Written by Andy DuBois - http://www.mile27.com.au

Feb 18 2014

For many years we have been lead to believe that a dehydration level of more than 2% will negatively affect performance and therefore we should drink to limit dehydration to this level. This advice has even been given out by coaches and trainers for years and was the official recommendation of the American College of Sports Medicine in 2007.

Where this 2% rule came from no-one is really sure as there is no research to show that this is the case when applied to athletes in actual races. In fact there is an abundance of research to show the opposite.

A study(1) that looked at competitors in the South African Ironman showed there was a significant relationship between the degree of weight loss of competitors and performance time. The competitors that lost the most weight finished the fastest. The dehydration levels of the top athletes exceeded 7% .

Another study(2) looked at marathon finishing times and body weight loss and also concluded that the fastest finishers in a marathon were the most dehydrated.

If you look at what happens in longer races the same applies. Analysing results in 12 and 24 hour races researchers (3) found a linear relationship between weight loss and distance run. The more body weight lost the further the distance covered.

The same can be found in studies looking at the Rio Del Lago 100mile race (4) and the Marathon de Sables (5). Dehydration levels of up to 10% were reported in the fastest finishers.

In all of these studies the more dehydrated the faster the finish time. Clearly dehydration more than 2% isn’t detrimental to performance.

Dehydration and Core Temperature

It is also believed that hydration helps to prevent core temperature from rising yet several studies (6,7,8) have shown that running speed not percentage of dehydration was the determining factor in core body temperature

Is thirst a good indicator of fluid requirements?

We are often told that our sense of thirst is not sufficiently well tuned for us to rely on to determine our water intake. This may be true if we want to maintain zero to two percent dehydration but since performance isn’t hindered by dehydration levels this low, thirst can be used as the primary means of determining your water intake.

As Tim Noakes points out in his book Waterlogged every other living creature manages to use thirst as an indicator to regulate fluid balance so why would we be the only one incapable of doing this?

Should you replace all weight lost during a run?

The practice of weighing yourself before and after a workout to determine your fluid requirements is not only misleading it’s dangerous. A study (8) on the errors in estimating hydration status from changes in body mass concluded “body mass change is not always a reliable measure of changes in hydration status and substantial loss of mass may occur without an effective net negative fluid balance”

For example for every gram of glycogen in your muscles you need 3-4 grams of water to store it. As the glycogen is used for energy the water is released. This water wasn’t part of the water required for optimal function of tissues and cellular processes so doesn’t need to be replaced. If you burn up 400-500g of glyocgen thats 1.2-2 litres of weight loss you could suffer before even dehydrating even 1%.

Replacing 100% of your weight loss with water means you are effectively over-hydrating and the consequences of over-hydrating are far more severe than dehydration.

Thats not to say that you should deliberately try to dehydrate yourself just because the elite runners do. Elite runners have higher levels of dehydration because they are generating more heat since they are running faster and therefore have higher sweat rates. If you are working at a lower intensity you will sweat less and therefore wont be as dehydrated.

The point is that high levels of dehydration (5-10%) are NOT detrimental to performance or health and may even have a positive affect on performance due to reduction in body mass. It has been reported that Hallie Gebresallsie lost 10% of his body weight when he set his World Marathon Record. A weight loss of almost 5kg must have been advantageous in the later stages when fatigue set in.

Once again I’ll say that becoming that dehydrated isn’t the goal, its the consequence of drinking to thirst during a marathon, ironman or ultra-marathon and there are no side affects except becoming thirsty.

Of course if you ignore thirst or have no access to water when you are thirsty then dehydration can be a much bigger problem but in almost all endurance races around the world there is access to enough water to avoid becoming severely dehydrated.

Drink to thirst

The simple take home message is drink to thirst. Don’t impose a set amount of fluid to consume especially one based on replacing all lost weight with fluid.

If its hot drink more and if its cold drink less. Of course your thirst will tell you that very clearly. Listen to it.


1. Sharwood Collins, Goedecke, et al Weight changes, medical complications and performance in the South African Ironman Triathlon, Br.J Sport Med 2004

2.Cheuvront, Carter, Sawaka Fluid Balance and endurance exercise performance. Curr Sports Med Rep. 2003

3. Kao, Shyu, Yang et al. Athletic performance and serial weight changes during 12 and 24 hour ultra-marathons. Clin. J. Sports Med 2008

4. Lebus, Cassaza Hoffman et all. Can changes in body mass and total water accurately predict hyponatremia

5. Zouhal, Groussard Vincent et al. Athletic performance and weight changes during the ” marathon of Sands” in athletes well trained in endurance. Int J Sports Med 2009

6. Bryne, Lee et al. Continuous thermoregulatory responses to mass participation distance running in the heat Med. Sci. Sports Exer. 2006

7. Leo, Nio, Lim et al. Thermoregulation, pacing and fluid balance during mass participation dustance running in a warm humid environment Eur. J. APp. Physiol. 2010

8. Maughan, Shirreffs, Leiper. Errors in estimation of hydration status from changes in body mass. J Sports Sci 2008

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