General Information

Water is the most important nutrient in any diet – the so-called first limiting factor – meaning that a lack of it proves fatal long before starvation would. In many physical activities, water is also the most important performance-limiting factor. A drop in performance can already occur with a water loss of 2 % of body weight (1.5 l for a body weight of 75 kg). The level of basic fluid intake is guided by the extent of fluid losses. It amounts to roughly two to three litres of water per day for healthy adults. This does not yet account for offsetting any losses through sweat. Half of the basic intake is usually taken in through drinking, which is why recommendations for drinking are lower than those for total fluid intake.

The rest of the basic water intake is covered by solid food and by the so-called oxidation water produced during the metabolic breakdown of carbohydrates, fats and proteins (approx. 300 ml per day). Losses caused by sweating must be offset by adding the same amount on top of the basic intake.

Requirements

Various factors influence a healthy person's daily water requirement. For athletes, the most important factor is physical activity. Depending on its duration and intensity, thermoregulation causes you to sweat more or less, and this sweating increases your water requirement.

Other factors that influence water requirement are the temperature and humidity of the environment, body mass and diet. For example, a high carbohydrate or salt intake reduces fluid loss through urine, because both nutrients bind water in the body through different mechanisms. This means that fluid requirement rises in these cases.

Functions in the body

At just over 60% water makes up the largest share of body mass. An 80 kg

man therefore consists of roughly 48 kg (litres) of water. Water's functions are wide-ranging: it is an essential requirement for chemical reactions to take place, it acts as a solvent, as a transport medium, as a building block and as a regulator of heat balance (see thermoregulation). Water balance is regulated both inside and outside the cells.

Regulation outside the cells is controlled by hormones, while the cell regulates its own water balance (autoregulation). The main organ controlling water balance is the kidney. The bloodstream contains pressure and concentration sensors that control the release of hormones (ADH, aldosterone and others). These hormones determine how much water is excreted as urine via the kidney.

Fluid imbalance

Normal fluid losses that cannot be prevented are losses through urine, the skin, the lungs and bowel movements. The formation of urine, for example, is necessary because the body has to excrete metabolic waste products. A state of reduced body water (dehydration) can occur when fluid intake is lower than the unavoidable fluid losses. This reduces blood volume, blood flow to the skin, the sweat rate and the ability to release heat, while increasing core body temperature, which in turn causes the body to tire more quickly. This can therefore indirectly reduce performance. Sweat losses equivalent to 2% of body mass or more can cause a drop in performance. If total body fluid decreases further, saliva production and urine output decline, and at a dehydration level of around 5% blood pressure drops. Pulse rate and body temperature, on the other hand, rise. With larger losses of 10 % of body weight or more, there is a risk of severely health-threatening effects, up to and including death from the pronounced volume deficit.

In healthy people, dehydration is mainly caused by sweat losses. However, certain factors can also mean that fluid losses through breathing likewise reach very high levels. With low physical activity and optimal ambient conditions, fluid losses through the lungs amount to only around 0.4 litres per day. If the ambient temperature or relative humidity drops, these losses can increase significantly. In dry, cold air combined with high, sustained physical activity, maximum losses through breathing amount to around 2.5 litres per day.

Taking in too much fluid is difficult and can only happen if you drink an extremely large amount in a short space of time. In addition, during exertion lasting several hours with heavy sweat losses, a state of hyponatraemia can develop if you consume very large amounts of low-sodium fluid. Dizziness, feeling unwell, vomiting and increasing confusion up to unconsciousness are some of the symptoms of the so-called hyponatraemia that then develops: because of the high salt/sodium losses through sweat and the lack of replacement of this mineral through drinking low-sodium water, osmotic processes cause a shift of water from the extracellular space into the cells. This causes the cells to swell, including in the brain. Because the skull is a closed space, this quickly leads to a rise in intracranial pressure with the symptoms described. However, health problems occur far more often as a result of too little fluid intake than of too much.

Measuring water losses

Weighing yourself before and after activity is the simplest and most accurate method of measuring fluid losses during physical activity. It is best to do this naked, so that any remaining sweat in your clothing is also taken into account. Fluid losses correspond fairly precisely to the difference in body mass before and after the activity, although any fluid intake during the exertion must be factored in.

Thermoregulation

Water plays a central role in regulating heat balance (thermoregulation), which is especially important during physical exertion.

Muscle work is made possible by releasing energy stored in nutrients. However, only around 20 to 25% of this energy is converted directly into muscle work. The remaining 75 to 80% is converted into heat, and the longer and more intense the activity, the more heat is produced. The body overheats if this heat is not released. Sweating serves to prepare the body for cooling down. The sweat then evaporates from the surface of the skin, which requires heat that is drawn from the body. This cools the body down, although the body can only be cooled if the sweat actually evaporates. However, the body must contain enough water for sufficient sweat to be produced in the first place.