Hyponatremia

1998 saw a record number of DNF’s ('Did Not Finish') at Ironman Canada (an 'Ironman' is a triathlon consisting of a 2.4 mile swim, 112 mile bike, and 26.2 mile run). Approximately 250 athletes - well-trained Ironman athletes - didn’t reach the finish line. The primary cause was attributed to dehydration and hyponatremia - as was evidenced by the number of IV bags the medical team went through. Granted, it was hotter than usual and the headwinds that plagued the cyclists for 2/3 of the bike course made for longer bike splits, but what did these unfortunate DNFers do wrong that cut their day short and landed them in the Medical tent?

 At the Ironman level, everyone knows the importance of hydration. Just count the number of porta-potties in a transition area and the number of athletes in line and you’ll agree that no one forgot to drink in the days leading up to a race. But that’s not enough in a race the length of Ironman. You need to understand the importance of sodium and it’s relationship with water in order to prevent a trip to the medical tent at your next ultra-distance race.

Hyponatremia means ‘low concentration of sodium in the blood’. Sodium is an important electrolyte (an element with an electrical charge - in this case Sodium is positive and represented as Na+) which plays a role in water balance and muscle contraction. Sodium is required to draw water through permeable membranes in the body and thereby distribute fluid throughout the body. When you sweat, you lose water and salt, and salt is made up of sodium and chloride and is represented as NaCl, so lots of sweating means you are depleting your sodium stores in the body. If your sodium levels in the blood get too low (hyponatremia) you will no longer be able to move water across permeable membranes and you will become dehydrated - even if you are drinking enough water. You can drink all the water you want, but if you don’t have the sodium present to move it from the gut to the bloodstream, you don’t stand a chance.

A common complaint among Ironman athletes is that they get off the bike, after hydrating regularly throughout the ride, and they feel bloated, their abdomen is swollen, and they have an upset stomach. It’s a good guess that they didn’t take in enough salt and now all that water/sports drink has pooled in their stomach and they are going to have to run/walk with a watermelon in their gut until they’re offered chicken soup broth - which is quite salty - at which point they’ll begin to feel like running again. Don’t laugh, I read about this happening to some poor souls every year. So what went wrong? Don’t sports drinks have sodium in them? We need to look at a few numbers in order to answer these questions.

  1. Sweat contains between 2.25 and 3.4 grams of salt per litre, and in a long race an athlete could easily lose 1 litre per hour. In a 12 hour race, that adds up to between 27 and 41 grams of salt.

  2. While everyone is different, a general rule of thumb is that you should try to ingest 1 gram of sodium per hour during a long event. You should also increase your sodium intake in the days leading up to the race. Aim for between 10-25 grams of salt per day pre-race.

  3. To ingest 1 gram of sodium from sports drinks alone, you would have to drink 2.18 litres of Gatorade. You can’t drink that much per hour for the duration of an Ironman race.

So, even if you drink sports drinks during a hot race it is very likely that you will sweat out more salt than you can replace. You will need to replace salt specifically. This can be accomplished by either eating salty foods or taking salt tablets. There are pros and cons to both: Salty Foods will taste good and the fact that you are ‘tasting’ the salt will help to trigger a thirst response which will make you drink more. But, you will need to eat a lot of salty food to get in enough sodium. One gram of Sodium equals 2.5 grams of table salt. Now, one tablespoon of salt weighs about 6.6 grams, so you will need less than half a tablespoon (per hour), but if you are trying to ingest that from pretzels or crackers, that’s a lot of food you’ll have to carry. On the other hand, Salt Tablets are convenient little ‘pills’ made up of sodium and chloride (some electrolyte tablets may have other ingredients, but basic salt tablets are just table salt) which are easy to carry and ingest. But, because they don’t stimulate a thirst response you will have to make sure that you are drinking enough during the race. The nice part about ingesting salt while you race is that it allows you to drink plain water for a change from the sports drink of the day. That can be a welcome relief on a hot day because you can wash your face, head, etc. and the water is often colder than the sports drink because it doesn’t have to be mixed ahead of time.

There are a couple of other important things to remember when discussing hyponatremia:

To summarize, when preparing for an Ironman it is important to practice salt replacement while training, to increase your salt intake in the days leading up to the race, to drink an amount of fluids which is appropriate to the race climate, and to ingest salt during the race if the day is hot. Follow these simple guidelines and hopefully you’ll never see the inside of the medical tent.

References

  1. Fluid replacement during exercise. [Review], Timothy Noakes, MD Source = Exercise & Sport Sciences Reviews. 21:297-330, 1993.,

  2. Dehydration and hyponatremia during triathlons. [Review] Hiller WD. Source = Medicine & Science in Sports & Exercise. 21(5Suppl):S219-21, 1989 Oct.

  3. Fluid replacement during prolonged exercise: effects of water, saline, or no fluid. Authors, Barr SI. Costill DL. Fink WJ. Source = Medicine & Science in Sports & Exercise. 23(7):811-7, 199I Jul.

  4. Renal and Electrolyte Disorders, Third Edition. Edited by RobertW. Schrier, MD. 1986. Little Brown and Company

  5. Symptomatic hyponatremia during prolonged exercise in heat. Armstrong LE. Curtis WC. Hubbard RW. Francesconi RP.Moore R. Askew EW.Source = Medicine & Science in Sports & Exercise. 25(5):543-9, 1993 May.

Hyponatremia Continued

The following was taken from the Runner’s World website:

South African sports medicine expert Tim Noakes, author of the widely respected "The Lore of Running," practically "invented" athletic hyponatremia. His studies and observations at the 54-mile Comrades Marathon led him to realize that some runners were actually drinking too much water during long, slow endurance events, and that this could bring on a dangerous medical condition. Hyponatremia has recently been discussed more often in the context of 26-mile marathons, with the medical director of the Rock 'n' Roll Marathon stating that the RnR race might have seen as many as 12 cases of hyponatremia. Noakes discussed hyponatremia with Runner's World Daily shortly after returning home to Cape Town from a lecture at Harvard Medical School.

Runner's World Daily: What is hyponatremia and what are its physical effects?

Tim Noakes: Hyponatremia means a reduced blood sodium concentration. When the sodium level falls below 129 mmol per liter, it creates, in mild cases, a general clouding of consciousness not unlike the slowing of brain function that occurs in drunkenness. This is caused by swelling of the brain that results from the general state of fluid overload. In more severe cases, the athlete lapses into unconsciousness, develops epileptic-like seizures and may stop breathing or suffer cardiac arrest. Fluid overload of the lungs may produce pulmonary edema that leads to shortness of breath and coughing up blood-stained sputum. In Ironman triathletes suffering from hyponatremia, I have observed gross swelling of the hands and forearms.

RWD: How and why do distance runners get hyponatremia?

TN: By drinking too much fluid during very prolonged exercise. We usually find that athletes who develop the condition drink between 1,000 and 1,500 ml per hour [between one and one and a half quarts] during exercise but sweat at much lower rates, perhaps 700-1,000ml per hour. As a result they develop a progressive fluid overload.

RWD: Who gets it most commonly and in what kinds of events?

TN: Women are at much greater risk than men for reasons that we don't yet understand. I think it is purely a size effect; women are smaller and more likely to develop a fluid overload simply because it takes less fluid for small people to become overloaded. Alternatively, it is clear that a big part of the problem is the inability of the athlete to excrete the excess fluid perhaps because of high levels of fluid-retaining hormones. It may be that woman have larger amounts of these hormones, the nature of which remain uncertain.

RWD: How can marathoners make sure they are getting enough fluids but not so much as to be at risk for hyponatremia?

TN: You have to drink a lot for a long time to develop a fluid overload. If runners are drinking less than 1 liter per hour, they are unlikely to develop the condition. Since you need to keep drinking for 5 to 6 hours or more, only very slow marathon runners and ultradistance endurance athletes are at risk.

The best thing for athletes to do is to weigh themselves before and after a hard training workout to determine their usual sweat rate. Then they can plan their fluid intake during a race accordingly.

RWD: How can runners or medical personnel spot hyponatremia in another athlete?

TN: Aside from some medical conditions that are usually well recognized, there are really only two conditions specific to sport that cause an altered level of consciousness during prolonged exercise: heat stroke and hyponatremia. Measuring body temperature is the first step in the differential diagnosis. If the body temperature is above 42 degrees Centigrade, the diagnosis is heatstroke, and the athlete must be placed in an ice-water bath for 5-10 minutes to lower his or her body temperature. If the temperature is normal [i.e., 38-40 degrees Centigrade], then the most likely diagnosis is hyponatremia. The diagnosis can be confirmed by measuring the blood sodium content, and obtaining a result below 129 mmol per liter.

RWD: What should be done for a stricken runner?

TN: The best treatment of the hyponatremia of exercise is masterful inactivity. Given time, the body will start to get rid of the fluid excess by increasing urine production. Full correction of hyponatremia requires that the athlete gradually ingest some salt over the next 10 to 12 hours. If medical help is available, the physicians may choose to manage the condition by using a urine-producing drug [a diuretic] and replacing the lost salt with a very concentrated [3 percent] salt solution given intravenously at a very slow rate [less than 50 ml of fluid per hour].

RWD: What should not be done to a runner with hyponatremia?

TN: Athletes with an altered level of consciousness should never be given intravenous fluids until it has been determined that the individual is not suffering from hyponatremia. We need more people to understand that the mild levels of dehydration experienced by modern marathoners does not cause loss of consciousness. Giving fluid to hyponatremic athletes will, at best, worsen the condition and delay recovery. At worst, it may produce respiratory and or cardiac arrest as a result of a sudden worsening of the brain swelling.

Post-Script (by Dr. Mark Steckel)

You’ll notice that this interview discussed hyponatremia - low sodium or salt levels. It is the primary concern in ultradistance events where the athlete has been sweating. No mention was made of hypokalemia - or low potassium levels. This is because low sodium is the main culprit in these types of dyhydration/electrolyte imbalances. In some Ironman races, like Ironman New Zealand, the medical staff warns against drinking too much water. This is because the weather is typically cooler and therefore sweat rates are lower and drinking too much will decrease the concentration of blood sodium - giving the same effect as if the athlete had sweat out too much salt.

It should be noted that drinking sports drinks that have electrolytes added to them will delay the onset of hyponatremia because you are adding back some of the lost salt, however in ultra events this still may not be enough therefore replacement via salt tablets may also be required.

Conclusion - know your body, have a feel for your own sweat rate, and replace lost sodium with sodium chloride (NaCl) tablets in ultra events and in the days preceding and following the event. Sodium chloride tablets are more important than electrolyte tablets (which may contain potassium and other minerals along with sodium) because your main concern is with avoiding hyponatremia.


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