~Written by Sports Performance Dietitian, Kristin Foreman MS RDN~
It’s been years since I collected my own sweat rate data. The last time I did, a dozen eggs cost about $2.00. I’ve been doing a lot of mid-afternoon runs this summer, and like many of you, I generally do okay on shorter workouts in the heat. But I was really feeling it on my hour-long runs--my heart rate would climb and I couldn’t get it down, I felt more fatigued, and I just wanted to be done.
Sports Dietitians are scientists by training. I wondered what patterns I might see in the data. I also wondered if anything had changed. Part of me really doubted it had, but I was curious to know what the numbers revealed.
A lot, it turns out…
Kristin’s sweat rate data (you can also view the file by clicking here)
The first thing I noticed was how much I sometimes sweat. This is my sweat rate. I wasn’t coming home drenched from my runs, so it wasn’t my obvious first thought. But this Colorado, where humidity is low and the air is drier than a popcorn fart. Sweat evaporates. Duh.
I’m also a moderately “salty” sweater. I lose nearly 1,000 mg of sodium per liter of sweat. This is my sweat sodium concentration. It’s an easy test that anybody can have done*. Sweat sodium concentration is fairly constant over our adult lives, but sweat rate is highly variable; it’s based on duration, intensity, weather, fitness, hydration, etc. We need both sweat rate and concentration when we’re helping athletes plan their training and racing hydration strategies.
In my case, a high sweat rate combined with moderate sodium losses adds up to significant sodium loses per hour. I was getting away with it because my runs have been relatively short (around 60 minutes), but as my marathon training increases, I knew my training would suffer. This data clearly showed that I was not doing a good job managing my hydration and electrolytes. I was experiencing significant performance declines as a result.
The second thing that jumped out at me was the two data points on lines 5 & 8 (in pink) where I didn’t sweat much at all. My current weight generally hovers around 118-121 pounds. If you look at the days where my starting weight was 119-120, my sweat loses were around 2.1 – 2.4 liters per hour. On the two days when I started my runs at around 115 pounds, my sweat rate was significantly lower. Why? Because I was dehydrated even before I started running.
Research has demonstrated that a majority of athletes begin their training sessions or competitions in a somewhat dehydrated state (1, 2, 3). A 2015 study of elite female soccer players revealed that only 2% were adequately hydrated (4).
Even mild dehydration during training and competition is known to reduce athletic performance. We can experience increased physiological stress with as little as 1–2% reduction in body mass through sweat. These loses, or level of dehydration, can also lead to increases in heart rate, a rise in core body temperature, increased muscle glycogen use, with decreases in cardiac output, sweat, time to exhaustion, mental acuity, and power. Additionally, inadequate replacement of sodium, the predominant electrolyte lost through sweat, can sometimes exacerbate these declines.
The final trend I noticed was how little I was actually drinking on some runs. I generally run with a hydration bladder (I love this one) and I thought I was drinking closer to 24 ounces an hour. Turns out I was really under-hydrating some days. And for the record, you cannot train your body to “adapt” to dehydration. Don’t even try. It’s a no-win strategy.
I’m really happy I pulled out the spreadsheet and bathroom scale because, as it turns out, my body has changed. I was indeed surprised by the data--more like, OM[freaking]G, I sweat like a horse sometimes! Needless to say, I’ve made some changes, like, ahem, drinking more water during the day, and guess what, I feel a TON better and my training runs are strong again. Yay water!!!
Here are a few things that might help you:
Learn your sweat rate and how it trends in different conditions
Stay hydrated during the day (you should have at least 4 good straw-colored pees a day)
Alcohol is dehydrating (and it disrupts sleep). Avoid it the night before long workouts
Drink a glass of water after you wake up. If you’re thirsty, you may be mildly dehydrated
“Preload” the night before and the morning of long workouts (I love Skratch Hyperhydration & Precision Hydration 1500)
For many athletes, a body weight change of 2% will result in performance declines. These loses are cumulative.
We tolerate dehydration more in cool conditions than in hot conditions
Get sweat sodium tested here
Call the Sports Dietitians at The Nutrition Mechanic or book your free 20-minute session to see how we can help you have your best race ever!
*Sodium losses in athletes range from about 200mg to 2200mg per liter of sweat. Are you curious about how much salt you lose in your sweat? We can sweat test you and it doesn’t involve heavy breathing or working out. You sit and we chat. Easy peasy. Book your sweat test here.
Happy Trails,
Kristin
Email Kristin
1) Arnaoutis G, Kavouras SA, Angelopoulou A, Skoulariki C, Bismpikou S, Mourtakos S, et al. Fluid balance during training in elite young athletes of different sports. J Strength Cond Res. 2015;29(12):3447–52. https://doi.org/10.1519/JSC.0000000000000400
2) Magee PJ, Gallagher AM, McCormack JM. High prevalence of dehydration and inadequate nutritional knowledge among university and Club level athletes. Int J Sport Nutr Exerc Metab. 2017;27(2):158–68. https://doi.org/10.1123/ijsnem.2016-0053.
3) Magal M, Cain RJ, Long JC, Thomas KS. Pre-practice hydration status and the effects of hydration regimen on collegiate division III male athletes. J Sports Sci Med. 2015;14(1):23–8.
4) Castro-Sepulveda, M., Astudillo, J., Letelier, P., & Zbinden-Foncea, H. (2016). Prevalence of Dehydration Before Training Sessions, Friendly and Official Matches in Elite Female Soccer Players. Journal of Human Kinetics, 50(1), 79–84. https://doi.org/10.1515/hukin-2015-0145