How much dehydration is too much?
Hello! Last week we talked about hyponatremia (which is a dangerous and potentially deadly condition that arises when sodium concentration levels get too low in the body).
Today we are going to talk about dehydration. It’s normal to dehydrate during long events and long training sessions but how much is actually too much?
Not long ago the general advice during athletic events was to drink ahead of thirst.
These guidelines did not consider the fact that, during long events, if one were to replenish every ml of fluid loss it would be an enormous volume of liquid to take (more than the body is prepared to metabolize at once).
In that matter, the body can cope with a certain level of dehydration (or we would not be here, considering our antecessors hunters in the African desert).
That mindset (drink, drink, drink ahead of thirst) - has been blamed for the rising hyponatremia cases seen in amateur sports.
A study conducted at Ironman Frankfurt a few years ago found that as many as 10% of finishers had hyponatremia to some extent, which could have impacted their performance.
Due to the increasing rate of hyponatremia the guidelines from ACSM (American College of Sports Medicine) were later revised to;
“The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance.”
These new guidelines were based on studies about dehydration and performance. The problem with these studies was that most did not take into account real world conditions such as wind (cooling effect), pre event dehydration, supplements that could eventually act as diuretics, among other factors.
In spite of that, it’s common sense today that during events and long training sessions a certain level of dehydration does occur and it’s ok.
A 2013 study at the 100 mile Western States ultra-marathon running race showed that bodyweight changes of 1-6% were reported in the top finishers with the overall winning time of 14 to 15 hours.
Nowadays is common practice to measure fluid loss by the weight difference from before and after workouts or events. Most of the time it’s ok but for long workouts or races it may not work too well as one would expect (depending on the length and intensity of the race or workout to see some weight loss as well). The body uses carbohydrates, fat and protein (to a less extent).
Many authors talk about a 1L fluid replacement per Kg lost. The math can sometimes get confusing due to many factors such as glycogen levels (depending on muscle mass the body can store about 500g of glycogen, which is the carbohydrate stored in the liver and muscles. These sotores account to about 1-3g of water per gram of glycogen) and pre race and workout bodyweight.
Most people, during tapering, increase carbohydrate consumption and sodium. It’s something to think about when evaluating the data based on training sessions only (it’s not common to prep that way before training sessions).
Obviously there’s a great deal of difference between athletes (as far as how much dehydration is safe), assuming they are properly hydrated before the event. Many factors come into place like race condition, training. The range has been about 2-3% weight loss.
There are many adaptations going on in the body during harsh climate conditions, the body usually adapts the sodium concentration in the urine according to it’s concentration in the blood but the sweat adaptation usually does not follow the same speed.
A recent study in Spain looked at fluid and sodium intake during a hot half-Ironman event in 2015. Their data showed that athletes who took in more sodium tended to lose less bodyweight in percentage terms than a control group who only took a placebo.
In fact it also showed a positive correlation between increased performance and losing less bodyweight overall by using sodium supplementation to absorb and retain more fluid.
The more recent general guidelines suggest taking between 300-400ml per half an hour or drinking on demand according to your thirst.
There are a few important points to consider:
Thank you for your attention! If you lost the post about hyponatremia click here.