Bloating with low blood sodium is extremely dangerous

by Lulu Wescher

Lulu Weschler is physical therapist, ultra cyclist and regular contributor to UltraCycling.


A potentially very dangerous situation in endurance athletes is low blood sodium. During an ultra event, an athlete may drink plenty of water, but not replace the sodium lost in sweat. If the athlete starts retaining water (bloating), then the decrease of sodium in the body and the increase in fluid will result in a too low concentration of sodium in the blood.

Bloating with low blood sodium is extremely dangerous because it can lead to a potentially fatal brain swelling. Mental changes and/or seizures (convulsions) in a bloated athlete are signs of brain swelling and represent a dire medical emergency.

Low sodium is called hyponatremia. “Hypo-” means too little and “-natremia” means sodium status. Hyponatremia means that the concentration of sodium in blood plasma is too small. If the plasma volume increases, then even if the amount of sodium has not decreased, the concentration of sodium will have decreased.

Bloating means increased water weight. In an exercising athlete, bloating is a red flag suggesting hyponatremia. In some athletes, the kidneys, for reasons that are not understood, fail to increase the rate of urination to dump a moderate fluid overload. The same athletes at rest may properly excrete the fluid overload that caused them to bloat during exercise. It is also possible to ingest more fluid than any kidneys could possibly excrete, for example, a 100 lb athlete consuming two quarts per hour at 80 degrees F and riding at a slow speed.

Be vigilant about bloating. Signs include puffiness (at feet and ankles, watchband, ring, and shorts elastic band), and perhaps a forehead-type of headache, which is aggravated by riding on a bumpy road. The rider begins to feel (and look) like the Michelin Man.

A bloated athlete should not consume water and should not consume any sports drink until the bloat has been urinated. (All sports drinks are too dilute in sodium and will only increase the fluid overload.) Stopping exercise will allow urination to start (the athlete must continue to be closely monitored for signs of brain swelling until he/she has urinated a significant volume).

Oral ingestion of salt may get urination to “kick in”. On long rides I monitor the symptoms of bloating and my experience suggests that I can reverse bloating by taking in salt and restricting fluid intake. I am also able, I think, to prevent bloating by topping up on salt proportional to my water intake (~1000 mg sodium per quart water).

The only definitive diagnosis of hyponatremia is a blood test for sodium. Keeping track of weight can tell whether an athlete has bloated (weight increases during the ride), or dehydrated (weight decreases during the ride). Watch also for the signs of bloating described above.

Riders and crews may erroneously view bloating as indicative of too high a sodium concentration (hyper-natremia). Bloating is the opposite of dehydration. In the exercising athlete it suggests an existing or imminent hyponatremia, not hypernatremia. The danger of bloating hyponatremia is greatly increased if it is misdiagnosed as dehydration and treated as such. All fatal and severe cases of hyponatremia reported in the literature have involved bloating.

More on Hyponatremia.