Water Retention During Endurance Sports

author : AMSSM

comments : 0

Member asks about source of measurable weight gain during events

Member Question from KL:

Hi folks, I gain 1.5 to 3kg (3 to 7 lb) of weight after long endurance events, for example an 8 to 10 hour vigorous day-hike, an all-day mountain climb, or 50km endurance trail run. I've always had this effect. Can anyone recommend things I can do to reduce the amount of weight gain? This weight gain slows me down because of the extra mass I am carrying during these events.

I'm assuming it's caused by water retention since I can't possibly gain that much weight from what I eat on race day. This is backed up by the fact that I typically seem to drink more than my friends do during events like these.

Typically my weight is greatest the day after the event, and reduces to even lower than starting-weight within 1 to 3 days afterwards (takes longer if I'm unfit or if it's very challenging trip).

I'm attaching a chart showing my weight fluctuations over the last 3 weeks, where I did all-day challenging trips each weekend during this period.

I am female 40 years old, and a little chubby (73 kg / 161 lb) but I am quite fit.

Answer by James Suchy, MD
Member AMSSM

Hi KL,

I agree, your weight changes are likely due to fluid changes in your body.  Rapid changes (i.e. days to weeks) in weight are often attributable to changes in our body’s fluid composition.  While gradual changes (i.e. weeks to months) in weight are often attributable to changes in our body’s solids composition, like fat and muscle mass.  Net changes in bodily fluid occur due to abnormal fluid intake or abnormal bodily excretion, or a combination of the two.  For example, if your body excessively urinates you will have a net fluid loss and if your body excessively conserves urine you will have a net fluid gain.  Now that we understand the theoretical concept, let’s return to your situation to determine where this imbalance might be occurring.

Perhaps the most telling part of your story is that you “drink more than my friends during events like these.”  A possible cause of your weight gain after exercise is due to over-hydration or a related condition known as exercise-associated hyponatremia (EAH).  Hyponatremia is a disorder where blood plasma sodium concentrations become abnormally low.  While hyponatremia can occur in other situations, a combination of factors sets up exercising athletes for developing this problem.  Intense exercise and heat causes the body to naturally want to conserve water, so urine output decreases.  Simultaneously large amounts of sodium are lost through sweating.  Then excessive pure water consumption during these prolonged events leads to a buildup of total body water without replacement of lost sodium.  The imbalance in sodium concentration between our blood plasma and our cells causes fluid to shift into cells resulting in fluid retention even after exercise finishes.  Aside from sudden weight gain after exercise, people may also experience swelling in the extremities, fatigue, dizziness, among other symptoms.  Studies have found some risk factors for EAH: female sex, exercise lasting more than 4 hours, slow running, race inexperience, excessive drinking behavior, and high availability of drinking fluids.  It should also be noted that nonsteroidal anti-inflammatory medications such as Ibuprofen alters the kidneys’ water excretion capacity, potentially worsening this problem.

Despite all this doom and gloom, preventing EAH and over-hydration is rather simple: focus on reducing overall fluid consumption, and replace pure water with electrolyte-rich fluid during long bouts of moderate to intense activity.  But how much is enough hydration you ask?  The American College of Sports Medicine’s position used to be that athletes should drink “as much [fluid] as tolerable” to prevent dehydration.  This sort of mindset likely originated from prior decades of fear of dehydration in our athletic population.  Alternatively, some organizations more recently have advocated for “drinking when you feel thirsty.”  However, this is subjective, and more often than not, could lead to dehydration.  I’m an advocate of hard data, and based on your chart, it seems like you are as well.

Here’s a simple way to determine if you are hydrating too much or too little during exercise.  Weigh yourself before and after an hour of exercise.  Continue to drink fluids as normal.  Make sure to re-create similar conditions you would normally exercise under: temperature, intensity, clothing, etc.  All of these factors can affect how much water your body loses.  Ideally your change in weight will be near zero, or where your fluid loss will equal your fluid intake.  If you weigh a lot less than when you started, you are under-hydrating.  If you weigh a lot more than when you started, you are over-hydrating.  Each kilogram of weight equates to approximately 16 ounces of fluid.  So if you gain 0.5 kilograms of weight after an hour, then try decreasing your intake of fluid by 8 ounces for every hour you are exercising.  This may take some trial and error.  If the intensity of your exercise increases (e.g. short, hard race) you may find yourself not able to comfortably consume your target fluid level.  That’s okay, assuming you’re not losing more than a few pounds or feeling dehydrated.  Just remember to hydrate after your effort to replace any losses.

As a physician, I should also mention some potentially dangerous medical problems that limit the body’s ability to excrete fluids and thus can cause rapid weight gain.  Medical conditions commonly associated with rapid fluid shifts include heart failure, kidney disease, and liver failure.  This fluid retention may manifest as visible swelling in the legs or feet (especially if you are standing for long periods).  Small amounts of fluid, say a pound or two, may be difficult to detect visually, especially since fluid accumulation can be dispersed throughout your whole body.  Patients with these conditions have to carefully monitor their fluid and salt intake because their bodies are more prone to fluid retention.  If you are otherwise healthy and not experiencing other new symptoms in conjunction with these weight changes, chances are you don’t have any significant medical issues.  However, an extended history, physical exam, and some simple lab tests can be used by your primary care physician to rule in or out these conditions and I would encourage you to see one to make sure these aren’t related to your problem.  

I hope this information empowers you and doesn’t frighten you.  Best of luck with your races and getting to the bottom of this problem.  

References:

1. Hew-Butler T, Rosner MH, Fowkes-Godek S, Dugas JP, Hoffman MD, Lewis DP, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clin J Sport Med (2015) 25(4):303–20.


2. Convertino, V. A.; Armstrong, L. E.; Coyle, E. F.; Mack, G. W.; Sawka, M. N.; Senay, L. C.; Sherman, W. M. (1996-01-01). "American College of Sports Medicine position stand. Exercise and fluid replacement". Medicine and Science in Sports and Exercise. 28 (1): i–vii.

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date: September 1, 2017