initial absorption, which is why plain water is absorbed rapidly from the gut. The catch: the absorbed water then dilutes plasma sodium, and at high volumes the dilution effect overwhelms the speed advantage.

A hypertonic fluid has a higher concentration than blood. Water moves in the wrong direction: out of the cells lining the gut, trying to dilute the drink before it can be absorbed. Gastric emptying slows. In the short term, a hypertonic drink can actually draw fluid into the gut and make dehydration worse before it gets better. Most carbohydrate-heavy sports gels, taken without water, are hypertonic.

Plain water sits at essentially zero milliosmoles. It is the most hypotonic fluid you can drink, and at high volumes in a setting where ADH is elevated, it is precisely the wrong thing to be drinking in large quantities. Most bottled “electrolyte waters” with trace mineral additions sit in the low hypotonic range: better than plain water, but not by much if the sodium dose is negligible. A genuinely isotonic drink requires a real sodium load.

The target for hydration during exercise is isotonic, or mildly hypotonic with adequate sodium: a drink where the concentration closely matches blood plasma and where enough sodium is present to prevent the dilution problem. Everything else is a trade-off between absorption speed and concentration management.

The cholera discovery that explains your sports drink

In 1970, a paper in the Lancet described a field trial of a simple oral solution given to cholera patients in a rural treatment centre in Bangladesh. The solution contained water, sodium, glucose, potassium, and bicarbonate. The results were dramatic: death rates fell sharply. Patients who could not receive intravenous rehydration, which required trained staff and sterile equipment unavailable in rural areas, could be kept alive on a drink they mixed themselves from a sachet.^[4]

The mechanism behind this had been worked out six years earlier in laboratory experiments by Schultz and Zalusky at Cornell.^[5] The small intestine, they discovered, contains a specific transport protein, now called SGLT1 (sodium-glucose linked transporter 1), that moves sodium and glucose across the gut wall simultaneously and together. The transporter requires both to function. When glucose is present, it dramatically accelerates sodium absorption. And because sodium is the solute that drives osmotic water absorption, dramatically accelerating sodium absorption means dramatically accelerating fluid uptake.

Adding glucose to a rehydration solution was not adding calories. It was activating a molecular pump that the gut has evolved to use. The WHO Oral Rehydration Salts formula that emerged from this research, approximately 75 mEq/L sodium and 75 mmol/L glucose (~1.35% by weight) plus potassium and bicarbonate, was calibrated to maximise this cotransport effect.^[6]

The Lancet later called oral rehydration therapy “potentially the most important medical discovery of the century.” The estimate that it has saved tens of millions of lives from diarrhoeal disease is credible. It remains the backbone of cholera treatment, infant diarrhoea management, and emergency rehydration in resource-limited settings worldwide.

This is the mechanism that underlies the sugar in sports drinks. It is not marketing. It is a well-characterised molecular transporter with a clinical track record measured in millions of lives.

What this means for sports drinks: the real role of sugar, and where it ends

The glucose-sodium cotransport mechanism is genuine. In the right circumstances, adding glucose to an electrolyte drink accelerates sodium and water absorption from the gut, which means faster rehydration. This is a real physiological advantage.

The relevant circumstances, however, are narrower than the sports nutrition industry implies.

The SGLT1 mechanism matters when gut absorption is the limiting factor: when you need to move fluid from the gut into the bloodstream faster than it would move without help. This is most relevant during sustained exercise lasting more than 60 to 75 minutes, particularly in heat; during rapid post-exercise rehydration when the recovery window matters; and during situations of acute large fluid loss.^[7]

Below that threshold, the mechanism is largely irrelevant. If you are training for 45 minutes at moderate intensity in a temperate environment, your gut is not the limiting factor in your hydration. You are not losing electrolytes fast enough, or long enough, for the cotransport advantage to matter. The glucose is doing nothing useful. It is calories.

There is a further complication. The ORS formula, optimised for maximum absorption rate, sits at roughly 1.35 percent glucose by weight. Commercial sports drinks typically contain 6 to 8 percent carbohydrate, four to six times higher. At that concentration, the drink is isotonic to mildly hypertonic, and the priority has shifted from absorption speed to energy delivery. That is a legitimate trade-off for a marathoner who needs both fuel and fluid over four hours. It is the wrong product for someone who is primarily trying to rehydrate after a 45-minute fasted training session.

The practical consequence: the sugar in a standard sports drink is doing something real when you are two hours into a long run in the heat. It is doing essentially nothing, and adding roughly 50 grams of sugar per litre, when you are drinking it at your desk or after a short workout.

Practical protocol

The principles above resolve into a fairly simple set of decisions.

The primary variable is sodium. This is what most people under-prioritise and what most commercial electrolyte products get wrong. The minimum dose that meaningfully affects plasma sodium maintenance during exercise is approximately 500 to 600 mg per litre. Products at this level are adequate for moderate conditions. For heavy sweaters, heat, sauna, or extended sessions, 1000 mg/L or above is the right target. Most flavoured electrolyte tablets, powders, and hydration drinks sold in supermarkets contain 100 to 200 mg/L: enough to justify the label, not enough to do the job.

Potassium and magnesium are secondary but real. Sweat contains roughly 150 to 200 mg/L of potassium, and magnesium losses are smaller but relevant for muscle contraction and sleep quality. A product that contains meaningful sodium but nothing else is better than one with no sodium. A product that also contains potassium and magnesium is better still for prolonged sessions or heavy sweating days.

Sugar: match the scenario.

Scenario	What to drink
Training under 60–75 min, normal conditions	Plain water is fine
Fasted training, sauna, heat, or sessions over 75 min	Electrolyte drink with real sodium; no sugar needed
Long endurance over 90 min, racing	Isotonic drink with glucose; both fuel and fluid matter
Post-exercise rapid rehydration	Electrolyte drink; small glucose addition can help absorption speed
Daily resting hydration	Plain water; electrolytes if you sweat heavily during the day

How to gauge whether you are hydrated. Body weight is the most reliable proxy. Weigh yourself each morning after using the bathroom, before drinking. Track the number. A 1 to 2 percent drop from your training baseline is normal. Above 2 percent begins to impair performance. Above 3 percent is meaningful dehydration that affects cognition, thermoregulation, and cardiovascular strain. Urine colour is a useful field test: pale yellow is well-hydrated; dark yellow or amber means drink more; colourless may indicate overdrinking.^[8,9]

Most people who think they are dehydrated are actually under-sodiumed. Most people who think they need sports drinks are drinking them in scenarios where plain water and a salty meal would do the same job at no extra cost.

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The hydration setup in my personal protocol is described in detail in the longevity protocol article: at least one litre of high-sodium electrolyte water around training, three or more litres total per day, nothing after 6pm to protect sleep. The reasoning behind the sodium target comes directly from the mechanism described here.

References

1. Almond CS, Shin AY, Fortescue EB, et al. (2005). Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 352(15), 1550–1556. https://www.nejm.org/doi/10.1056/NEJMoa043901
2. Montain SJ & Coyle EF (1992). Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise. Journal of Applied Physiology, 73(4), 1340–1350. https://pubmed.ncbi.nlm.nih.gov/1447078/
3. Noakes TD, Goodwin N, Rayner BL, Branken T, & Taylor RK (1985). Water intoxication: a possible complication during endurance exercise. Medicine & Science in Sports & Exercise, 17(3), 370–375. https://pubmed.ncbi.nlm.nih.gov/4021469/
4. Cash RA, Nalin DR, Forrest JN, & Abrutyn E (1970). Rapid correction of acidosis and dehydration of cholera with oral electrolyte and glucose solution. The Lancet, 2(7679), 549–550. https://pubmed.ncbi.nlm.nih.gov/4195653/
5. Schultz SG & Zalusky R (1964). Ion transport in isolated rabbit ileum. II. The interaction between active sodium and active sugar transport. Journal of General Physiology, 47(6), 1043–1059. https://pubmed.ncbi.nlm.nih.gov/14193290/
6. World Health Organization (2006). WHO position paper on Oral Rehydration Salts to reduce mortality from cholera. https://www.who.int/cholera/technical/en/
7. Coyle EF (2004). Fluid and fuel intake during exercise. Journal of Sports Sciences, 22(1), 39–55. https://pubmed.ncbi.nlm.nih.gov/14971432/
8. Sawka MN, Burke LM, Eichner ER, et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377–390. https://pubmed.ncbi.nlm.nih.gov/17277604/
9. Casa DJ, Armstrong LE, Hillman SK, et al. (2000). National Athletic Trainers’ Association position statement: fluid replacement for athletes. Journal of Athletic Training, 35(2), 212–224. https://pubmed.ncbi.nlm.nih.gov/16558633/