Smart Hydration for Your Marathon

by Karl Gruber
If there is a running-related topic that leads to much confusion and misconception —especially when training for longer races such as a half or full marathon — it’s how to correctly hydrate your body.  Hydration is relative to the distance and time you are running, as well as your overall effort as a runner. Gender and fitness level also play key factors in the amount of liquid you lose during intense athletic activity such as running.

Being a running coach and having had many coaches myself, the words “drink, drink, drink” continue to reverberate in my brain. I’ve also heard, “The water is better in you than on you!” in regards to running in hot weather. In other words, it is smarter to put the water in your body — to keep your internal organs cooler and promote better and easier blood flow — than pouring it over your extremities and head in an attempt to cool off.

Let’s start by taking a look at the coach’s mantra to drink and drink a lot. While hydration is super important to help you run well, it is possible to drink too much liquid and end up with a life-threatening condition called hyponatremia. This generally occurs when running for long periods of time. Many years ago when I was coaching a charity runner group, a couple female runners experienced swelling in their hands and fingers during their longer runs. I wasn’t sure of the cause, until I learned about hyponatremia. According to a 2015 article in Runner’s World on the topic:

“Hand swelling can be found in hyponatremia, which occurs in runners who drink too much fluid over a four- to six-hour run. During exercise, circulation increases, and the hand has a large network of small blood vessels that open up. With the increased blood flow, there is some fluid leak between the cells. (We think of blood vessels like a garden hose, but in reality they have links between the cells that allow some leakage, and that leakage increases with the increased circulation from exercise.) This leakage is probably the cause of your finger swelling. The arm swing of running increases air movement across the skin (to improve heat exchange with the air), and this arm swing motion may also contribute to fluid retention in your hands.”

While this finger swelling is certainly not life-threatening, emergency action in necessary if a runner is severely over-hydrated. I saw it first-hand when a good running buddy of mine (a very experienced, excellent runner) had to be life-flighted out of the Grand Canyon with hyponatremia when he was attempting the Rim-to-Rim run.

I am by no means trying to scare you; I only want you to be fully informed so that you can train and race well. The bottom line is that smart hydration will always help you to run well! 

According to Dr. Jack Daniels, the author of my go-to running book is Daniels’ Running Formula, ”When fluid loss causes body weight to drop 3% to 5%, adverse effects on performance will occur. Certainly, people have had greater fluid losses, but they are getting into dangerous territory when they lose more than about 5% of body weight in fluid. For runners who perspire a great deal, it is important to monitor body weight closely.” 

Dr. Daniels suggests one very good way to monitor your fluid loss while running is to weigh yourself without clothes prior to your run, then go for a 30 minute run and weigh yourself (again, sans clothing) immediately post-run. If you lost one pound, you can extrapolate that you will have a two-pound weight loss via sweat if you run one hour. Still, with so many variations in the amount of fluid lost while running, you can only gauge how much you should drink through trial and error and running experiences over time. 

Another very important factor to take into account regarding hydration is your gender. Believe me, I know and run with a number of female runners who sweat buckets every mile — in fact, one of them is nicknamed “The Human Rain Machine” — but overall, women tend to sweat less than their male counterparts. In a very revealing New York Times article Phys Ed:  Do Women Sweat Differently Than Men? reporter Gretchen Reynolds sited an in-depth study on this very topic from researchers at Osaka International University and Kobe University in Japan: 

“What the researchers found was that the fit men, unsurprisingly, perspired the most, significantly more than the fit women, especially during the more intense exercise. But the athletic men weren’t using more sweat glands. The fit women had just as many glands active and pumping; they produced less sweat from each gland. Meanwhile, the unfit women, by a wide margin, perspired the least, especially during the strenuous cycling, and became physiologically hotter — their core temperatures rising notably — before they began to sweat at full capacity. These results, the scientists concluded, ‘revealed a sex difference in the effects of physical training on the sweating response’ and, just as important, ‘a sex difference’ in ‘the control of sweating rate to an increase in exercise intensity.’ In other words, the women, whether fit or not, were less adept of ridding themselves of body heat by drenching themselves in sweat.”

As your training schedule has you running for longer and longer periods of time, be sure to figure out how much you need to drink to stay properly hydrated! As I mentioned before, smart hydration will always help you continue to run well.

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