An evidence-based review of hydration and electrolytes for runners.
Written by Dr. Josh Mularella, DO, FACEP, FAWM, DTM&H - Medical Director, G2G & M2M
Back in 2010 I was deployed to California’s Sierra Nevada mountain range for cold weather and mountain warfare training with the US Marines. They would line us up before a run and have us drink water until one of us vomited to fully hydrate us. This is obviously no longer recommended, as overhydration is the major cause of hyponatremia (low sodium) which can be deadly.
On the other hand, we also know that dehydration, or even hypohydration, can lead to decreased aerobic performance. So what is the right answer? How do you reach that sweet spot? Unfortunately there is no simple answer. The current recommendation is to “drink to thirst”, but in reality everyone is different. Some sweat more than others, some sweat more salt than others. Even at the individual level the quantity and composition of your sweat will change over time as your training increases and you become acclimated to the heat.
“The best laid plans of mice and men often go awry”
Perhaps you are one of those runners who picked up some new electrolyte powders for your water bottle on Amazon a few days before G2G only to realize day one that it tastes disgusting and makes you nauseous. You’re in trouble. Or maybe you did everything right yet you still feel bloated and nauseous by day two and not urinating. Something is wrong. My point is that even the most experienced of you may still run into problems (pun intended) from time to time. The rest of this post therefore breaks down hydration/electrolytes into what you need to know as a runner to finish G2G safely.
Stage one begins! As you run, blood and its oxygen/nutrients are going to be diverted to where it is needed most such as your legs (for energy) and your skin (to cool off). As a result, blood is diverted away from less important organs such as your stomach, intestines and kidneys.
When blood is shunted away from your stomach it doesn’t function well. If you try to drink too much water at once it will stay there and slosh around (often referred to as “slosh stomach”). This can be uncomfortable and make you nauseous. Since most water is absorbed in your small intestine, water trapped in your stomach isn’t doing much for your hydration either. You can try to make yourself vomit but then you have lost all of that fluid.
The stress of running in the desert will stimulate the production of various hormones and cytokines. Arginine vasopressin (AVP) is the main hormonal inhibitor of water excretion (ie urination). AVP secretion is stimulated by intense exercise, stress, pain, heat, nausea, and low blood sugar. In other words, G2G = AVP! While AVP serves a purpose to make sure you don’t get dehydrated, too much AVP can be a bad thing.
80-90% of the water you drink will have been absorbed before reaching your large intestines, meaning water is absorbed quickly into the bloodstream. If you drink more water than you excrete then you are in danger of “over hydration”. Everyone is different, but you can lose up to 2L of water an hour from breathing, sweating and urinating.
75% of the sodium in your body is locked away and not immediately physiologically available. Some runners can access this stored sodium better than others. But when there is a large increase in water absorption from your intestines, your sodium level gets diluted (low), known as hyponatremia. When this happens during/after exercise it is called Exertion Associated Hyponatremia (EAH).
Here come the “hills”! The strain/stress on your legs will cause small tears in those muscles, possibly even to the point of rhabdomyolysis (a condition where muscle fibers are damaged and broken down during strenuous exercise, releasing myoglobin into the bloodstream). This in itself activates secretion of AVP to some degree.
Dehydration, AVP, the heat and certain medications (especially NSAIDs like ibuprofen) all decrease blood flow to your kidneys. The myoglobin released from rhabdomyolysis can clog your kidneys, further decreasing flow. If blood doesn’t flow to your kidneys, they can’t filter out the excess water. Therefore it is no surprise that there is a high incidence of “Acute Kidney Injury” at staged ultramarathons. A study done on the Incidence and Prevalence of Acute Kidney Injury During Multi-stage Ultramarathons showed that by Stage 5 over 75% of runners had evidence of kidney injury (see below). Most runners quickly return to baseline after the race.
Where is base camp?!? The continued demand for more energy leads to the conversion of glycogen stores to glucose, releasing large amounts of free water in the process, further diluting sodium stores.
You have finished running and you think you are in the clear. You start cooling off, AVP secretion decreases. But as blood flow makes its way back to your GI tract, all the water hanging out in your stomach and intestines is quickly absorbed potentially causing a further precipitous dilution of your sodium levels!
WHAT DOES THIS MEAN FOR ME??
The good news is that you still have over a month to figure out your hydration/electrolyte plan for G2G. Whether or not you supplement your water with electrolytes is up to you .There has been a lot of research over the years by Marty Hoffman, MD at the Western States Endurance Run that shows sodium supplementation doesn’t prevent EAH. However I know from past experience that most of you will take them (I probably would too). In that case you need to figure out NOW what product you want to use and how often you will need to take it. You should take your own supply of electrolytes.
Our recommendation on the course is to drink to thirst. It is important to try and drink gradually between checkpoints otherwise you may end up with exertion associated gastroparesis (“slosh stomach”) which can be debilitating.
There are many reasons why you may be feeling nauseous. All of you should have been prescribed nausea medication by your primary care provider to take as needed out on the course. Ondanzetron (Zofran) is great for nausea and is typically our first treatment. Metoclopramide (Reglan) also works for nausea and has some “promotility” effects making it at least theoretically better for “slosh stomach” issues.
Don’t be surprised if we ask you when you last peed, what it looked like, etc. They used to think that if you aren’t urinating then you were dehydrated, and if you were urinating too much you were overhydrated. But as we talked about previously, inappropriate AVP secretion can contribute to kidney injury… therefore it is possible to have overhydration with hyponatremia and not urinating as well. This is also why we don’t recommend using any NSAIDs such as ibuprofen or naproxen while running on the course since they can further exacerbate kidney injury and the development of EAH.
EXERTION ASSOCIATED HYPONATREMIA (EAH)
From my perspective, this is what it all comes down to. The symptoms of mild EAH (nausea, headache, dizziness, fatigue) are the same as dehydration, low blood sugar, impending heat stroke and even altitude sickness! We know that overhydration is the major cause of EAH and many races use weight as a screening tool (you are expected to lose weight). However, we also know that EAH is more complicated than JUST too much water. Getting reliable weights out on the course is also impractical and unreliable. We do not have any fancy lab equipment with us at G2G to measure sodium levels on the spot either.
If you are not urinating or have symptoms of EAH you need to let us know. We will do our best to try and determine whether you are just dehydrated (need to drink more) or possibly are overhydrated with hyponatremia (need to drink less), or whether something else is going on entirely. The current recommendation for mild hyponatremia is fluid restriction or, preferably, hypertonic (very salty) oral fluids such as beef bullion cubes. The medical team also carries hypertonic saline for emergencies.
I think I have given you enough to think about. Good luck with your training!