The Missing Piece of the Hydration Puzzle
My mom loves working outside in her garden.
There’s a peaceful waterfall, a koi pond, and a plethora of plants, flowers, and trees she can tend to her heart’s content. But it’s hot in the summer (and she sweats about as much as I do).
I’m constantly reminding her that she can’t just drink plain water or she risks becoming dehydrated. It’s a common mistake many people make that can lead to dizziness, headaches, and fatigue.
It’s important for my mom, but it’s even more crucial that endurance athletes like you understand what they’re losing when they sweat and how to replace it effectively.
Why you can’t drink just plain water
When you sweat during a training session or a race, you’re losing more than water. You’re also losing electrolytes, including sodium. A visual sign of this is white, streaky marks that appear on your kit on a hot day.
What does sodium do? Sodium helps regulate fluid balance, nerve signals, and muscle function.
If you replace the water you lose by sweating, but not the sodium, the levels in your blood can become diluted. In some circumstances, this can lead to a dangerous condition called hyponatremia, or low sodium concentration in the blood. Symptoms include headaches, nausea, confusion, seizure, or death.
Therefore, the first important takeaway is:
Takeaway #1: Hydration isn’t just about replacing water. We must replace fluid and sodium.
Drinking only plain water, especially during long endurance events can cause several problems:
Increased urine output without actually hydrating cells
Fatigue, cramping, dizziness, or bonking due to improper fluid and nutrient balance
Poor gut absorption, which can leave you feeling bloated or with a sloshy stomach
These are all bad things. We don’t want this to happen. Now, you can drink some plain water to supplement fluid intake, but it can’t be the only source of fluid. A sports drink is essential.
The sodium–carbohydrate connection
Now, here’s where things get interesting. Your body has a specific way of absorbing sodium from your gut into your bloodstream. It uses a co-transport system, which means sodium and glucose (carbohydrate) are absorbed together through the same channel in your intestines.
Think of it like a buddy system. Sodium needs a bit of carbohydrates to get into your bloodstream effectively.
So if you’re drinking water with salt, but no carbs, your body can’t absorb the sodium as well.
Takeaway #2: That’s why most sports drinks include both sodium and some carbohydrates.
Understanding this relationship is the missing piece of the hydration puzzle—and it’s what makes sweat testing such a powerful tool for endurance athletes. It helps you dial in your sweat rate so you can hydrate strategically and perform at your best.
Q&A with Lexi Kelson of Precision Fuel & Hydration
Recently, our monthly webinar featured guest speaker, Lexi Kelson, “Sweat Scientist” at Precision Fuel & Hydration, who talked to us about all things sweat! Triple Threat Life Club members were able to attend live and can watch the replay here in the Expert Interview Archive.
Lexi is a Registered Dietician based in Atlanta who manages the PF&H Sweat Test Centers. She visits pro and college sports teams to help train their staff on hydration protocol, test athletes, and strategize hydration needs.
Brittany: Can you explain the purpose of sweat for the human body?
Lexi: When you sweat, your body is trying to cool itself down. I think about my dog as a good example. She pants. That's her natural mechanism. But as humans, we don't pant. We're going to sweat, and that's what our body is doing when it’s trying to cool ourselves down.
But when you sweat, you're going to be losing fluid, and then within that fluid, sodium. There’s a variance in terms of how salty your sweat is, but then also how heavy of a sweater you are.
Brittany: Can you explain how two athletes might do the same training session in the same weather conditions, but have entirely different hydration needs. For example, my husband, Matt, doesn't sweat a lot, but he gets white crusty marks on his kit. Whereas, I look like I just got out of the swimming pool when I get done with a run, but I'm not as salty.
Lexi: So if you're a heavier sweater, you're going to be losing more fluid, with or without the sweat sodium. Just because you're a heavy sweater, that doesn’t mean you're going to be a saltier sweater.
You have to take into account both of those variables, as well as weather conditions. The hotter the weather, the more total sweat you're going to lose. Workout intensity also falls into that same category. You're probably going to sweat more when you're working harder.
Brittany: There are different forms of sweat testing. There’s testing to determine sweat rate and testing to determine sodium concentration. What’s a basic version of a sweat rate test someone could do at home?
Lexi: You can measure your sweat rate really easily at home by weighing yourself before and after a training session. You're just looking for the change in fluid loss based on your weight. So you would weigh yourself, fully nude, before your training session. Then, do the training session. Factor in anything you drink during the session. Afterwards, weigh yourself without your clothes on. Then you’ll know, “Okay, I've lost 24 ounces of fluid in an hour in these conditions at this intensity.”
If you weigh yourself, before and after a session in the heat, versus the cold, your sweat rate is going to be different. But your sweat sodium concentration is going to be the same. Knowing how heavy of a sweater you are in different conditions is what you really want to tailor for your race.
Brittany: What type of information do you get from the Precision Fuel & Hydration sweat testing process?
Lexi: The sweat test is actually the same test they use in the hospital for cystic fibrosis, so it’s been around for a long time. It's a passive at-rest test, which means you don't have to exercise, because we're not looking at how much total fluid you're sweating out. We’re looking at the sodium concentration, and that’s pretty stable because it’s largely genetically determined.
During the test, we put electrodes on your forearm that stimulates your sweat glands. Then, we put a collector disc on your forearm. It sucks up the sweat into these little tubes. There’s no needles and nothing painful. Then, we run that sample through an analyzer machine to see how many millimoles of sodium, translated into milligrams, how many milligrams of sodium you’re losing per liter, or per 32 ounces of fluid.
Finally, we tie the fluid and sodium component together. We're not saying that you lose X amount of sodium over the course of a race. We're saying that for every 32 ounces of fluid you sweat out, you’re losing this amount of sodium.
Brittany: Now, athletes have to take that information with a grain of salt, because they need to figure out how their bodies are going to respond when working out in real-life conditions. For example, if there’s high heat or humidity, or how much their stomach will allow them to consume. I know I can drink a lot more while riding the bike than when running.
Lexi: Your sweat sodium concentration is the easiest, most consistent variable that's going to be the same. So even if it's a hotter day, or your gut isn't trained to tolerate more carbohydrates or more fluid, all of that is going to be separate from your sweat sodium concentration.
This is based on the sweat rate test you did at home, measuring how much fluid you lost, and thinking about the practicality of being able to drink more on the bike than the run. So you might go from drinking closer to 100% of fluid losses on the bike to 50-60% of fluid losses on the run.
Front-loading hydration on the bike is a big thing we see triathletes do a lot, just because of the logistics. You still want to hydrate and fuel on the run as well, but you can give yourself a bit of a buffer by doing a little bit more on the bike. Then, you’re averaging the gold standard of about 70 to 80% replenishment of your total fluid losses.
Brittany: I think another thing to consider with hydration, or the other side with nutrition, is that you can have the perfect plan, but you must have the ability to adapt when you get out there. You have to be able to adjust on the fly, and I think that's something a lot of athletes struggle with.
Lexi: That’s always why I recommend having a range. If you give yourself a range, then you know what your non-negotiables are. You can hit those and then try to increase where you can.
Brittany: Sweat rate can be affected by a variety of factors: weather, body size, gender, fitness, ect.?
Lexi: Body size can definitely be a factor. If you have a larger body, you're probably going to be a heavier sweater, because there’s more mass. Another thing is fitness. Especially with heat adaptation protocols, we see sweat rate increasing with heat adaptation, because your body becomes more efficient with sweating.
So it’s going to start sweating sooner. This also applies if you live in the heat, like Georgia or Florida. We know that very well! Also, physical fitness has an effect. The body is training itself to do the performance of the workout, but it's also training itself to do it as efficiently as it can, because the body wants to take care of itself. Part of that means using sweat to your advantage and cooling yourself down.
Brittany: Why is understanding sweat loss so important for triathletes?
Lexi: You guys are a rare breed in the sense that you're doing all these things at once. There are so many variables that can happen within a triathlon, because you're covering three different sports. If you can control something as important as your hydration, that goes a long way. Knowing your data and how to adjust it on the fly can be really helpful in terms of going from discipline to discipline. Your sweat sodium concentration and sweat rate are two huge parts of that. So for triathletes, sweat becomes a very controllable variable.
Brittany: Now in talking about distance of triathlon, like a 70.3 (5-7 hours) or Ironman (10-17 hours), at that point it’s almost impossible not to really pay attention to your hydration. You’re going to get dehydrated throughout the course of the race, but trying to delay, or prevent the effects, of too much dehydration from getting to a point where it starts to affect performance, heat regulation, fatigue, ect. is crucial.
Lexi: I think the biggest problem we see is athletes not practicing their strategy. That’s a big factor with carbohydrate intake and gut training. A lot of times, in training, you can get away with lower fluid intake and less sodium, because you're not training for as long. But on race day, you want your body to do what it’s been used to doing, and training with your hydration is part of that. There's nothing new on race day!
Also, you're giving yourself a chance to recover well from training. And if you're recovering well, you're probably getting more out of your training than if you aren’t recovering. Part of recovering well is fueling and hydrating your training sessions.
Brittany: Definitely! People tend to focus on the isolated part of, “I’m going to train hard,” and they neglect recovery or hydration, especially when running. For any run over 15 minutes, we carry our hydration with us. It’s for several reasons: to train our stomachs to take in fluid, to fuel the effort properly, and to ensure we’re not getting behind.
Most triathletes are doing two sessions a day. At that point, if you’re completely behind after session 1, and you’re going into session 2 under-hydrated, by the time you’re done, you’re completely in the hole. It’s a snowball effect.
I know when we do these big workouts where we’re riding for 3+ hours on the bike and then doing a 30-minute run, we’re exercising for close to 4 hours. If I’m not very cognizant of making sure I replace fluid and sodium throughout the rest of the day, I will get horrible headaches.
Lexi: Yes, as an endurance athlete, your sweat losses mount up really fast because you're out there for so long. If you were doing a shorter distance, you could replenish it easier, but you're still losing a high volume. That's just the nature of the sport. So staying on top of it goes such a long way.
Brittany: Talking about the weather component, why do athletes have to be careful in conditions with high heat and humidity?
Lexi: Your body is trying to cool itself down when it's sweating, but you're not able to really have the full cooling effect because it's humid. Your core temperature is going to be a bit higher, which means that's going to put extra stress on your GI system.
We see more GI issues in hotter temperatures, because your body is trying to cool itself down, and that takes a lot of energy and blood flow. But it also needs to send blood flow to the gut to help digest and absorb the fuel you're taking in. So staying hydrated is a way to help your GI system as well, because if you're dehydrated, that's putting a strain on your gut.
Sometimes we see those two things go together, where if someone isn't hydrating properly from a sodium and fluid standpoint, they may be at a higher risk, especially in hot races, for having really bad GI distress and not being able to fuel properly, which obviously causes a whole host of other issues.
It’s kind of a domino effect if you're not staying hydrated, and it can really have a knock-on effect on so many parts of your race and recovery.
Brittany: I think that's something a lot of people struggle with. When they have GI problems, they automatically think it’s because of carbs. But it can also be a result of dehydration. I know this personally.
Brittany: So people can do a traditional sweat rate test at home, but if they want to do the PF&H sodium concentration test, how would they do that?
Lexi: If you go to the Precision Fuel & Hydration website, and click on sweat testing, you'll see a map of all the test centers in the world so you can find one closest to you. Also, check our Instagram for where we'll be popping up at events, like the 70.3 Ironman World Championship in Spain.
Next Steps
If you’re interested in learning more about the other important component of fueling- carbohydrate intake, check out this post I wrote with help from my coach, Marni Sumbal, Board Certified Sports Dietician and owner of TriMarni Coaching and Nutrition.
It goes into much greater detail, but a great starting point for 70.3 athletes out there is:
60-70g carbs per hour (if new, start at 40-50 g/hr and work up)
400-600mg sodium
24-28 ounces of fluid per hour
The Most Common Triathlon Nutrition Mistakes (You Don't Know You're Making)
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