by Katie Elliott

Ed. note—Katie Elliott graced these pages a few months back, with her helpful look at recovery techniques through a nutrition lens. She is back for a more tactical approach, demystifying the evergreen topic of hydration in endurance sports. If you'd like to hear more from her and other women in the endurance and triathlon space, check out the Women.Thrive Symposium, which comes up in the next few weeks.

It’s summertime. Training and virtual racing are in full swing and temperatures are increasing in many parts of the world. During this patch of increased activity, you've found the perfect opportunity to fine tune your hydration strategy so that you aren’t leaving performance on the table. Before I jump into testing and figuring out your individual hydration needs, however, let’s start with everybody's favorite: a little science. 

Why is Hydration Important in Endurance Sports?

In short, staving off a reduction in performance. Proper hydration from a combination of fluid and electrolytes prevents performance declines and preserves health. The American College of Sports Medicine (ACSM) states that as little as a 2% loss of body water through sweating may negatively impact athletic capacity. This reduction happens because your body has to work harder due to decreases in blood pressure alongside increases in core temperature and heart rate when you are dehydrated. When these physiological changes happen, you get tired, everything seems harder, and your performance capacity drops. Muscle cramping is also associated with both dehydration and electrolyte deficits, which can certainly put a damper on your race.

From a health standpoint, a dehydrated athlete can experience negative consequences like headaches, nausea, and vomiting. More serious consequences include heat exhaustion and heat stroke. In addition, dehydration increases the likelihood of kidney failure in exertion-related rhabdomyolysis. Perhaps the most serious of all of the health consequences associated with hydration is hyponatremia. Simply put, hyponatremia happens when there is too much fluid ingested and not enough electrolytes. This can result in brain swelling, pulmonary edema, and death. You might be thinking that the health consequences I just mentioned are pretty rare. Sadly, they are not statistical anomalies or scare scenarios. A study by Danz et. al. found that 10.6% of triathletes in the Ironman European Championships suffered from hyponatremia. The authors also cited a previous study on marathon runners in which 12-13% of finishers experienced hyponatremia. 

Didn't we just suggest that you need to avoid dehydration, though? Yeah, we understand the possible confusion. Note that hyponatremia is both too much fluid and not enough electrolyte—athletes who drink only water (and lots of it) and don't include salt or other electrolytes in their bottles.

Hydration is not a One-Size-Fits-All Equation 

Another important concept to understand is that different athletes sweat differently. Athletes vary in sweat lost per hour during exercise (sweat rate) and amounts of electrolytes lost per liter of sweat (sweat concentration). The ACSM position paper on electrolyte and fluid replacement shows sweat rates ranging from .5 liters/hour to 2.5 l/hr in athletes participating in a wide range of sports. Other studies have shown that sweat sodium concentration among athletes ranges from 200mg per liter sweat lost to 2200mg per liter of sweat lost. A ten-fold difference! 

Here is some math to further illustrate the need for individualization: 

Athlete #1: 
low sweat rate of .55 liters lost per hour of exercise at sub-threshold intensity, low sweat sodium concentration of 240 milligrams lost per liter sweat 
Total sodium losses over a five-hour event: 660 mg sodium lost  

Athlete #2: 
average sweat rate of 1 liter lost per hour of exercise at sub-threshold intensity, medium sweat sodium concentration of 900 milligrams lost per liter of sweat. 
TOTAL sodium losses over a five-hour event: 4,500 mg sodium lost 

Athlete #3: 
very high sweat rate of 2.5 liters lost per hour of exercise at sub-threshold intensity, very high sweat sodium concentration of 1610 milligrams lost per liter of sweat. 
TOTAL sodium losses over a 5-hour event: 20,125 mg sodium lost in a five-hour event 

Sweat Rate Testing

Now that we’ve established that people sweat differently, let’s talk about determining your individual needs. The first step is to do some sweat rate testing. Here is a link to a protocol that you can use to calculate your sweat rate and a spreadsheet you can use to keep track of different tests. Remember, sweat rate is affected by environmental factors such as intensity, ambient temperature and humidity, altitude, your clothing choices, genetics, fitness level, and heat acclimatization status. This means you should test more than once. I also recommend testing in similar conditions (temperature, humidity and exercise intensity) to those you will experience on race day if you are looking to inform your race fueling plan. 

The second part of the equation is what is in your sweat. Given that there can be a ten-fold difference in sweat sodium concentration between athletes, this test is an important piece of the puzzle. There are a few ways to measure sweat sodium concentration. The gold standard is called whole-body washdown. This is done in a controlled lab setting, requires researchers to collect all sweat lost, generally must be done on a stationary bike and requires a high level of skill. Thus it is not especially practical. Other more user-friendly ways to measure sweat sodium concentration include patch testing and pilocarpine iontophoresis. At our lab, we use Precision Hydration’s pilocarpine iontophoresis technology because the test is accurate, the equipment is medical-grade, you only have to test once, it's easy test to perform, and done at rest (which means I don’t have to interfere with an athlete’s training schedule). If you don’t have access to sweat concentration testing, you can guesstimate your sodium losses by qualitatively determining how salty a sweater you are. Here are several signs suggesting you are a salty sweater.

  • Your eyes sting when you sweat
  • Your sweat tastes salty
  • You have white marks on your body after a training session
  • You crave salt post-training
  • You experience cramping (salt is not the only factor that effects cramping, but it has been associated with significant electrolyte losses)
  • You feel awful after exercising in the heat
  • You feel faint when you stand up after hard exercise

Please note: If you are experiencing feeling faint after exercise, I recommend checking in with your doctor in addition to evaluating your sweat concentration.

If you answered yes to these questions and need to guesstimate your levels, first understand that the average person loses around 949mg sodium per liter sweat. If you are experiencing these symptoms, you are potentially above average and should estimate your sweat sodium concentration as being higher than average.

Putting It All Together

Once all data is collected, you can estimate how much you should be drinking in like conditions as well as how much sodium you lose per hour. Knowing sodium lost per hour gives you a good idea about the concentration of drink you need to be using. Sports drinks vary considerably in terms of how much sodium they contain. Here are a few examples: 

Hammer Nutrition Heed: ~120mg/liter
Gatorade: ~427mg/liter
Skratch Labs Exercise Hydration: ~760mg/liter
EFS Pro: ~1125mg/liter
Precision Hydration 1500: ~1500mg/liter

If you have a high sweat rate and are a salty sweater, you likely want to choose a drink with more sodium per liter. While you want more sodium in your beverage, you want to be careful not to have too much other stuff in your drink (meaning a high concentration of sodium AND a lot of carbohydrate). Ideally, you will want to take in a drink that is a hypotonic or isotonic solution to ensure that you absorb fluid adequately. Here is an article that explains different drink solutions and why tonicity is important to hydration. If you need more sodium in your drink, You will want to get more carbohydrate calories from food. However, if you are at the low end of sweat rate and concentration, you might want to choose something that doesn’t have as much sodium so that you don’t have GI issues from taking in excess electrolytes. It is also important to note that you don’t need to cover 100% of your needs. While you don’t want to rack up a large deficit, particularly for activities over four hours, you will be fine if you aim to cover the majority of needs (70% is a good place to start, but not an absolute). Problems typically arise when athletes are at very significant deficits. For example, if Athlete #3 in the example above drank a beverage that only covered 2100mg sodium, when he needed 20,125mg over the 5-hour period, that could be problematic.

Ways to Gauge Hydration Status

Now that you know your stats, you will want to monitor your hydration status, particularly across big training weeks. Ideally, you would weigh after your first morning void several days in a row to get a baseline for first morning weight. From there, you can see how things are trending—if you have lost three pounds in one day, that is likely fluid loss as that much true weight loss would be impossible in that time frame. You can also weigh before and after workouts. You want to ensure you are not losing more than 2% of your body weight from sweat loss (if you losing more than this, consider more fluid intake during training sessions). For every pound you do lose, drink 16-24 ounces of fluid post-workout along with electrolytes to replace those losses. Along with taking weight measurements, you can purchase urine specific gravity (USG) reagent strips online to quantify your urine concentration. A USG of ≤ 1.2 indicates being hydrated, although most strips offer the information based on color squares rather than having you figure out numbers. If you aren’t into collecting urine and using strips, you can also measure urine color. While this isn’t a perfect science for a few reasons, it can be a good indicator of where you are hydration wise. If your urine is clear and you are having to pee a lot, you can probably slow down on drinking. If your urine is consistently dark and you are often extremely thirsty after training, you might need to consider having more fluids during your workouts.

image courtesy of Nils Nilsen

Do Women Sweat Differently?

ACSM’s Position Paper: Exercise and Fluid Replacement does state that, in general, women have lower sweat rates and electrolyte losses. There are exceptions to this, but as a general rule women do lose less than their male counterparts. This difference is likely because women tend to be smaller in body size and have lower metabolic rates when they are exercising. The menstrual cycle and, specifically, perturbations in estrogen and progesterone can also have effects on hydration status. In the high hormone phase (luteal phase) right before a period begins, progesterone levels increase significantly. The net result is greater sodium loss. In addition, thirst sensation also decreases during this phase. To offset any issues resulting from hormone changes right before the period begins, female athletes can potentially add sodium to foods. If your race falls within this phase, you might also consider using a pre-load solution such as Precision 1500 or Skratch hyper-hydration (as long as you’ve done a trial run prior to race day). As an aside, there is a lot to learn about differences between female and male physiology when it comes to performance. In this article, I have barely scratched the surface in terms of what is out there. If you want to learn more, here are a few recommendations for more information:

Leah Roberts’ Women.Thrive Symposium Session: Active Women, the Menstrual Cycle and Beyond: How to Work with Your Physiology 
(Live session is on July 17, 2020 and this recorded session will be up until July of 2021. Women.Thrive is donating 100% of proceeds to COVID-19 relief, so you can hear Dr. Roberts’ session for $25 and give to a good cause at the same time)

Roar, Stacy Sims

Your Period and Exercise: What to do when Aunt Flo is Visiting on Race DayLeah Roberts, M.D.

In Conclusion

Hydration is especially important to athletes in endurance sports because of the length of time on course. Nailing your drinking strategy can move the needle significantly in terms of results, quality of the experience, performance, recovery, and—obviously—health. I always talk about nutrition (which includes hydration) as the fourth discipline in multisport. Optimizing how you fuel for training and racing really can take your performance and results to the next level if you are willing to put in the time. As you go through the steps and tests I discussed in this article, remember that your hard work will be worth it. Obviously, we have covered a lot here. If something isn’t clear or if you have any further questions, feel free to reach out to me on Instagram.  

Katie Elliott, MS, RD, CSSD, USA Triathlon Coach is a board certified sports dietitian and the founder of Elliott Performance and Nutrition, based in Aspen, Colorado. Katie works with clients nationwide in-person and via telehealth, and also does exercise physiology testing at Achieve Health and Performance. Katie’s practice specialties include sports nutrition, wellness nutrition, nutrition for executives and professionals and weight loss. Katie enjoys working with athletes of all levels and abilities from weekend warriors to professional athletes. In addition, Katie played Division I tennis at Davidson College, is a two-time Tour of New Zealand champion, is a proud Wattie Ink Hit Squad member and has competed on more than 10 amateur world championship triathlon teams.