There is quite a bit of confusing information floating about on hydration strategies for marathon and other long-distance events. Misleading data can even be found in otherwise very reliable sources. For instance, a few months ago I read an article on running.competitor.com (a website I visit regularly and which usually has great content and provides solid advice) that gave outdated recommendations on this very subject. My intent here is to clarify the prevailing science behind this perplexing topic. For the sake of simplification, I will focus mostly on marathon running, but this applies to longer distance triathlons as well (half-ironman and ironman).
For a long time dehydration was the nemesis of participants in marathon. A lot of studies done in the latter half of the twentieth century noted the negative effect dehydration has on performance. Others focused on the role that dehydration may play in the development of heat illness. After that information came out, along with the fact that hydrating while exercising can enhance performance, there was a big push to have people aggressively hydrate during exercise, especially in marathons. At the same time, marathons were becoming en vogue among the "non-elite" population, exploding in popularity with several thousands in the larger events.
With this new data came companies such as Gatorade which helped push this doctrine to the public through advertising. However, a deadly side effect of this is what is called Exercise Associated Hyponatremia (EAH), hyponatremia meaning low sodium concentration within the bloodstream. What started to happen was that participants, mostly in marathons, were actually over-hydrating, diluting out their sodium so severely that in the most dire cases several actually died. Some may have heard of this as "water poisoning".
Over-hydrating is an oversimplification actually. There are a couple of other theories about why this happens (abnormalities in the secretion of water-conserving hormones and transfer of sodium between tissues) but there's no getting around the fact that the majority of EAH patients actually weigh more at the end of the race than they did at the beginning, and they report an intake of several liters of fluids during it.
It is also worth mentioning that the loss of sodium through sweat has NOTHING to do with EAH. Sweat is hypotonic, meaning that it has a lower sodium concentration than our blood, and thus it would have a greater chance of increasing the sodium concentration than decreasing it. The important thing to remember here is not the actual quantity of sodium ions in our body but rather its concentration within our blood and other tissues.
If you watch enough of the Gatorade and Powerade commercials you may be led to believe that you can prevent electrolyte abnormalities if you drink these products during and/or after exercise. False. There have been a few studies that looked at the difference between drinking water and sports drinks (those containing carbs, sodium, and potassium) and the incidence of EAH: there was NO difference. So drinking Gatorade or Powerade or a similar beverage during a marathon instead of water will not prevent EAH. There's not enough sodium in them to make a difference, they are hypotonic relative to our body fluids. However, I have to say that they are likely better than water alone simply because of the carbs in them, but they do little to restore electrolytes if imbibed during a race or workout. In fact, they contain sodium mostly to improve the taste and also to help with the absorption of the carbs; the receptors in the intestinal cells that absorb glucose need to also have a sodium ion attached in order for it to function.
So what should you do? Here are a few items that I hope will prove useful:
1. Do NOT listen to anyone who says you need to drink a set amount of fluids per hour of a marathon or who says you must drink at every aid station
This is outdated information! Do not pay any attention to it!
2. Do drink fluids while exercising longer than one hour
It's ok to drink during these races and training runs, in fact it is still recommended, but err on the side of drinking only to thirst. This will prevent severe dehydration but minimize risk of over-hydration. Heck, sometimes I drink during shorter runs of maybe 30-40 minutes if it's hot or dry enough out or if I'm doing a hard interval workout.
3. Take in carbs during the race and in longer training bouts
You are better off drinking sports drinks than water, if your stomach can tolerate it, but only because of the carbs in them, not the electrolytes. If you can't tolerate the sports drinks, then try eating energy gels or bars or other sources of carbs while drinking water for fluids. Again, it is best to try out these things during training. Don't wait until race day to try something you haven't already gotten used to.
4. Monitor your urine output
You should not find yourself urinating very light colored urine or very frequently in the latter half of a marathon. This means you are over-hydrated and risking EAH.
5. Monitor for swelling
You should also not be getting swollen ankles, hands, or fingers during a marathon, this is another potential sign of EAH.
6. Stay cool
If it's really hot out, the water at the rest stops may be more beneficial dumped over your head than down your throat, as long as you're not overly dehydrated. This will do far more to cool you down and prevent heat illness.
Lastly, there have been very few reports of EAH in endurance cycling events, so there's less need for concern in this area. However, the same hydrating principles apply.
As they say, all things in moderation.
