UPWr Expert's advise – what to drink during heatwaves

Dr hab. Robert Gajda from the Department of Human Nutrition explains that the body’s need for fluids depends on the cause of dehydration.

– If the outside temperature is high and you are also engaging in physical activity, hydrating the body with just spring water, low-mineralized mineral water, herbal teas, or infusions may be insufficient. This is because you could be experiencing isotonic or hypotonic dehydration – explains dr Gajda. He clarifies that in such cases, the body loses not only water but also electrolytes. He reminds us that most of the water loss — especially in high temperatures and during physical exertion — occurs through the skin, rather than the urinary or respiratory systems.

During heatwaves, athletes, physically active individuals, children, and the elderly are particularly at risk of dehydration.

Generally, under optimal environmental conditions for the human body — meaning lower temperatures and low physical activity — we typically encounter hypertonic dehydration. In this case, we mainly lose water through the urinary tract. This type of dehydration is usually caused by insufficient fluid intake but can also result from fever and excessive sweating. In such situations, it’s recommended to drink fluids with low electrolyte content, such as spring water, low-mineralized mineral water, unsweetened weak tea, or herbal infusions. It’s also important to remember that excessive intake of extracellular electrolytes, especially sodium, is not advisable, so it’s best to avoid foods high in salt.

– With isotonic dehydration, where we lose water and electrolytes from the body in proportional amounts — whether through the urinary tract, such as when taking diuretic medications (for hypertension) or with certain kidney diseases, through the skin with extensive burns, or through the gastrointestinal tract during vomiting or diarrhea — we need to hydrate the body with moderately mineralized water frequently and in small portions. It’s also necessary to include isotonic drinks, i.e., fluids with electrolytes (mainly sodium, magnesium, potassium).

– But when we’re dealing with high ambient temperatures, as is often the case during extreme heatwaves, combined with intense or prolonged physical exertion, the focus must be on consuming large amounts of fluids and electrolytes simultaneously, as the consequences for the body can be severe. This is when we face hypotonic dehydration, with a significant loss of water from the body (sometimes even several liters per day) and a disproportionately large loss of electrolytes, especially sodium. In such situations, hydration should be planned – explains Dr. Gajda, noting that hypotonic dehydration is caused by high ambient temperatures, intense or prolonged physical activity, but also vomiting, diarrhea, or burns with a predominance of electrolyte loss.

In cases of hypotonic dehydration, it is important to consume highly mineralized water, but special attention should be given to hydrating fluids — such as isotonic drinks or electrolyte preparations dissolved in water. An alternative to isotonic drinks could be diluted fresh fruit juices or skim milk. With significant electrolyte loss, a well-balanced diet rich in fruits and vegetables, as sources of potassium and magnesium, is also crucial. Salt intake should not be particularly limited, as long as it is not consumed in excess.

– Professional athletes are particularly vulnerable to hypotonic dehydration, but young children and the elderly are also at risk during hot days – emphasizes the UPWr expert.

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