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StillnessHealth fact

Drinking a very large amount of water in a short time can dilute the sodium in your blood to a dangerous level, a condition called hyponatraemia. Healthy kidneys can clear only about 0.8 to 1 litre of water an hour, so drinking much faster than that can overwhelm them.

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Sodium, the main salt in blood, keeps water balanced between the inside and the outside of cells. Normal blood levels are about 135 to 145 millimoles per litre, and anything below 135 counts as hyponatraemia. When the blood becomes too dilute, water moves into cells and they swell. Brain cells are most at risk because the skull leaves no room, so symptoms include headache, nausea, confusion and, in severe cases, seizures. The problem is rare in everyday life but well known in endurance sport. In a study of the 2002 Boston Marathon, published in 2005, 13% of the 488 runners tested at the finish had hyponatraemia, and the main risk factors were taking more than four hours to finish and putting on weight during the race, a sign of drinking more than was lost in sweat. International sports medicine guidelines now advise drinking according to thirst.

Türkçe oku
Kısa sürede çok fazla su içmek, kandaki sodyumu tehlikeli ölçüde seyreltebilir; bu duruma hiponatremi denir. Sağlıklı böbrekler saatte ancak 0,8-1 litre kadar suyu atabilir; çok daha hızlı içilen suya yetişemezler.

Kanın başlıca tuzu olan sodyum, hücrelerin içiyle dışı arasındaki su dengesini korur. Kandaki normal düzeyi litrede yaklaşık 135-145 milimoldür; 135'in altı hiponatremi sayılır. Kan fazla seyrelince su hücrelerin içine geçer ve hücreler şişer. En büyük risk beyin hücreleri içindir, çünkü kafatasında genişleyecek yer yoktur; bu yüzden baş ağrısı, bulantı, bilinç bulanıklığı ve ağır durumlarda nöbet görülebilir. Sorun gündelik hayatta nadirdir, ama dayanıklılık sporlarında iyi bilinir. 2002 Boston Maratonu üzerine yapılıp 2005'te yayımlanan bir çalışmada, bitiş çizgisinde kanı incelenen 488 koşucunun yüzde 13'ünde hiponatremi bulundu. Başlıca risk etkenleri, yarışı dört saatten uzun sürede bitirmek ve yarış sırasında kilo almaktı; kilo almak, koşucunun terle kaybettiğinden fazla su içtiğini gösterir. Uluslararası spor hekimliği kılavuzları bugün susadıkça içmeyi öneriyor.

Sources and further reading

Background reading: Hyponatremia
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