Too much water could be dangerous


We grew up with the notion that doctors’ standard advice to drink eight glasses of water per day is cast in stone. For years we also believed that we – and particularly runners, walkers, hikers, sportspeople and recreational exercisers - should drink water even before we are thirsty, because by the time we feel truly thirsty, we are already quite dehydrated.

People believed that dehydration was the single greatest risk to the health of runners because it would cause the body temperature to rise, leading to heat illness, including heat stroke.

Now, almost twenty years after Prof Tim Noakes of the Sport Science Institute of South Africa and the University of Cape Town first challenged this line of thinking, other experts have come to recognise that he was right after all.

A closer look at the three major findings that lead to paradigm shifts can help you to understand and prevent dehydration and heat illness.

1. The intensity of the exercise leads to the rise in body temperature – not the lack of water

According to Noakes, newer scientific research has revealed that dehydration is not the major factor in hyperthermia or heat illness. In fact, heat stroke can occur even when the athlete is well-hydrated.

Who are at risk to develop heat illness?

  • People running as fast as possible, even if they are adequately hydrated. The faster the person runs, the more heat is generated by his body, and this happens particularly in shorter distance races, like 10 km runs
  • Heavier and fatter runners, walkers and hikers. The fatter and heavier the person, the more heat will be generated by his body
  • Running, walking or hiking on hot, humid and windless days. In these conditions the cooling effect on the skin is less and these conditions attribute to heat stroke.

Prevent your risk of heat stroke by slowing your pace as heat and humidity rise, regardless of how much you may be drinking.

2. Don’t worry too much about dehydration

Becoming dehydrated to the extent that modern runners usually do, carries no documented medical risk. You get a bit thirsty and have a dry mouth, but that's it, says Noakes.

According to Noakes, the body is adapted for conditions of mild dehydration. “We evolved from hunters – we had to run and chase animals on the hot African plains. We didn't have time to pause for a drink! Physiologists who did not understand either humans’ prehistory or the history of running then came along with the unproven hypothesis that to become even the slightest bit dehydrated during exercise would kill you. And then the sports drinks industry in the United States used this bad, indeed non-existent science to market their products."

3. Drinking too much water can do more harm than good to athletes doing prolonged exercise. In fact it can be fatal!

Doctors at major marathons around the world are hospitalising more people as a result of over-drinking, and even some deaths from hyponatremia, commonly known as water intoxication. This condition is characterised by too much water in the blood and too low levels of sodium (less than 130 mmol/L). These athletes didn't need any salt to make them better, so they weren't salt deficient, but purely water overloaded, Noakes says.

Who are most at risk?

Recognise the signs of water intoxication

It causes the body - and brain - to swell. The pressure of the brain against the skull increases, leading to convulsions, heart failure and cessation of breathing.

Water intoxication can mimic the signs of heat stroke such as nausea, vomiting, extreme fatigue, respiratory distress, dizziness, confusion, disorientation, coma and seizures.

Watch out for:

  • a progressively worsening headache
  • body temperature rising higher than 39.9 degrees C during or immediately after exercise
  • swelling of the hands and feet (wedding bands, watches and shoes may feel tight)
  • coughing pink, frothy sputum.

How to prevent heat illness and/or water intoxication

  • Runners should be sensitive to the onset of thirst as the signal to drink, rather than staying ahead of thirst.
  • Elite runners should drink between 200 ml to 800 ml per hour, depending on individual requirements. Slower runners are less likely to “overheat” and should only drink when really thirsty.
  • There is no need to take salt before, during or after exercise, says Noakes. All Western athletes have too much salt in their diets, and the reason the salt appears in their sweat is because the body is trying to get rid of it.
  • Pain-relievers such as paracetamol, aspirin and non-steroidal anti-inflammatory drugs (NSAIDS, such as ibuprofen) may contribute to developing a water overload. Discuss this with your doctor if you use this medication for a condition. Otherwise, avoid them before and during your long walks.
  • Women should take extra care because they appear to be more vulnerable to water intoxication than men. This may be due to the fact that women are smaller and need even less fluid, and they generally run slower and therefore don't sweat as much as heavier men running even at the same speed. Women also tend to follow advice too well and subsequently drink too much water, Noakes says.

1. British Medical Journal (2003) 327;113-114
2. Sports Science Exchange 88 (2003) 16(1). GSSI
3. The Physician and Sportsmedicine (2000) 28(9); 71-76
4. Medicine and Science in Sports and Exercise (1985): 17(4); 456-61
5. Journal of Applied Physiology (2001) 90; 1057-1064
6. The Physician and Sportsmedicine (2003) 31(7)
7. IMMDA-AIMS Advisory Statement by IAAF (2002) 17(1);15-24

(article updated October 2011)

Photo:  Glass of water from Shutterstock

Read more:

How much fluid do you need?
Proper hydration during exercise
How much should we drink?

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