Hydration and electrolytes: understanding fluid balance

Water is the largest component of the human body, and even moderate water loss affects thermoregulation, the cardiovascular system, and performance. However, excessive drinking is also dangerous. The editors explain how to find a balance between dehydration and hyperhydration and why an athlete should think about sodium and potassium.

Water and electrolytes in the body

In an adult, water makes up approximately 50–60% of body weight, and it is much more in muscle tissue than in fat. Therefore, in muscular athletes, the total water content in the body is usually higher than in people with the same weight, but a higher percentage of fat.

Water is distributed between intracellular and extracellular space. The balance between them is maintained by electrolytes — primarily sodium (the main extracellular ion) and potassium (the main intracellular ion). The osmolality of the blood, which the body keeps within very narrow limits, depends on the concentration of sodium.

During physical work, up to 75–80% of energy is converted into heat. The main method of its removal, especially in the heat, is the evaporation of sweat. Together with sweat, the body loses water and electrolytes, primarily sodium and chlorine, to a lesser extent potassium, magnesium and calcium.

Water balance is regulated by thirst, antidiuretic hormone (vasopressin) and the renin–angiotensin–aldosterone system. In ordinary life, these mechanisms work accurately, but during long-term loads in the heat, thirst can lag behind real losses.

How dehydration affects the result

The ACSM position statement on fluid replacement (Sawka et al., 2007) indicates that a loss of more than 2% of body weight through water can impair aerobic performance and cognitive function, particularly in warm environments. In cool conditions, the sensitivity threshold is usually higher.

The mechanism is primarily related to the cardiovascular system: the volume of plasma decreases, the stroke volume of the heart falls, the heart rate and body temperature increase. The body is forced to "distribute" blood between the working muscles and the skin for cooling.

For strength and speed-power indicators, the effect of moderate dehydration is less pronounced than for endurance, but with significant losses, for example, when "cutting the weight" before weighing, strength, power and concentration also suffer.

A review by Cheuvront and Kenefick (2014) emphasizes that the combination of dehydration and heat stress is the worst case scenario. Additional risk factors are high humidity, insufficient acclimatization, tight clothing or equipment.

A separate problem is deliberate dehydration in martial arts and bodybuilding: sharp restriction of water, saunas, and diuretics. Such methods can lead to heatstroke, heart rhythm disturbances and acute kidney damage, and diuretics are also prohibited by WADA.

Individual calculation of needs

Sweat rates in athletes vary widely, from about 0.5 to over 2 liters per hour, depending on intensity, climate, and genetics (Baker, 2017). Therefore, the universal advice "drink 2 liters per day" says little about training needs.

The easiest way to estimate your own losses is to weigh yourself before and after training:

1. Weigh yourself without clothes before training, after using the toilet.
2. Record how much liquid you drank during the class.
3. After training, dry yourself with a towel and weigh yourself again without clothes.
4. Sweating rate (l/h) = (weight before − weight after + fluid consumed) / duration in hours.

Practical guidelines from the position of ACSM: about 4 hours before training, drink about 5-7 ml/kg of liquid; while working, drink so that weight loss does not exceed 2%; after - gradually replace the losses, usually drinking about 1.25-1.5 liters for each kilogram of lost weight, because part of the liquid is excreted with urine.

A simple daily indicator is the color of urine. A light straw color indicates adequate hydration, a dark one indicates a lack of it (the urine color scale was described by Armstrong et al., 1994). Note that B vitamins and some foods can change color regardless of hydration.

Sodium, potassium and sports drinks

The concentration of sodium in sweat varies several times between people. There are so-called "salty" sweat eliminators, on the clothes of which white stains remain after training. Sodium replacement is especially important for them during long sessions.

The sodium in the post-workout drink helps the body retain the fluid it drinks: without it, part of the water is quickly excreted by the kidneys. Therefore, ordinary food with a sufficient amount of salt is often no worse than special drinks.

Potassium comes mainly from food - vegetables, fruits, potatoes, legumes, dairy products. Losses of potassium through sweat are relatively small, so for most athletes it does not need to be specially supplemented. High doses of potassium supplements can be dangerous, especially with kidney disease.

Muscle cramps are often associated with electrolytes, but current evidence suggests that their causes are multifactorial: muscle fatigue and neuromuscular control play as much a role as salt loss.

Hyponatremia: the danger of excessive drinking

Exercise-related hyponatremia is a decrease in blood sodium concentration below 135 mmol/L during or after prolonged exercise. The main reason is excessive drinking of water or hypotonic drinks in excess of sweat losses.

A study of participants in the Boston Marathon (Almond et al., 2005) revealed hyponatremia in 13% of the examined finishers. The strongest risk factor was weight gain during the race, meaning more was drunk than lost.

Mild forms are manifested by nausea, headache, edema, disorientation. Severe forms can lead to brain swelling, seizures, coma and death. The exercise hyponatremia consensus (Hew-Butler et al., 2015) recommends drinking according to thirst and avoiding weight gain during competition.

Groups at increased risk are slow participants in marathons and ultramarathons, people with low body weight, those who drink "at every point", despite the lack of thirst. For them, an excess of water is more dangerous than moderate dehydration.

Editorial conclusion

The goal of hydration is not to "drink as much as possible", but to keep losses within about 2% of body weight and not gain weight due to fluid during exercise.

Individual sweating rate is easily determined by weighing before and after training; this is the best basis for a personalized drinking plan.

Sodium is needed during long work and for recovery after heavy sweating. Hyponatremia is a real threat that can be fatal.

We also recommend reading our articles "Nutrition before and after training", "Carbohydrates and glycogen recovery" and "Micronutrients for men's health".

The material is exclusively informative and does not replace a doctor's consultation. In case of diseases of the heart, kidneys, hypertension or taking diuretics, the drinking and salt consumption regime is determined by the doctor.

References

1. American College of Sports Medicine; Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–390.
2. Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Compr Physiol. 2014;4(1):257–285.
3. Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Med. 2017;47(Suppl 1):111–128.
4. Almond CSD, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. 2005;352(15):1550–1556.
5. Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med. 2015;25(4):303–320.
6. Armstrong LE, Maresh CM, Castellani JW, et al. Urinary indices of hydration status. Int J Sport Nutr. 1994;4(3):265–279.
7. World Anti-Doping Agency. The World Anti-Doping Code International Standard: Prohibited List (S5. Diuretics and masking agents). Montreal: WADA; current edition.