IB Sports, exercise and health science A.2.1.2 Imbalances in Water and Electrolyte Balance and Their Effects on Exercise Performance Notes

1. Muscle contraction during exercise generates a large amount of metabolic heat, most of which is lost through evaporation of sweat from the skin.
2. As a result, sweat losses are far greater during exercise or in a warm climate than they are at rest, and fluid intake must rise in step to maintain balance.
3. When the body loses large volumes of water, the balance between water and electrolytes is also disrupted because electrolytes leave the body in sweat alongside the water.

Factors That Determine Sweat Rate

Five factors set how much sweat an athlete produces during exercise.

1. Environmental temperature: hotter air means the body must lose more heat to stay at 37°C, so sweat output rises.
2. Humidity: higher humidity slows evaporation, so the body sweats more in an attempt to lose the same amount of heat. Fluid is lost without the matching cooling benefit.
3. Air velocity: still air slows evaporation. A breeze or a tailwind speeds it up and lowers the sweat load needed for cooling.
4. Body size: larger athletes produce more total metabolic heat at a given pace and have more surface area to sweat from.
5. Metabolic rate: working harder generates more heat, so faster running and higher cycling power both push sweat rate up.

How Exercise Disturbs Water and Electrolyte Balance

1. At the onset of exercise, water moves from the plasma into the interstitial and intracellular spaces of the working muscle.
2. The amount that shifts depends on how much muscle is active and how intense the effort is.
   1. Driver 1: metabolic by-products inside the working muscle raise local osmotic pressure, so water diffuses passively from plasma into the muscle.
   2. Driver 2: the rise in blood pressure during exercise forces water out of the bloodstream, and sweating drains plasma directly.
   3. Net result: plasma volume falls during exercise.

Consequences of Imbalance

1. Dehydration

Dehydration occurs when the body loses more water than it consumes, leading to a reduction in blood plasma volume, impaired thermoregulation, and decreased oxygen delivery to tissues.

Causes of Dehydration

1. Increased sweating due to high temperatures results in significant water and electrolyte loss.
2. Intense physical exercise leads to excessive sweating, which depletes fluid and sodium reserves.
3. Gastrointestinal illnesses, including vomiting and diarrhea, cause rapid fluid and electrolyte depletion.
4. Inadequate fluid intake throughout the day prevents proper rehydration and maintenance of fluid balance.
5. Diuretic use from medications, alcohol, or caffeine can lead to excessive urine production, reducing fluid levels.

Effects of Dehydration on Performance and Health

1. Decreased blood volume results in lower stroke volume and cardiac output, reducing oxygen and nutrient delivery to muscles.
2. Reduced thermoregulation impairs the body's ability to release heat through sweating, increasing core temperature.
3. Diminished endurance and strength occur due to inadequate fluid availability for muscle function.
4. Increased heart rate results from compensatory mechanisms as the heart works harder to maintain circulation.
5. Cognitive impairment affects reaction time, focus, and decision-making abilities.

Symptoms of Dehydration

1. Mild to moderate dehydration presents as thirst, dry mouth, reduced urine output, headaches, and dizziness.
2. Severe dehydration results in confusion, rapid heartbeat, dangerously low blood pressure, fainting, and, in extreme cases, organ failure.

2. Hyponatremia

1. Hyponatremia is a plasma sodium concentration below 135 mmol/L (normal range is 135 to 145 mmol/L).
2. In sport, the dominant cause is drinking too much fluid with too little sodium during long-duration exercise. Some affected athletes actually gain weight during the event.
3. Severe symptoms arise because as ECF sodium drops, water moves into cells by osmosis.
4. Brain cells swell inside the rigid skull, causing seizures, coma, and potentially death if untreated.

Causes of Hyponatremia

1. Overhydration (water intoxication): drinking excessive water without replacing lost electrolytes is the most common cause of exercise-associated hyponatremia.
2. Excessive sweating leads to sodium loss, particularly in endurance athletes who do not replace electrolytes.
3. Kidney disorders can impair the body's ability to regulate sodium concentration.
4. Hormonal imbalances, such as excess antidiuretic hormone (ADH) secretion, cause water retention and sodium dilution.

Who Is at Greatest Risk

Hyponatremia is most often seen in endurance sport, but prevalence varies sharply with five factors.

1. Duration of the event: risk is low in a marathon and high in an ultramarathon.
2. Sport discipline: rare in cycling, frequent in running and triathlon, and very frequent in swimming.
3. Biological sex: increased risk in biological females, with several reported deaths in endurance events.
4. Ambient temperature: risk increases in hot temperatures, where athletes tend to drink more.
5. Country of competition: very common in the USA, much rarer in Europe, and almost never reported in Africa, Asia, or Oceania (Knechtle et al., 2019).

Symptoms of Hyponatremia

1. Mild cases present with nausea, headache, bloating, and fatigue.
2. Moderate cases result in confusion, dizziness, and difficulty concentrating.
3. Severe hyponatremia can cause brain swelling, seizures, respiratory distress, and coma.

3. Hypernatremia

1. Hypernatremia is a plasma sodium concentration above 145 mmol/L, reflecting a deficit of total body water relative to total body sodium.
2. It arises in two situations in sport: dehydration leading to a hypohydration state, or excessive sodium intake from the diet.
3. In both cases ECF sodium concentration rises, water moves out of cells along the osmotic gradient, and the cells themselves become dehydrated.
   1. Timing in sport: hypernatremia tends to occur during or up to 4 hours after prolonged physical activity.
   2. Diagnosis: symptoms overlap heavily with hyponatremia, so the only reliable way to tell them apart is a blood sodium reading.

Causes of Hypernatremia

1. Dehydration due to inadequate water intake causes a relative increase in sodium concentration in the blood.
2. Excessive sweating results in water loss without sufficient electrolyte replacement, increasing sodium levels.
3. High dietary sodium intake without enough water can contribute to hypernatremia.
4. Kidney dysfunction affects the body's ability to regulate sodium balance, leading to excessive retention.
5. Diabetes insipidus: massive urine output that drives sodium concentration up if water is not replaced.

Symptoms of Hypernatremia

1. Early symptoms include extreme thirst, dry mouth, muscle weakness, and irritability.
2. Moderate cases present with headache, confusion, and restlessness.
3. Severe hypernatremia may result in high blood pressure, seizures, and coma.

Cardiovascular Drift

1. During prolonged steady-state submaximal exercise (walking, cycling, or running held at the same pace for an hour or more), heart rate slowly creeps up while stroke volume slowly falls.
2. This phenomenon is called cardiovascular drift, and it shows up in both thermoneutral and hot environments.
3. The cause is the combination of water loss from the body (mainly through sweat) and a rise in core body temperature.
4. Both reduce circulating plasma volume.
5. Stroke volume falls because there is less blood to push per beat, and heart rate has to climb to try to maintain cardiac output.
6. Mean arterial blood pressure also declines slightly over time because the rise in heart rate cannot fully cover for the fall in stroke volume.

Mechanism in Detail

1. Stroke volume falls: as the body loses fluid and core temperature increases, less blood is available to pump per beat and blood is shifted to the skin for cooling, so stroke volume decreases.
2. Heart rate rises: the body compensates for the drop in stroke volume by raising heart rate to defend cardiac output.
3. Mean arterial pressure declines slightly: the rise in heart rate cannot fully cover for the fall in stroke volume, so blood pressure also drifts down over time.
4. Performance cost: oxygen delivery to muscles becomes less efficient, muscle fatigue increases, and it becomes harder for the body to sustain exercise at the same intensity.

Hydration, Cooling, and Pacing Reduce Cardiovascular Drift

Three groups of strategies help athletes limit cardiovascular drift during prolonged exercise.

1. Hydration: adequate fluid intake before, during, and after exercise helps maintain blood volume and prevent dehydration.
2. Electrolyte replacement: sports drinks containing sodium, potassium, and chloride help maintain osmolarity and prevent the drift caused by electrolyte imbalance.
3. External cooling: cooling vests, ice towels, and cooling sprays help manage core temperature and reduce the sweat rate needed for thermoregulation.
4. Environmental timing: in hot conditions, athletes may train during cooler parts of the day or seek shaded areas to reduce the risk of overheating.
5. Pacing and rest: adjusting intensity or including rest periods limits heat build-up and fluid loss, helping preserve performance.