Physical activity and fluid supply
J. Tina Sentočnik: Water is not enough!
We often discuss the most appropriate nutrition for individual sports activities and ideal menus that should enable the best results for recreational and top athletes. But we tend to forget about proper hydration during activity, just like we often forget about adequate fluid supply in healthy adults during normal daily activities. Of course, we don't have to drink liters of water and force fluids, but it's wise to remember that at least 70 percent of adult women live on the edge of dehydration!
Water reserves in the body
Water is key for metabolic processes in the body. It enables the transport of substances needed for growth and energy production, which arises from the exchange of substances between organs and the external environment.
Water is the largest "department" in the body, accounting for 45 to 70 percent of total body weight. Even muscles contain 70 to 75 percent water, while fat tissue contains 10 to 15 percent. Thus, athletes with a lot of muscle mass have relatively a lot of water in the body, since they have little fat tissue.
If we drink normally, we have a stable amount of water in the body; due to kidney function, which excretes all excess, we cannot accumulate water in the body or have it "in reserve". Dehydration occurs if we consume too little water; the water content decreases both in cells and outside cells. Two-thirds of the total body water is in the cells (about 30 liters).
Water in the body and physical activity
During physical activity, muscles contract, thus performing muscle work, and the result is accumulation of metabolic products in muscle cells. This causes increased water entry into cells. Later, cell membrane permeability changes, metabolic products and later water exit the cells, and blood pressure increases. Due to increased blood flow to muscles during physical activity, muscle volume increases. During intense anaerobic muscle work, blood flow to muscles increases the most, as a large amount of lactate (lactic acid) is produced in them, which then accumulates in the muscles.
Dehydration
If fluid intake during physical activity is too low, hemoconcentration or blood thickening occurs. The phenomenon is more pronounced if we lose a lot of sweat during activity. During physical activity, we also lose water through breathing. Depending on exercise intensity, fitness, external temperature and air humidity, and body weight, the amount of water lost through sweating varies – from a few hundred milliliters to two liters per hour!
If the amount of blood plasma decreases significantly due to dehydration, blood flow to working muscles will decrease, and consequently oxygen supply and substances needed for muscle work. Waste removal from working muscle will decrease, as well as from the whole body, since they won't reach the kidneys. Then muscle capacity for work decreases and premature muscle fatigue occurs. The body overheats excessively, because lower blood flow does not remove heat from the working muscle. Consequences can be severe: dehydration is followed by fatigue, overheating, and possibly heat collapse. This of course happens only in case of inadequate fluid intake during activity.
It must be known that fluid losses from sweating should not be replaced only with plain water. In marathon runners, cases of so-called water poisoning are common; replacing sweat by drinking pure water results in decreased sodium levels in body fluids, and the worst complication is coma.
Rehydration
Thirst and drinking depend on food and especially on the amount of sodium (salt) in food. The most obvious sign of thirst is dry mouth. We often don't feel thirst or feel it only when we are already dehydrated.
Every day we should consume at least as much fluid as we excrete – about 4 percent of body weight (for healthy adults). Physical activity greatly increases "turnover" of fluid in the body, and temperature, air humidity, and altitude also affect it.
Taking all this into account, a 70 kg man needs 1.5 to 2.0 l of fluid per day for normal kidney function.
People doing sedentary work need 1 ml of water per kilocalorie of their daily calorie needs; a person who should consume 1800 kcal per day to maintain body weight should thus drink 1.8 l of fluid per day.
The same applies during sports activity: an athlete or recreational athlete who will use 6000 kcal during a several-hour intense cycling race should consume 6 liters of fluid. A marathon runner uses about 3000 kcal, so must drink 3 liters during the run.
Expert recommendations (National Research Council) on daily electrolyte needs state: a healthy adult needs 500 mg sodium, 750 mg chloride, and 2000 mg potassium. With ingested food we consume much more of these key electrolytes, so no need to add them to food or replace with supplements. But this is necessary in case of very increased needs or electrolyte loss (really heavy sweating, diarrhea ...).
Classic rehydration solution with combination of electrolytes and carbohydrates
Recommended composition
Carbohydrates: 30 to 100 g/l
Sodium: max 1100 mg/l
Osmolality: < 500 mosmol/l
(recommended: isoosmolality)
Recommended carbohydrates
Fructose, sucrose, glucose, maltose,
maltodextrin, soluble starch
Possible additives (optional)
Chloride: max 1500 mg/l
Potassium: 225 mg/l
Magnesium: 100 mg/l
Golden rules of rehydration
1. Fluid and electrolytes are important for maintaining fluid balance during physical activity.
2. Loss of body fluids through sweating and breathing during activity results in smaller blood volume and lower flow through working muscles. Inadequate fluid replacement during intense activity can have severe consequences: heat stroke and collapse.
3. Dehydration (fluid loss) exceeding 1.5 liters reduces oxygen supply to muscles to the extent that muscle fatigue and intestinal complications occur.
4. Rehydration with rehydration solutions containing electrolytes and carbohydrates in addition to water is more appropriate than with plain water
5. The most important electrolyte in rehydration solutions is sodium, which acts rehydrating and also reduces water loss through urine. Adding other electrolytes to rehydration solutions is not mandatory.
Rehydration solutions should be isotonic and not hypertonic.
J. Tina Sentočnik, MD, specialist in internal medicine