Weight Change During a 100-Mile Race


by Marty Hoffman, MD
and Tamara Hew-Butler, DPM, PhD
As is often the case in life, a little information can be dangerous. Then, as we learn more, we begin to recognize how complicated an issue really is and that we still have a lot to learn. Certainly, that’s been the situation with fluid and electrolyte balance during endurance exercise.

One thing that’s evident is that 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. Furthermore, just following your weight during an event doesn’t give an adequate picture of what might be happening with your blood electrolyte concentrations. Weight gain doesn’t equate to the development of hyponatremia nor does weight loss eliminate the chance of hyponatremia. For instance, in our recent studies at 100-mile races, we saw runners with weight gains over four percent who had normal sodium levels. We also saw runners with five – six percent weight loss who were hyponatremic. In fact, in the 2009 Western States Endurance Run, we found that half of the study participants developing hyponatremia had lost over three percent of their body weight during the race. So, the weight scale is of no value as a sole measure to estimate electrolyte concentrations.

What we can say about weight is that maintaining a stable weight during exercise will mean that one is likely to be over-hydrated. That’s largely because water is stored with glycogen (about three grams of water for each gram of glycogen), so when you utilize glycogen stores during exercise, you are releasing a considerable amount of water into the body. Most of that water moves into the blood stream. Assuming you start an event with good glycogen stores and appropriate hydration levels, you should actually lose around two – three percent of your body weight to maintain a stable hydration level. If your weight remains constant or increases during exercise, then you are likely taking in too much fluid.

We also now know that, contrary to conventional wisdom, there is no evidence that weight loss of a few percent impairs performance in ultramarathons considering the baseline weight is in the hydrated and carbohydrate-loaded condition. In fact, we’ve seen a direct relationship between running speed and percent body weight loss in our recent studies at 100-mile races. In other words, there is a trend for the faster runners to lose more weight than slower runners. Some of the fastest runners have lost five – seven percent of their starting body weight by the time they reach the finish line, feeling fine and without medical problems.

With this information in mind, imagine a situation where a well-intentioned race medical staff holds and forces a runner to drink because his weight is down five percent. The runner drinks a combination of water and energy drinks and his/her weight increases a little at which point he/she is allowed to continue. The concern is that we cannot know if the race medical staff might have just caused the runner to develop dilutional hyponatremia from the forced hydration. We certainly wouldn’t want to induce or exacerbate hyponatremia, and then send the runner out on the course where they could develop hyponatremic encephalopathy (brain swelling due to the hyponatremia) and progress to impaired consciousness, seizures and death. Conversely, the well-intentioned race medical personnel who advises a runner who has gained weight to increase sodium intake could also make matters worse. In such a case, the additional sodium could increase the runner’s thirst and cause him/her to increase fluid intake further and gain even more weight.

Therefore, it imperative for runners, race staff and medical personnel to understand and recognize that: 1) the scale is NOT a diagnostic tool and 2) body weight is NOT an accurate indicator of electrolyte status during an ultramarathon race. The signs and symptoms of emergent medical conditions such as dehydration/ hypovolemia, hypernatremia, hypoglycemia, heat illness and hyponatremia all overlap and can only be diagnosed through blood tests and core temperature measurement. Body weight changes should not be the sole determinant whether or not a runner is in medical “danger” but alternatively considered as adjunct information if the runner is symptomatic and onsite electrolyte analysis is unavailable.

Given this information, it’s evident that we need to adjust the conventional thinking that our goal should be to maintain body weight during an endurance event, that we are likely to be hyponatremic if we gain much weight, and that we are unlikely to be hyponatremic if we lose weight. But, how then do we use the weight scale during a race? The following guidelines are offered:

Runners and medical personnel should use caution in interpreting the measured weights at aid stations. Scales are not always calibrated or may read inaccurately if not on a level surface. It is more important to pay attention to the trend being observed.

Runners and medical personnel should recognize that they cannot use weight as a sole measure for the estimation of electrolyte status or for the provision of definitive recommendations. But, these loose practical guiding principles should be considered with caution:

Weight gain. The runner should be advised to reduce fluid and salt intake until urinating, particularly if there are signs of swelling (rings are tight, runner feels puffy).

Up to five percent weight loss. This is an appropriate weight loss, and the runner should be allowed to continue unless there are signs of distress.

Five – seven percent weight loss. The runner should be encouraged to increase fluid and salt intake and allowed to continue unless there are signs of distress.

Seven percent or greater weight loss. If mental status is satisfactory, the runner should be advised to increase fluid and salt intake. If vomiting or the inability to rehydrate is evident, the runner should attempt to recuperate until rehydration is possible before continuing.

In summary, runners need to understand the importance of responsible drinking during long periods of exercise – avoiding dehydration as well as over-hydration. A two – three percent weight loss during long endurance events is appropriate, will generally keep one at normal hydration levels and will not impair performance. Feedback from intermittent monitoring of body weight can help runners achieve this goal of two – three percent weight loss. But, the weight scale cannot be used to define electrolyte status, and any recommendations regarding fluid and salt intake based on changes in body weight should be provided with caution. Nonetheless, it would be appropriate for a runner to increase sodium and fluid intake when weight has decreased by five percent and to reduce sodium and fluid intake when weight is stable or increasing.


Comments are closed.