Body Weight Changes During Endurance Events: An Accurate Indicator of Electrolyte Balance and Predictor of Performance?

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How body weight changes during a long-distance race. Body Weight Changes During Endurance Events: An Accurate Indicator of Electrolyte Balance and Predictor of Performance?

Previously thought to be rather clear-cut, the link between hydration and endurance performance recently came under scrutiny. While many of us grew up with the notion that exercise-induced dehydration resulting in a body weight loss of 2% or more impairs performance, recent research findings have demonstrated quite the opposite.

Proof is fast accumulating that, in real-world endurance racing conditions, exercise-induced body weight loss of more than 2% does not hinder performance. In fact, some studies have even found an inverse relationship between these two variables.

So what exactly is the latest on the issue? Are changes in body weight during an endurance event an accurate indicator of an athlete’s hydration status and electrolyte balance? And can this be used as an accurate predictor of endurance performance? Here, in short, are some of the latest findings.

A New Era: What Recent Research Findings Say

According to clinical professor of health sciences Martin Hoffman and associate professor of exercise science Tamara Hew-Butler, “we’ve got to get beyond the idea that achieving proper fluid and electrolyte balance is simply a matter of replacing the water and electrolytes that are lost in sweat”. They add that measuring your weight before and during an endurance event may not give an accurate indication of what is actually happening with blood electrolyte concentrations. Why? Because, in their words, “weight gain doesn’t equate to the development of hyponatremia, nor does weight loss eliminate the chance of hyponatremia”.

For instance, during some of the duo’s recent studies at 100-mile races, runners with body weight gains of more than four percent were found to have normal sodium levels. In contrast, some runners with a five to six percent body weight loss were found to be hyponatremic. It is therefore clear that body weight cannot be used as a sole measure for estimating electrolyte concentrations.

And what about other studies? Here are some highlights:

1. The impact of body weight loss on performance during a 161 km ultra

In a 2014 study co-conducted by Hoffman, 383 participants of a 161 km ultra-marathon underwent body weight measurements before, during and after the event. The majority (67%) of the study participants drank to thirst during the event, and most (95.6%) used a sodium supplement during at least one segment of the race. The findings? Out of the top-10 race finishers, half had lost more than 2% of their body weight after 90 kilometers. The research team, therefore, concluded that “weight loss greater than 2% does not necessarily have adverse consequences on performance”. They also added that, even in taxing environmental conditions (the maximum temperature during the race was 39 degrees Celcius), drinking beyond thirst is not required to maintain hydration.

2. Nutritional challenges faced by athletes participating in ultra-endurance running and walking events

A study published in Extreme Physiology & Medicine in 2016 looked at the nutritional challenges faced by athletes participating in ultra-endurance running and walking events. Through reviewing numerous recent publications on the subject, the author made the following statements regarding optimal hydration during such events:

  • Changes in body weight during and after an ultra event “do not provide an accurate indication of hydration status”.
  • Body weight loss of more than 2% during an endurance event does not necessarily have a negative impact on performance.
  • Body weight gain or loss during an ultra-distance event is caused by a number of highly complex processes within the body. As a result, measuring an ultra-endurance athlete’s body weight before and after an ultra-endurance event is not considered an effective strategy for determining his or her hydration levels and needs.
  • “To prevent over or underhydration, current available research suggests that the most suitable strategy to maintain hydration is to ‘drink to thirst’”.
  • Although as many as 90 to 96 percent of ultra-endurance runners use sodium supplements, “sodium supplementation beyond that taken in food and fluids, even when exercising in high ambient temperatures”, is not recommended.

  • Lastly, in order to maintain a normal state of body water content during an ultra-distance endurance event, it is recommended that an athlete achieves hydration balance by consuming 5–10 mL/kg body weight from water or carbohydrate-supplemented beverage two to four hours before the event.

Why is an exercise-induced body weight loss of more than 2% acceptable?

But why is an exercise-induced body weight loss of more than 2% regarded as acceptable by this new generation of researchers? Hoffman and Hew-Butler explain that water is stored with glycogen in the body, at a ratio of approximately three to one. When glycogen stores are then utilized during endurance exercise, a considerable amount of water is released along with it. If you, therefore, start an event properly hydrated and with good glycogen stores, it would be normal to lose about 2 to 3% of your body weight to maintain a stable hydration level. If, on the other hand, your body weight remains constant or increases, the chances are good that you’re taking in too many fluids.

A complex issue that requires more investigation

So while the latest research findings clearly challenge conventional wisdom on the matter, experts agree that even more research is needed. But, until such time as more light is shed on this complex issue, it appears that a) drinking to thirst and b) toeing the starting line in a properly hydrated state and with good glycogen stores should serve you well.


  1. M. Hoffman, MD & T. Hew-Butler, PhD, Weight change during a 100-mile race, Online publication
  2. E. Williamson, Nutritional implications for ultra-endurance walking and running events, Scientific journal
  3. E.D.B. Goulet, Dehydration and endurance performance in competitive athletes, Scientific journal
  4. M.D. Hoffman & K.J. Steumpfle, Hydration strategies, weight change and performance in a 161 km ultramarathon, Scientific journal