A.2.1 Water and electrolyte balance

Learning objectives

• Outline where water is held in the body and the functions of water and of the main electrolytes.

• Describe osmosis and osmolarity, and apply them to the movement of water between body compartments.

• Describe how water and electrolytes enter the body and are absorbed, and how they are lost by evaporation and excretion.

• Explain how the hypothalamus, the pituitary gland and the kidneys regulate water and electrolyte balance.

• Distinguish between dehydration, hypernatraemia and hyponatraemia, and explain their effects on health and performance.

• Evaluate methods of measuring hydration status, and calculate sweat rate from changes in body mass.

• Explain cardiovascular drift during prolonged submaximal exercise and sketch the changes in heart rate, stroke volume and cardiac output.

• Evaluate hydration strategies before, during and after exercise, including sports drinks, and explain how the environment changes fluid needs.

Introduction

Water is the largest single component of the human body, and the electrolytes dissolved in it control how that water is distributed and how nerves and muscles work. Water and electrolyte balance means that the amounts gained and lost over time are equal, so the volume and concentration of the body fluids stay within narrow limits.

Exercise disturbs this balance quickly. A runner in a warm marathon can lose more than a litre of sweat per hour and become dehydrated, while a recreational runner who drinks at every station can gain water and dilute the sodium in the blood. Either imbalance can reduce performance and, in severe cases, endanger health.

Water in the body

In a lean adult man, water makes up about 60% of body mass; in an adult woman the average is closer to 55%. The total volume of water in the body is called total body water. For a 70 kg man this is about 42 L.

The proportion varies between people mainly because different tissues hold different amounts of water. Skeletal muscle is about three-quarters water, whereas adipose (fat) tissue holds much less. A person with more muscle and less fat therefore has a higher percentage of body water. Infants are about 75% water, and the proportion falls with age to about 55% in older adults, partly because muscle mass declines.

Intracellular and extracellular fluid

Body water is held in two main compartments, separated by cell membranes. About two-thirds is inside cells as intracellular fluid (ICF). The remaining third is outside cells as extracellular fluid (ECF).

The extracellular fluid is itself divided by the walls of the capillaries. Most of it is interstitial fluid, which bathes the cells, and a smaller part is plasma, the liquid part of the blood. Plasma is the compartment that carries water, nutrients, hormones and heat around the body, so changes in plasma volume quickly affect the heart and circulation.

Water moves freely between the compartments, so a loss from one is shared by the others. When an athlete sweats, water is first lost from the plasma and interstitial fluid; as the extracellular fluid becomes more concentrated, water is then drawn out of the cells. The rules that decide which way water moves are those of osmosis.

Functions of water

Water has several properties that make it essential for life and for exercise. It dissolves many substances, it holds a lot of heat, and it absorbs a great deal of energy when it evaporates.

Temperature regulation is the function most challenged by exercise. Working muscles turn most of the chemical energy they use into heat, and in a warm environment the main way to lose that heat is by evaporating sweat. The water used for cooling must then be replaced, which links thermoregulation directly to fluid balance.

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Electrolytes

Many substances dissolved in body water split into electrically charged particles called ions. These ions are known as electrolytes because a solution that contains them conducts electricity.

The two compartments have very different electrolyte contents. Sodium and chloride are the main ions of the extracellular fluid, so they largely set the concentration of plasma. Potassium is the main positive ion inside cells. The difference in sodium and potassium across the cell membrane is what allows nerve and muscle cells to carry electrical impulses.

Because sodium is the main ion of the extracellular fluid, the body's sodium content controls the volume of that fluid. Water follows sodium by osmosis, so a loss of sodium in sweat therefore reduces the volume of the extracellular fluid as well as its concentration, and a gain of water without sodium dilutes it.

Sweat is less concentrated than plasma. Sweat sodium concentration varies widely between people, from about 10 to 70 mmol L⁻¹ for the whole body, compared with about 140 mmol L⁻¹ in plasma. Two athletes who sweat the same volume can therefore lose very different amounts of salt, which is one reason why fluid and electrolyte advice is individual.

Osmosis and osmolarity

Cell membranes let water pass through easily but restrict the movement of many dissolved particles. When the concentration of dissolved particles is different on the two sides of a membrane, water moves across it until the concentrations are equal. This movement is called osmosis.

The concentration that drives osmosis depends on the number of dissolved particles, not on what they are. One molecule of glucose and one sodium ion each count as one particle. The total concentration of dissolved particles in a fluid is its osmolarity.

Normal plasma osmolality lies between about 275 and 290 mOsm kg⁻¹. The two terms osmolarity and osmolality are often used interchangeably in sport, because for body fluids their values are almost identical. Laboratories usually measure osmolality.

Hypotonic, isotonic and hypertonic

A fluid can be compared with plasma by its tonicity. A hypotonic fluid has a lower solute concentration than plasma, an isotonic fluid has about the same, and a hypertonic fluid has a higher one. Water moves by osmosis from the hypotonic side to the hypertonic side.

These rules explain how body water shifts during exercise. Sweating removes more water than solute, so plasma becomes hypertonic and water is drawn out of the cells into the blood, which partly protects plasma volume. Drinking a large volume of plain water makes plasma hypotonic, so water moves into cells. In the brain, which is enclosed by the skull, this swelling is dangerous.

How water and electrolytes enter the body

A sedentary adult in a temperate climate gains roughly 2.3–2.5 L of water a day. Most comes from drinks, but food also supplies a large share, about a fifth of the total in a typical Western diet, because fruit, vegetables, soups and cooked grains are mostly water. A smaller amount is produced inside the body.

Water produced inside cells by aerobic respiration is called metabolic water. When glucose or fat is oxidised in the mitochondria, the hydrogen it contains is combined with oxygen to form water. A sedentary adult makes about 200–300 mL a day, and the volume rises during long exercise because more fuel is oxidised.

Electrolytes come almost entirely from food and drink. Sodium is mostly eaten as salt (sodium chloride) in processed food, and potassium comes from fruit, vegetables, milk and meat. Sports drinks and electrolyte tablets add sodium during long exercise.

Absorption in the intestines

Very little water is absorbed in the stomach. The rate at which a drink is emptied from the stomach into the small intestine (gastric emptying) is therefore the first limit on how quickly fluid reaches the blood. Large volumes empty quickly, whereas concentrated drinks, fat and high-intensity exercise slow emptying.

Each day, the gut receives the water from food and drink plus about 8 L of digestive secretions such as saliva, gastric juice, bile and pancreatic juice. Nearly all of this is reabsorbed. The small intestine absorbs most of it. Sodium and glucose are carried across the intestinal wall together, and water follows them by osmosis. This is why a drink that contains a little sodium and glucose is absorbed faster than plain water.

The large intestine (colon) recovers most of the water and electrolytes that remain. It absorbs sodium and chloride, and water follows by osmosis, so that only about 100 mL of water a day leaves in the faeces. When this process fails, as in diarrhoea, water and electrolyte losses can be large enough to cause serious dehydration, especially in children.

How water and electrolytes leave the body

Water is lost by two main routes: evaporation, through the skin and the respiratory tract, and excretion, in urine and faeces. The figure shows a typical daily balance for a resting adult in a temperate climate.

Evaporation from the skin

Water evaporates from the skin in two ways. Insensible perspiration is water that diffuses through the skin and evaporates without being noticed, about 300–400 mL a day. Sweating is the active secretion of fluid by sweat glands, controlled by the hypothalamus as part of thermoregulation. At rest in a cool room sweat loss is about 100 mL a day, but during exercise it can rise to more than a litre per hour.

Whole-body sweat rates in athletes usually range from about 0.5 to 2.0 L h⁻¹, and rates above 3.0 L h⁻¹ are rare. Sweat rate rises with exercise intensity, air temperature and humidity, body size, clothing and protective equipment, and with heat acclimatisation. Trained and heat-acclimatised athletes start sweating sooner and sweat more, which improves cooling but increases the fluid that must be replaced.

Evaporation from the respiratory tract

Air breathed in is warmed and fully saturated with water vapour in the airways, and much of this water is breathed out. At rest the loss is about 300–400 mL a day. It rises during exercise because minute ventilation increases, and it is larger when the inspired air is cold and dry, as at altitude or in winter sport.

Excretion in urine and faeces

The kidneys filter the blood plasma and then reabsorb most of the water and useful solutes back into the blood. Water returns to the blood by osmosis, and the volume reabsorbed is adjusted by hormones. What is not reabsorbed leaves as urine. Typical urine output is about 1.5 L a day, but it can fall to well under 1 L when the body needs to conserve water and rise to many litres when large volumes are drunk.

The kidneys also excrete excess sodium, potassium and other electrolytes, so they control both the volume and the composition of the body fluids. Faeces contain about 100 mL of water a day under normal conditions.

Sources: Guyton and Hall, Textbook of Medical Physiology; Baker, Sports Medicine, 2017; Popkin et al., Nutrition Reviews, 2010.

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Regulation of water and electrolyte balance

Water and electrolyte balance is a homeostatic system controlled by negative feedback. The controlled variable is the osmolarity of the plasma. The receptors are in the hypothalamus, the hormone that adjusts water loss is released by the pituitary gland, and the effector organ is the kidney.

Detecting a change: the hypothalamus

Specialised cells in the hypothalamus, called osmoreceptors, detect the concentration of the plasma flowing past them. When an athlete sweats without drinking, plasma osmolarity rises and these cells shrink slightly as water leaves them by osmosis. The change in their activity signals that the body is short of water.

Pressure receptors in the large blood vessels and the heart add a second signal. A large fall in blood volume, as in severe dehydration, also stimulates water conservation, even if osmolarity has not changed much.

The response: ADH and the kidneys

The hypothalamus makes antidiuretic hormone (ADH), which is stored in and released from the posterior part of the pituitary gland. When osmolarity rises, more ADH is released into the blood. ADH acts on the collecting parts of the kidney tubules and makes them more permeable to water, so more water is reabsorbed into the blood by osmosis. The result is a small volume of concentrated, dark urine.

When the body has too much water, plasma osmolarity falls, ADH release is suppressed, less water is reabsorbed, and a large volume of dilute, pale urine is produced. Both responses push osmolarity back towards its normal value, which makes the system an example of negative feedback.

Aldosterone and sodium

Sodium balance is controlled mainly by aldosterone, a hormone released by the cortex of the adrenal glands. When blood volume or blood pressure falls, or plasma potassium rises, aldosterone release increases and the kidneys reabsorb more sodium (and excrete more potassium). Because water follows sodium by osmosis, aldosterone also helps restore blood volume. It also reduces the sodium concentration of sweat in heat-acclimatised people.

Thirst

The second response to a rise in osmolarity is behavioural. The hypothalamus generates the sensation of thirst, which makes a person seek and drink fluid. Thirst is triggered by a rise in plasma osmolarity of only a few per cent and by a fall in blood volume. A dry mouth adds to the sensation, and drinking relieves it quickly, often before the fluid has been absorbed.

Dehydration, hypernatraemia and hyponatraemia

When intake and loss of water and electrolytes do not match, three states of imbalance can develop. Dehydration is a deficit of body water. Hypernatraemia is an abnormally high concentration of sodium in the blood, and hyponatraemia an abnormally low one. The spellings hypernatremia and hyponatremia are used in American sources.

Dehydration and hypohydration

Strictly, dehydration is the process of losing body water, and hypohydration is the state of having less body water than normal. The normal, balanced state is euhydration. In everyday use and in most sports science writing, "dehydration" is used for both the process and the state.

Hydration is always judged against a person's normal state. In healthy adults who are eating and drinking normally, body mass varies from day to day by only about 1%, so this narrow band is used as the reference for normal hydration.

In sport, the commonest cause is sweating faster than fluid can be, or is, replaced. Other causes are deliberate fluid restriction to "make weight" in weight-class sports such as boxing, judo and rowing, vomiting and diarrhoea, illness with fever, and simply not drinking enough before training.

How dehydration affects the body

A loss of body water reduces plasma volume. Less blood returns to the heart, so venous return and stroke volume fall, and heart rate rises to compensate. At the same time, the body has to share a smaller blood volume between the working muscles and the skin. Less blood reaches the skin, so less heat is lost, sweating is reduced, and core temperature rises faster.

The effects therefore form a chain. Lower plasma volume leads to higher heart rate and lower stroke volume, which leads to less skin blood flow and higher core temperature, which leads to a greater sense of effort and earlier fatigue. Each 1% of body mass lost as water raises heart rate and core temperature a little more, so the effects are graded rather than all-or-nothing.

The 2% threshold and its limits

A loss of more than about 2% of body mass during exercise is commonly cited as the point at which endurance performance starts to fall, and it is the limit used in many drinking guidelines. The figure comes mainly from laboratory studies in which participants exercised at a fixed intensity, often in the heat, until exhaustion.

Results from field and time-trial studies are more mixed. A meta-analysis of laboratory studies found that, when athletes could set their own pace in a time trial, losses of up to about 4% of body mass were very unlikely to reduce performance, and successful marathon runners often finish with losses well above 2%. The threshold is therefore best seen as a useful warning level, especially in the heat and in long events, rather than a fixed limit that applies to everyone in every condition.

Sources: Goulet, British Journal of Sports Medicine, 2013.

Hypernatraemia

Hypernatraemia is a plasma sodium concentration above about 145 mmol L⁻¹. In exercise it usually develops when much more water than sodium is lost, as in heavy sweating without drinking, because sweat is more dilute than plasma. The plasma becomes hypertonic, water is drawn out of the cells by osmosis, and the cells, including brain cells, shrink.

Symptoms include intense thirst, weakness, irritability and, in severe cases, confusion and seizures. In sport, mild hypernatraemia often accompanies dehydration at the end of long events. It is most dangerous in people who cannot respond to thirst, such as infants and frail older adults.

Hyponatraemia

Hyponatraemia is a plasma sodium concentration below 135 mmol L⁻¹. When it occurs during or up to 24 hours after prolonged exercise it is called exercise-associated hyponatraemia. The main cause is drinking more hypotonic fluid (water or most sports drinks) than the body loses, so that total body water rises and the sodium in the extracellular fluid is diluted.

Three factors combine. The athlete drinks to excess, usually because of a belief that more fluid is always better. The kidneys fail to excrete the extra water, because ADH release stays higher than the low osmolarity would justify (exercise, nausea and pain all stimulate ADH). Some athletes also lose a large amount of sodium in sweat. Because the extracellular fluid becomes hypotonic, water moves into cells by osmosis, and brain swelling causes the dangerous symptoms.

• Early or mild symptoms: nausea and vomiting, headache, bloating, swollen hands and feet, and a gain in body mass during the event. Many cases have no symptoms.

• Severe symptoms: confusion, disorientation, seizures, coma and fluid in the lungs. Severe hyponatraemia (often below 125 mmol L⁻¹) is a medical emergency and has caused deaths in marathons, triathlons and military training.

The main risk factors are a long exercise duration (more than about four hours), slow pace with frequent drinking opportunities, gaining body mass during the event, a small body size, inexperience and, in some studies, female sex, largely because women are on average smaller and slower in mass-participation events. Hot weather adds risk when athletes drink heavily, and so does unusual cold, when sweat losses are small.

Comparing the three states

Measuring hydration status

No single method measures hydration perfectly, so athletes and scientists choose methods by balancing validity (does it measure hydration?), reliability (does it give the same answer when repeated?), cost and practicality. The main methods use body mass, urine, thirst or blood.

Change in body mass and sweat rate

Over a single exercise session, almost all the change in body mass is water, because the mass of fuel used is small. A loss of 1 kg corresponds to about 1 L of fluid. The athlete is weighed, ideally nude or in minimal dry clothing after emptying the bladder, before and after exercise. The volume drunk is added and any urine passed is subtracted to estimate the total sweat loss.

Body mass change is cheap, quick and the most practical way to estimate sweat losses in the field, and it is the basis of individual sweat testing. Its accuracy depends on careful procedure: sweat trapped in clothing, food eaten, toilet visits that are not recorded and small errors in the scales all distort the result. In events lasting several hours, the mass of fuel burned and water breathed out also become noticeable.

Urine colour

Urine becomes darker as it becomes more concentrated, because the same amount of pigment is dissolved in less water. A urine colour chart compares the colour of a sample with a numbered scale, from pale straw (well hydrated) to dark amber or brown (dehydrated).

Urine colour is free and easy for athletes to use daily. It is less precise than laboratory methods, and colour is affected by vitamin supplements (especially B vitamins), some foods and medicines, and the lighting. It also lags behind rapid changes: an athlete who drinks a large volume quickly can pass pale urine while still hypohydrated, because the kidneys respond to the sudden fall in plasma osmolarity.

Urine specific gravity and urine osmolality

Two laboratory measures give a number for urine concentration. Urine specific gravity (USG) compares the density of urine with that of pure water and is measured with a refractometer from a few drops. Urine osmolality measures the number of dissolved particles and is measured with an osmometer.

Both measures agree well with each other and with urine colour, which allows colour to be used in the field when a close estimate is good enough. Like colour, they are best on a first-morning sample and are poor at tracking rapid changes during and just after exercise.

Thirst, blood sodium and plasma osmolality

Plasma osmolality and blood sodium concentration are the most direct measures of the concentration of the body fluids, and blood sodium is the only way to diagnose hyponatraemia or hypernatraemia. They require a blood sample and laboratory equipment or a portable analyser, so they are used in research, in medical tents at endurance events and in clinical settings rather than for daily monitoring.

Thirst is the body's own signal of water need. It costs nothing and responds within minutes, but it is a subjective sensation that is affected by the taste and temperature of drinks, by habit, by dry mouth and by age.

Evaluation of hydration measures

No single measure is valid in every situation. A practical combination for athletes is a first-morning urine check to confirm that training starts euhydrated, plus body mass before and after key sessions to estimate sweat losses. Blood measures are kept for research and for diagnosing athletes who are unwell.

Cardiovascular drift

During prolonged exercise at a constant, moderate intensity, heart rate does not stay steady. After about 10 minutes it starts to rise slowly, while stroke volume falls, and cardiac output stays roughly the same. This pattern is called cardiovascular drift.

Why the drift happens

Cardiac output is the product of heart rate and stroke volume, so a fall in one must be matched by a rise in the other if cardiac output is to be maintained.

1. Sweating removes water from the plasma, so plasma volume falls and less blood returns to the heart.

2. As core temperature rises, more blood is directed to the skin for cooling, and the skin veins hold a larger volume of blood.

3. Venous return falls, so the ventricles fill less and stroke volume falls.

4. The sympathetic nervous system increases heart rate to maintain cardiac output and blood pressure.

5. The rise in heart rate shortens the time available for the ventricles to fill, which reduces stroke volume further.

Two explanations have been proposed for the fall in stroke volume, and both are supported by evidence. The traditional view is that rising skin blood flow and falling plasma volume reduce venous return. A later view, from studies that manipulated heart rate directly, is that the rise in heart rate itself shortens the time for the ventricles to fill, so stroke volume falls as a consequence. In practice, dehydration and heat strengthen both effects.

Evidence: fluid replacement prevents the drift

In one classic study, ten endurance-trained cyclists rode for two hours at about 70% of VO₂ max in a 22 °C room, once without fluid and once drinking enough to prevent any loss of body mass. Without fluid they lost 2.9% of body mass; with full replacement they lost none.

The chart shows that stroke volume fell by 15% and cardiac output by 7% between 20 and 120 minutes when no fluid was drunk, while heart rate rose by 10%. When fluid was replaced, stroke volume was maintained and the smaller rise in heart rate increased cardiac output. After two hours, rectal temperature was 0.6 °C higher in the no-fluid trial.

Fluid loss is therefore a major cause of the drift, and drinking can largely prevent it in moderate conditions. The 7% fall in cardiac output without fluid also shows that, once dehydration approaches 3% of body mass, the rise in heart rate no longer fully compensates for the fall in stroke volume.

Implications for training and competition

• Hydration: drinking to limit body mass loss reduces the drift, so heart rate stays lower at the same pace.

• Cooling: pre-cooling (ice slurry drinks, cooling vests) and exercising in the cooler part of the day reduce the demand for skin blood flow.

• Heart rate training zones: in long sessions, especially in the heat, heart rate rises even though the workload is constant. An athlete who holds heart rate steady will slow down. Pace, power or rating of perceived exertion may therefore be better guides in hot weather, and heart rate targets should be interpreted with the drift in mind.

• Testing: physiological tests that use heart rate (for example submaximal fitness tests) should be done in standard conditions and with athletes euhydrated, or drift will distort the results.

Hydration strategies

The aim of a hydration strategy is to start exercise euhydrated, to limit losses during exercise to a level that does not harm health or performance without gaining body mass, and to replace losses afterwards. Because sweat rates and sweat sodium vary so much between individuals, the most effective strategies are individual and are based on the athlete's own measurements.

Before exercise

Athletes should start exercise euhydrated with normal plasma sodium. If they are not, they can drink slowly for several hours before exercise, for example about 5–7 mL per kg of body mass about four hours before, so that excess fluid can be passed as urine before the start. A small amount of salt or salty food helps retain the fluid. A pale first-morning urine and stable morning body mass are simple checks.

During exercise

Two approaches are used. In a planned approach, the athlete knows their sweat rate from previous sessions and drinks a set volume to keep losses below about 2% of body mass without gaining mass. In a thirst-guided approach, the athlete drinks when thirsty, which works well for many recreational athletes in events of moderate length. Drinking is unnecessary in short sessions of less than about an hour in cool conditions.

Sodium becomes more important as duration increases, particularly when sweat losses are large or the athlete is a "salty sweater". Carbohydrate in drinks provides fuel in events longer than about an hour; carbohydrate fuelling is considered with the other nutritional strategies for carbohydrate and glycogen.

After exercise

Rehydration after exercise aims to replace both water and sodium. Because the kidneys excrete some of the fluid drunk, about 1.5 times the volume of body mass lost should be drunk over the following hours (1.5 L for every kilogram lost). Drinks or food containing sodium increase the fluid retained, and they also restore the thirst drive that plain water switches off too early.

Sports drinks: hypotonic, isotonic and hypertonic

Sports drinks are classed by their osmolarity compared with plasma, which depends mostly on their carbohydrate and electrolyte content.

An isotonic drink balances fluid and fuel, and the sodium and glucose it contains speed absorption in the small intestine. A hypotonic drink empties from the stomach quickly and its water is absorbed rapidly, but it supplies little fuel.

A hypertonic drink empties more slowly and can draw water into the intestine before it is absorbed, which can cause stomach discomfort during exercise. It is therefore used mainly after exercise or in small amounts during very long events.

Individual sweat testing

Teams and sports science services test athletes to find their sweat rate (by body mass change) and sweat sodium concentration (from absorbent patches on the skin or whole-body washdown). The results are used to set individual drinking volumes and sodium intakes. The method is only as good as its procedure. Patch measurements on the forearm or back can read considerably higher than whole-body sweat, and day-to-day variation is about 5–7% for sweat rate and larger for sodium, so repeated tests in similar conditions give the most useful guide.

Evaluation of hydration strategies

Planned drinking suits elite athletes in hot, long events with heavy sweating, who have measured their losses and have regular access to drinks. Drinking to thirst suits most recreational athletes, protects against hyponatraemia and is simple, but it may not keep up in very hot conditions or for athletes with high sweat rates.

Sports drinks benefit long sessions where fuel and sodium are needed, but they add energy that recreational exercisers, who are often exercising for weight management, may not need. The best strategy depends on the duration and intensity of exercise, the environment, the individual's sweat rate and sodium loss, the availability of drinks and the goal of the session.

How the environment changes fluid balance

The environment changes both how much fluid is lost and how much heat the body must remove. Its effects are summarised in the table; the physiology of heat, cold and altitude is considered in full under heat acclimatisation and acclimatisation.

Humidity has a particularly large effect, because sweat only cools the body when it evaporates. In hot, humid conditions, athletes may lose large volumes of sweat with little cooling benefit, which is why endurance events in such conditions are often moved to the early morning or evening.

Case study: hyponatraemia in the 2002 Boston Marathon

By the early 2000s, runners in mass-participation marathons had for years been encouraged to drink as much as they could tolerate, to avoid dehydration. At the same time, doctors were reporting runners who collapsed with low blood sodium, and some who died. A team of doctors from Boston Children's Hospital set out to measure how common hyponatraemia was in an ordinary marathon field and what predicted it.

What the study did

Runners entered in the 2002 Boston Marathon were recruited one or two days before the race. They completed a survey on their background and training, and were weighed before the race. At the finish line they gave a blood sample, were weighed again, and completed a questionnaire on how much they had drunk and how often they had passed urine during the race. Of 766 runners recruited, 488 provided a usable blood sample.

Sources: Almond et al., New England Journal of Medicine, 2005.

Explaining the findings

The pattern fits the mechanism of exercise-associated hyponatraemia closely. Gaining body mass during a race means that a runner has drunk more fluid than was lost in sweat, urine and breath, so total body water has risen. Because the extra fluid contained little or no sodium, the sodium already in the extracellular fluid was diluted. Slower runners are on the course longer, pass more drink stations and sweat less per hour because they run at a lower intensity, so it is much easier for them to drink more than they lose.

The finding that sports drinks did not protect runners also fits. Most sports drinks contain much less sodium than plasma, so a large excess of either drink dilutes plasma sodium. The link with female sex disappeared once body mass index and race time were taken into account, which suggests that women were at greater risk mainly because, on average, they were smaller and slower in this field, not because of their sex itself.

A larger study of 2,135 athletes in endurance events, published the same year, supported this explanation. Of the 231 athletes who gained weight during exercise, 19% finished with sodium between 129 and 135 mmol L⁻¹ and 11% below 129 mmol L⁻¹. The other 70% kept a normal sodium concentration despite gaining weight. Those authors concluded that hyponatraemia develops when athletes drink to excess and also fail to excrete the extra water, because ADH stays raised, or fail to release sodium from the body's internal stores.

What the case shows

This study and others like it changed race practice and drinking advice. Medical teams at large marathons now measure blood sodium before treating collapsed runners with fluid, and advice to runners has moved from "drink as much as possible" towards drinking according to thirst or a measured plan, and not gaining weight during a race. An international consensus statement on exercise-associated hyponatraemia in 2015 recommended using thirst to guide drinking, as a way to avoid overdrinking while still limiting dehydration.

For health, the case shows that a strategy designed to prevent one imbalance can cause another that is more dangerous. For performance, it shows that elite runners, who run fast and sweat heavily, face a different risk (dehydration) from slow recreational runners (overdrinking), so one drinking guideline cannot suit the whole field.

Limitations

• Fluid intake and urine output were self-reported after the race, so they are estimates that may be affected by memory.

• Only 64% of the runners recruited gave a usable sample, and volunteers may differ from the rest of the field.

• This is an observational study: it shows associations, not causes, although the link between weight gain and low sodium fits a well-understood mechanism.

• Conditions in one race (weather, drink-station spacing) may not apply to other events.

Sources: Almond et al., New England Journal of Medicine, 2005; Noakes et al., Proceedings of the National Academy of Sciences, 2005; Hew-Butler et al., Clinical Journal of Sport Medicine, 2015.

• Explain why a gain in body mass during a marathon is a warning sign of hyponatraemia rather than a sign of good hydration.

• Suggest why the risk of hyponatraemia is higher in a slow recreational runner than in an elite runner on the same course.

• Evaluate the use of self-reported fluid intake in this study, and suggest a more valid way to measure it.

Exam practice

Questions on water and electrolyte balance ask for definitions, the regulation loop, calculations of sweat rate and percentage body mass change, the interpretation of data from hydration studies, and evaluation of measures and drinking strategies.

Calculations follow a fixed pattern: change in body mass, corrected for fluid drunk and urine passed, divided by time.

Paper 1A-style questions

The next question tests the link between body mass and sodium.

Questions on cardiovascular drift usually test all three variables together.

Paper 1B-style data-based question

Eight endurance-trained cyclists each cycled for 2 hours at 62–67% of VO₂ max in a warm environment (33 °C, 50% relative humidity) on four occasions, in a random order. On each occasion they drank a different volume of fluid. The table shows the percentage of sweat loss that was replaced by drinking and the resulting loss of body mass.

Sources: Montain and Coyle, Journal of Applied Physiology, 1992.

The researchers also reported that the degree of dehydration after 2 hours was strongly correlated with the increase in oesophageal (core) temperature (r = 0.98), the increase in heart rate (r = 0.99) and the decline in stroke volume (r = 0.99).

Paper 2-style structured questions

Short structured questions often start with a definition or a "state" part and build up to an explanation.

Regulation questions reward each step of the feedback loop.

Calculation questions on hydration nearly always use body mass.

Explanation questions on cardiovascular drift need a mechanism, not a description.

Extended-response question

Summary

• Total body water is about 55–60% of body mass: about two-thirds is intracellular fluid and one-third extracellular fluid (interstitial fluid and plasma).

• Water transports substances, regulates temperature through sweating, lubricates joints and is the medium for metabolic reactions.

• Electrolytes such as sodium, potassium and chloride control fluid balance and nerve and muscle function; sodium is the main extracellular ion and potassium the main intracellular ion.

• Water moves between compartments by osmosis, from lower to higher osmolarity; normal plasma osmolality is about 275–290 mOsm kg⁻¹.

• Water is gained from drinks, food and metabolism, absorbed mainly in the small intestine with the large intestine recovering most of the rest, and lost by evaporation from the skin and respiratory tract and by excretion in urine and faeces.

• The hypothalamus detects a rise in plasma osmolarity, ADH from the posterior pituitary makes the kidneys reabsorb more water, and thirst increases; aldosterone controls sodium reabsorption.

• Dehydration is a water deficit; hypernatraemia is plasma sodium above about 145 mmol L⁻¹; hyponatraemia is plasma sodium below 135 mmol L⁻¹, usually from overdrinking in long events.

• Hydration can be measured by body mass change, urine colour, urine specific gravity or osmolality, thirst and blood sodium; each has different validity and practicality.

• Cardiovascular drift is the rise in heart rate and fall in stroke volume, with cardiac output maintained, during prolonged steady exercise, caused by fluid loss and rising core temperature.

• Effective hydration strategies are individual: start euhydrated, limit losses without gaining mass, replace water and sodium afterwards, and adjust for heat, humidity, altitude and cold.

Self-check

• To do:

  I can outline where water is stored in the body and describe the functions of water and of the main electrolytes.

• To do:

  I can describe osmosis and osmolarity and use them to predict the direction of water movement.

• To do:

  I can describe how water and electrolytes are gained, absorbed and lost, including the role of the large intestine.

• To do:

  I can explain how the hypothalamus, pituitary gland, ADH and kidneys regulate water balance, and the roles of aldosterone and thirst.

• To do:

  I can distinguish between dehydration, hypernatraemia and hyponatraemia and explain their effects on health and performance.

• To do:

  I can calculate sweat rate and percentage body mass loss, and evaluate methods of measuring hydration.

• To do:

  I can explain and sketch cardiovascular drift.

• To do:

  I can evaluate hydration strategies and sports drinks, and explain how the environment changes fluid needs.