Most of us have been trained to be terrified of dehydration – in fact, beverage companies would have us believe that any time spent in a state of thirst is a near-death experience. Now the hysteria has flipped us too far in the opposite direction and dangerous overhydration – or hyponatremia – is a real problem for many athletes.


I never even knew over hydration could be a problem. After all, you hear so many new age health nuts talking about drinking three litres, four litres, seven litres of water a day and all they do is power yoga. Then I spoke to Dr Shane Brun, former President of Sports Doctors Australia and Associate Professor of Sports Medicine at James Cook University.

“Over-hydration is just as dangerous as under-hydration,” he told me. “We see it a lot in fun runs, where athletes collapse because of over hydration – in fact, nowadays we’re probably seeing more people who are over hydrating than under hydrating.”

Drinking too much – even water – can be deadly, leading to a drop in sodium concentration in your body fluids that has the potential to be fatal. Kelly Barrett at the Chicago marathon in 1998, Cynthia Lucero at the 2002 Boston marathon, Hilary Bellamy at the 2002 Marine Corps marathon in Washington, DC, David Rogers at the 2007 London marathon – all died from hyponatremia. At the 1999 Houston Marathon, four runners were hospitalized with comas due to hyponatremia and at the 2002 Boston Marathon, 13% of 488 runners tested were hyponatremic. Yep, it’s that problematic.

Now in some ultra-endurance events, runners are weighed at the start then again at checkpoints, and they are held for observation or treatment if they have gained weight (believed to be due to water retention). Even at a moderate level, over hydration leads to reduced performance because your blood has to soak up some of the excess water in an attempt to equalize your body’s salinity. Cells begin to swell, causing all sorts of drama, from gastro to dizziness (yes, just like dehydration!), soreness and several of other symptoms that do nothing to make you faster.


Dr Brun explained that you need know what your baseline body weight is – that is, you jump on the scales in your undies before you go out and train – then weigh yourself the same weigh when you finish.

“Whatever you’ve lost in that hour or so is going to be fluid – not fat or anything else,” Dr Brun said. “Every gram difference is one millilitre of fluid. So ideally, we keep people within 1-2% of their pre-activity weight. Up to 2% more or less, people can still function fine, but once you go over 2% in either direction, people’s performance is adversely affected.”

Slower people in longer events seem to be most at risk, because they might be sweating less and they have more time to over-drink without taking in enough sodium. Sodium (salt and chloride) is an essential electrolyte because it enables water to permeate cell walls so that water can be redistributed through the body, allowing muscles to work and organs to function.


Some groups of athletes are more prone to over hydration and/or dehydration. For example, masters and veteran athletes can have decreased thirst sensitivity once they’re dehydrated, which can make them less likely to rehydrate voluntarily. With age the kidneys also tend to have slower response to water and sodium loads, which can also put older athletes at greater risk of over hydration.

Women in general tend to have lower sweat rates than men, which can make female athletes more prone to over hydration. However the group most at risk are those athletes with paranoia about dehydration – the real danger comes when they’re over hydrated and think they’re dehydrated because they haven’t urinated during training/competition (another symptom common to both dehydration and over hydration) and so they drink even more.


Indications of over hydration might include bloating, a feeling of fullness or a ‘sloshing’ in your stomach, nausea, poor absorption of food (you need adequate sodium concentration for digestion) and vomiting.

Neurological signs are bad news and indicate it’s time to see a medic. These signs include dizziness, incoherence, disorientation, irritability and possibly headache.

There are visible physical warnings, too – hands and wrists get tight and puffy (skin usually looks dry and loose with dehydration), urination may eventually come in high volume and crystal clear, and you might shiver when it’s not that cold.


Taking salt tabs during a race as a preventative measure to offset sodium depletion or over drinking is not recommended, because if you have adequate levels of sodium but you’re carrying too much water, adding a lot more sodium is not good. A deficiency of water or sodium can be corrected within minutes, but correcting excesses of either can take hours (and an IV drip!). Better to be conservative with the sodium so that if you have too much water on board then a one-time dose of electrolytes, along with sitting or walking, may prompt urination that will dump excess water. If you are craving salt, eat a salty snack. Your body will often tell you what it needs – you don’t always need to anticipate it!

So if you do long events, try to work out your ‘sweat rate’:

Litres per hour

( kg bodyweight pre-training – kg bodyweight post-training ­

+ litres drunk during training )

hours of training

Be aware that extreme conditions can throw off your usual sweat rate. For example, very dry or windy conditions can fool you into dehydration because the sweat evaporates too fast to be seen.


Remember that mild dehydration – around 2% of your normal weight (i.e. [0.02 x kg bodyweight] x litre) is not dangerous or even detrimental to performance – and it’s easy to quickly correct.