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When it comes to sport, the importance of optimal fluid intake is well known. However, an adequate supply is also crucial to maintaining good health.

Water is one of the essential nutrients and humans require a sufficient supply. Unlike with all other nutrients, humans can only survive for a very short time without water. It is therefore rather surprising that there are only rough estimates for fluid intake and no precise guidelines. 

However, there are good reasons why a generalised recommendation for fluid intake makes little sense. On the one hand, the need for fluid varies greatly individually, and on the other hand, the body has the ability to adapt relatively well to fluctuating water intake. It only becomes problematic when the control mechanisms responsible for this are thrown out of sync due to an insufficient – or excessive – fluid supply. In both cases, the consequences can be fatal.

Consumption depends on loss

The question of how much water a person needs is actually easy to answer: the intake must compensate for the loss every day. However, since this loss can vary greatly, a standard required quantity doesn’t make much sense. Nevertheless, in the context of water balance, most people typically lose two to three litres of fluid per day. For adults with an average body weight, minimal physical activity and cool to moderate ambient temperatures, this amount of loss is fairly accurate. However, as soon as you become physically active, increase your altitude or when warmer summer temperatures occur, these losses increase. 

Rule of thumb for basic intake

How do we get enough fluid? With a balanced, varied diet with many fresh foods, drinks deliver about one to one-and-a-half litres per day and solid foods about half to one litre. In addition, there is a good half-litre of "metabolic water", which is produced when the carbohydrates and fats in the body are broken down. This results in a daily fluid intake of two to three litres, which is approximately consistent for moderate temperatures and low physical activity without sweating – unless you are at altitude. Accordingly, it is commonly recommended to drink about one to two litres a day. 

However, with a physically active lifestyle, heat or altitude, the need increases. Sweat loss fluctuates between 0.1 and several litres per hour in the case of exercise lasting a few hours. And, at altitude in the Swiss Alps, you can lose up to an extra litre in a single day simply by exhaling water vapour. For a simulated climb to Mount Everest, the expected respiratory loss in addition to sweating is estimated at around two litres. 

What to drink?

For your basic daily intake, opt for drinks without sugar or artificial sweeteners. Water, ideally not from plastic bottles, is an obvious choice here but so are infusions such as coffee or tea. Fruit or vegetable juices also work well, even if they contain (natural) sugars. However, they should be seen as a supplement to other drinks rather than the only choice. 

It generally doesn’t matter if water is carbonated or not but it should be noted that different mineral waters contain greatly varying levels of minerals, especially calcium. An up-to-date table of mineral content can be found at www.mineralwasser.swiss (in German)

Drinking in sports

Sport has already experienced all possible drinking recommendations. From drinking nothing to drinking as much as possible, everything has been tried. For some years now, experts have agreed on at least one general aspect: The decrease in body weight due to sweating during exercise shouldn’t be too high, otherwise performance will decrease. 

On the other hand, there is no consensus on tolerable loss. If the latter exceeds two percent of your body weight, the risk of performance suffering also increases (especially during stop-and-go exercises). In the case of pure endurance exercise, however, we can tolerate slightly more, perhaps about three to four percent. In the case of an 80-kilogramme man, that would be around 2.5-3 kilogrammes of sweat loss.

What you drink and how often depends on your exercise goals. If the intensity or volume is low, performance does not matter, or the focus is on fat burning, then you should drink water and not carbohydrate-based sports drinks. You can then drink based on how thirsty you feel. 

However, if the opposite is the goal, i.e. at high intensities, larger volumes or when performance is decisive, then sports drinks are often advantageous because they supply not only fluids but also carbohydrates. The reasonable amount then moves between 0.4 and 0.8 litres per hour, which should ideally be adjusted in training sessions for different environmental conditions and drunk around every 15 minutes. 

Note: A predefined drinking plan with high drinking quantities can lead to a risk of "excessive drinking". This is especially possible with low sweat losses and longer loads, but can be easily checked on the basis of body weight. If your weight increases during exercise or remains the same (i.e. all conditions except weight loss), you have drunk too much during the workout. 

In the best case, this would be unhelpful and probably performance-reducing, in the worst case and if predisposed, however, this can be fatal due to blood dilution – known as hyponatremia. This can also happen outside of sports if the intake is higher than the amount of urine that can be excreted via the kidneys, i.e. more than 0.7 to 1.0 litres per hour is drunk without simultaneous sweat loss.

You can train to drink

In general, thirst in adults is a good control mechanism for maintaining a necessary fluid level in the body. You therefore don’t need to overthink your personal fluid requirements. The body takes good care of us here.

In old age, however, the sensation of thirst can decrease, including as a result of age-related diseases, and in older adults, insufficient fluid intake is not uncommon. Therefore, it is a good idea to consciously pay attention to a regular fluid intake before "old age" and get a good feeling of your thirst to train the right drinking habits.

*Nutritional expert Dr Paolo Colombani is a scientific consultant with his own company. Together with Dipl. Ing. ETH Christof Mannhart, he runs "Notabene Nutrition”, the online magazine with well-founded articles on foods, supplements & healthy living. www.colombani.ch; www.notabenenutrition.media