It’s hot. It’s humid. I’m in the southeast so from May until August, at least, this is the norm. Sometimes longer than that. Training for a fall marathon, reciting the mantra “humidity is the poor man’s altitude” and downing electrolytes.
But have you ever gotten late into a long run, somewhere past the 90 minute mark, and found yourself completely confused about how far you have to go before you turn around? Or not sure how you ended up in that neighborhood? Can’t remember that guy’s name as he waves and runs by?
It’s not you. It’s your hydration.
There is mounting evidence that being just slightly dehydrated can have subtle effects, including mood changes (ultra marathoners, anyone?) to cloudy thinking.
Logically, the southeast is a great place to study this topic, since we’ve all been sweating nonstop for the past three months now.
Mindy Millard-Stafford, director of the Exercise Physiology Laboratory at Georgia Institute of Technology, published an analysis in July of 33 related studies.
“Dehydration impairs cognitive performance, particularly for tasks involving attention, executive function, and motor coordination when water deficits exceed 2% body mass loss,” she wrote.
That’s 3 pounds for a 150-pound person. For reference, I’ve lost 3% during an hour run more than once this summer.
In fact, experts say if you’re running hard in the heat, you can lose 2% within 30 minutes. That’s about a liter of water for an average sized person — and I don’t know about you, but I don’t drink 33 oz of water in a half-hour run.
And, most of us can’t tell we’re dehydrated at that minute a level. If you drink only when thirsty — especially during summer running — you’re likely waiting too long.
Adding to this is the fact you will — voluntarily or not — slow down on hotter runs. Plus, as you become dehydrated, you slow down even more. This is a losing battle, because now your 12 mile run takes event longer.
When we look at dew points, there are huge drops in performance. Declines in performance due to dew point look like: 60° = -2%, 65° = -3%, 70° = -5%, 75° = -12%, 80° = -15%. Note, these are dew points and not simple air temperatures. And there’s a great calculator here.
Then, the general rule is you will slow down by 2% for every 1% in body weight loss due to dehydration.
How do we fight back?
First, make sure you stay hydrated on a daily basis. The National Academies of Sciences, Engineering and Medicine recommend 91 ounces total water per day for women, and 125 ounce for men. However, about 20% of this comes from your food, and they count coffee (thankfully) and other beverages towards this total. If you are used to caffeine, the diuretic effects are minimal; however, avoiding sugary drinks is a good choice in general.
Next, take in additional water while running. Stop at more water fountains or carry it with you, but start drinking sooner than you think. Hammer Nutrition recommends 20-28 ounces per hour, or about 5-7 ounces every 15 minutes. Don’t forget about electrolytes — there are a number of options out there in both tab and drink formulas. Make sure to choose one with a spectrum of electrolytes, not just salt.
Last, be prepared to slow down. It’s okay. Come November — maybe even October if we’re lucky — the dew points will drop, the mornings will be cool and crisp, and your runs fast.
Until then, drink up and don’t get lost on your way back!