Summer is hot! Doctor runner unlocks the secrets of scientific fluid replenishment before and after exercise

Written by Fu Jingbo (Chief Surgeon of Peking University First Hospital, Deputy Leader of Beijing Physician Running Group)

Editor/Ji Jingjing

Image source/Visual China (except those with signature)

Long-distance running is often accompanied by humidity and high temperatures, which lead to a large loss of water and electrolyte ions in the human body. How should we assess the loss? How can we reasonably replenish water and electrolytes before and after exercise to maintain the body's normal water and electrolyte balance and avoid dehydration and heat stroke?

With these questions in mind, this article refers to the research results of institutions such as the Chinese Sports Science Association CSSA (Chinese Sports Science Association) and the American College of Sports Medicine MCSM (American College of Sports Medicine) for reference by running enthusiasts.

Serious consequences of dehydration and electrolyte imbalance

In a relatively cool environment, human body heat is mainly dissipated through thermal radiation and convection, while in a hot environment, sweat evaporation is the main way to dissipate human body heat. In addition to water loss during sweating, electrolytes contained in sweat are also lost. During continuous exercise, especially in a hot environment, if the lost water and electrolytes are not replenished in time, the human body is very likely to become dehydrated.

▲Water and electrolytes are very important to the human body. Source: Beijing Daily Client

In hot environments, dehydration of more than 2% of body weight can reduce aerobic performance and damage the central nervous system, resulting in cognitive/mental symptoms. In cold weather, dehydration of more than 3% of body weight will affect aerobic performance. Skeletal muscle cramps are also caused by dehydration, electrolyte deficiencies, and muscle fatigue. Women and the elderly are at higher risk for water and electrolyte imbalances during and after strenuous exercise.

Intense exercise and excessive exercise may cause rhabdomyolysis, and dehydration is one of the factors that promote rhabdomyolysis. Rhabdomyolysis is the destruction of muscle cells, which causes the leakage of substances in muscle cells into the blood, resulting in a large amount of myoglobin and causing renal dysfunction. Dehydration can also cause and aggravate acute renal failure caused by rhabdomyolysis. Among hospitalized heat stroke patients, 25% of those with dehydration and electrolyte disorders also had rhabdomyolysis and 13% suffered from acute renal failure.

▲The symptoms caused by rhabdomyolysis are quite extensive. Source: ihealth.com.tw

In 1988, at the Massachusetts Police Training School in the United States, 50 trainees performed gymnastics and running exercises in a hot environment. On the first day of training, the trainees were required to limit their drinking water. One of the trainees died of complications of exercise-induced heat stroke, rhabdomyolysis, renal failure, and liver failure while running. The other 13 trainees were also hospitalized for dehydration, rhabdomyolysis, and acute renal failure, and 6 of them had to undergo hemodialysis due to acute renal failure. All trainees experienced rhabdomyolysis to some extent.

Before, during, and after a marathon, athletes who run slowly, lose little sweat, consume excessive amounts of water and other hypotonic beverages, and are small and thin may develop exercise-induced hyponatremia.

When the blood sodium level is ≤125 mmol/L, the human body will have obvious symptoms such as headache, vomiting, swelling of hands and feet, restlessness, abnormal fatigue, confusion (progressive encephalopathy) and wheezing with sound (pulmonary edema). When the blood sodium level drops further, such as below 120 mmol/L, symptoms such as respiratory arrest, coma, permanent brain damage will occur, and even death in severe cases. The occurrence of exercise-induced hyponatremia is related to many factors, common factors are excessive consumption of hypotonic beverages and too little total sodium in the body. Women are at a higher risk of exercise-induced hyponatremia than men.

How to replenish fluids scientifically before and after exercise

So how should we replenish fluids scientifically before and after exercise, and when should we replenish them?

Daily nutritious diet is the basis for ensuring normal water and electrolyte balance in the human body. Generally speaking, consuming meat can help promote water replenishment.

Before and after exercise, you need to drink enough fluids (electrolyte drinks) to replenish the electrolytes (sodium and potassium) lost with sweat. Caffeine intake does not significantly change the daily urine output or water and electrolyte status. Alcohol intake increases urine output and delays the process of restoring water balance. It is not recommended to drink alcohol after exercise, or to achieve water balance before drinking in moderation.

1. Rehydrate before exercise

The purpose of pre-exercise rehydration is to make the body fully hydrated before the start of exercise and to keep the body at normal electrolyte levels. It is recommended that for high-intensity training or competition, water should be gradually replenished according to body weight (about 5-7 ml/kg body weight) starting 4 hours before exercise. If there is no urination afterwards, or the urine is dark in color, water should be gradually replenished according to body weight (about 3-5 ml/kg body weight) 2 hours before exercise. Drinking sodium-containing electrolyte water (20-50 mg/L) can better preserve the water you drink into the body. In general, it is more appropriate to drink liquids at 15℃~21℃, but temperature and taste preferences often vary greatly due to individual and dietary culture differences.

2. Rehydration during exercise

The goal of water replenishment during exercise is to prevent dehydration (avoid water loss exceeding 2% of body weight) and maintain electrolyte balance to prevent reduced exercise performance. The amount and rate of replenishment should be determined by the individual's sweat loss rate, the duration of exercise, and the opportunity to replenish water. When you drink water when you are thirsty, your body is already in a state of dehydration, so you should take a phased approach to replenishment during exercise (when conditions permit). Especially in sports that last more than 3 hours, you should plan the frequency and amount of replenishment. The longer the exercise lasts, the more obvious the cumulative effect of the imbalance between the body's demand for water and replenishment, which can lead to dehydration and dilutional hyponatremia in severe cases.

Differences in sport (metabolic capacity, duration, clothing and equipment) and weather conditions, as well as other factors (such as physical fitness, heat acclimatization and training level) will affect each person's sweat loss rate and sweat electrolyte concentration (see table below), so it is difficult to provide universal guidelines for water and electrolyte replacement.

As can be seen from the table above, under different conditions, the expected sweat rate is 400 ml ~ 1800 ml / hour. It is recommended that athletes who are training/competing can monitor their weight changes to estimate their sweat loss and then find a suitable fluid replacement rule for themselves.

If you exercise for a long time in hot weather, it is recommended that sports drinks contain at least 20-30 mg/L of sodium chloride, 2-5 mg/L of potassium ions, and 5%-10% of carbohydrates. Intake of carbohydrates helps maintain exercise intensity. For continuous high-intensity exercise of more than 1 hour or longer, replenish carbohydrates at a rate of about 30-60 grams/hour. Usually, 1 liter of sports drink (6%-8% carbohydrate concentration) is consumed per hour. Carbohydrates are preferably mixed sugars (such as energy gels, which contain glucose, sucrose, fructose and maltose, etc.), and the carbohydrate concentration generally does not exceed 8%. Too high a concentration of carbohydrates will affect the emptying rate of the gastrointestinal tract and easily cause stomach discomfort and vomiting.

For marathon runners with normal water balance before the race, some studies suggest that they can freely drink 400 ml to 800 ml of beverages per hour during exercise. The faster the athlete runs, the heavier the weight, and the hotter the climate, the more water should be replenished; otherwise, the less water should be replenished. For example, if an athlete with a pace of 8.5 to 15 kilometers per hour and a weight of 50 kilograms drinks 800 ml of water per hour, his weight will increase by 0.7% to 3.3%, which will lead to excessive water; while an athlete weighing 90 kilograms will lose 2.6% to 3.9% of his weight if he drinks 400 ml of water per hour, leading to excessive dehydration.

Therefore, an appropriate fluid replacement plan should be developed based on actual conditions to prevent excessive dehydration (weight loss less than 2% of baseline body weight).

3. Rehydrate after exercise

The goal of post-exercise water replenishment is to fully restore the body's water and electrolyte reserves. If water and electrolytes have been properly replenished during exercise, a normal diet (containing an appropriate amount of sodium) plus water after exercise is sufficient to meet the requirements of restoring reserves. The appropriate amount of sodium at this time helps stimulate fluid replenishment and retain the water you drink. Simply replenishing water will prevent the body from restoring a state of adequate water, and drinking too much water will also stimulate the production of too much urine, resulting in the goal of scientific fluid replenishment being counterproductive.

If you are already dehydrated after exercise (loss of more than 2% of your body weight), you should take active measures to rehydrate within 12 hours. If you want to quickly and completely restore your body's functions, it is recommended to replenish and drink at least about 1.5 liters of beverages (containing electrolytes) based on your own body weight. Excess water will be excreted from the body in the form of urine, so additional water is needed to compensate for the amount of urine excreted.

If you experience severe dehydration after exercise (loss of more than 7% of your body weight), accompanied by nausea, vomiting, or diarrhea, or if you are unable to drink beverages, you should seek medical attention immediately and receive intravenous fluids.

References:

1.Exercise and Fluid Replacement（MSSE, 39(3),2007）

2.Study on the development and activity of anti-fatigue sports drink（Journal of Food Safety and Quality, 12(9),2021）

3.www.acsm.org (American College of Sports Medicine (ACSM)

4.The temporal effect of different types of sugar-supplementing beverages on exercise capacity and corresponding metabolic parameters in human endurance exercise. Compilation of abstracts of the 11th National Sports Science Conference. (Chinese Sports Science Association, (3), 2019)