Episode 159 - Estimate your sweat rate: New free tool!

Estimate your sweat rate: New free tool!

As a sports dietitian specialising in triathlon, one of the key elements I focus on with my clients is hydration strategy. Hydration plays a critical role in performance, and understanding your sweat rate is key to optimising your fluid intake.

Recently I was lucky enough to sit down with Lindsey Hunt, a senior sports scientist at Precision Fuel and Hydration, to discuss his work in this area and the great new tool that came from it. With an impressive academic background and hands-on research in human thermoregulation, Lindsey's insights are invaluable for athletes looking to fine-tune their hydration strategies.

Lindsey's research primarily focused on whole-body sweat rate prediction in both indoor and outdoor running and cycling. This involved numerous exercise trials with a broad cohort of participants to gather a comprehensive understanding of sweat rates across different conditions.

The culmination of this research is the sweat rate calculator, a free tool that helps you tailor your hydration strategy to meet your unique needs and perform at your best!

Links:

Sweat Rate Calculator

Heat Suit Training Preserves the Increased Hemoglobin Mass Following Altitude Camp in Elite Cyclists

Check how well you’re doing when it comes to your nutrition with our 50 Step Checklist to Triathlon Nutrition Mastery

Start working on your nutrition now with my Triathlon Nutrition Kickstart course 

It’s for you if you’re a triathlete and you feel like you’ve got your training under control and you’re ready to layer in your nutrition. It's your warmup on the path to becoming a SUPERCHARGED triathlete – woohoo!

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Episode Transcription

Episode 159: Estimate your sweat rate: New free tool!

Welcome to the Triathlon Nutrition Academy podcast. The show designed to serve you up evidence-based sports nutrition advice from the experts. Hi, I'm your host Taryn, Accredited Practicing Dietitian, Advanced Sports Dietitian and founder of Dietitian Approved. Listen as I break down the latest evidence to give you practical, easy-to-digest strategies to train hard, recover faster and perform at your best. You have so much potential, and I want to help you unlock that with the power of nutrition. Let's get into it.

[00:00:00] Taryn: Joining me on the podcast today is sports scientist, Lindsay Hunt, who comes all the way from Sydney, Australia, but he's about to make the big bold move to the UK. And I don't know why you're doing that, mate. Honestly, I spent a couple of years there and it's bloody freezing.

[00:00:34] Lindsey: well part of the reason is we built a heat chamber, so I'll be right at home.

[00:00:38] Taryn: So he's moving to the UK because he's taken on a new role as the senior sports scientist at Precision Fuel and Hydration. But his background, he has completed a Bachelor of Applied Science in Exercise and Sports Science back in 2016, and went on to do his Masters of Applied Science in Research in the UK.

[00:00:55] Taryn: In 2020, both of those at the University of Sydney, his master's thesis focused on the effects of caffeine on human thermoregulation. And then he went on because he's a total nerd to do his PhD in thermal physiology. He's, Thesis was titled Assessing Human Heat Stress Risk During Outdoor Summer Sports.

[00:01:15] Taryn: Now, that is a big broad term, but what it aimed to do is improve our understanding of the thermal environment in which we play sport as well as improve our understanding of the interactions between humans and their environment and the exchange of heat between them. Like, how cool is that? So that's the nerdy part out of the way and the main reason why I have him on the podcast talking to us today.

[00:01:38] Taryn: But this guy is also a little bit of a secret weapon athlete himself. He loves cycling, but he recently ran a three hour, 13 minute marathon at Berlin. Holy heck. And man, after my own heart, he did that consuming 128 grams of carbohydrate per hour with absolutely no gut issues. And he told me that he was feeling hungry, which is just crazy.

[00:02:05] Taryn: So you can read more about that case study on the Precision Fuel and Hydration website, if you do want to dive into some of those details. So welcome, Lindsay.

[00:02:14] Lindsey: No worries. Thanks for having me. It's great to be here, Taryn.

[00:02:16] Taryn: Ah, you're welcome. I'm excited to pick your brain. So the reason you're here is because two new papers have just been released that I would love to talk to you about. You're a co author and were part of the research studies. I'll link them in the show notes if you do want to get a little bit nerdy and, dive into the details on that.

[00:02:31] Taryn: The papers are on whole body sweat rate prediction in indoor and outdoor cycling and running. And there was a lot of research that went into that. And as a result of that research, they developed a very cool little tool, a sweat rate calculator, which I know you're going to love.

[00:02:49] Taryn: And we will talk about that as we go. But before we dive into that. Lindsay, can you walk me through those research papers? Just generally how it worked to get the research that you did to be able to create these prediction equations to understand somebody's sweat rate without having to do sweat testing.

[00:03:10] Lindsey: Yeah, for sure. So, the process was sort of the same for both running and cycling, where we had people turn up and come and complete a range of exercise sessions in a range of conditions at a range of intensities. , and we'd measure their body mass before and after they went for a run or a ride, in these different environments at different conditions, so we got a range of sweat rates. We use that information as well as some environmental data and some physiological data that we could then use to predict their sweat rates using heat transfer equations, biophysical heat transfer equations.

[00:03:45] Taryn: That sounds really hard. It also sounds like a lot of work. How many studies were there and how many participants were there to be able to get enough data to create the equations?

[00:03:55] Lindsey: we had quite a few trials for the indoor cycling trial, we had 168 people go through each trial and indoor running we had 266. For the outdoor cycling and running trials, we had 158 running and 182 running outdoors.

[00:04:12] Taryn: Yeah, that's a lot of people.

[00:04:13] Lindsey: So quite, quite a few, because we have a decent amount of variability in, sweat rates, we need quite a lot of trials and a lot of people to come back through and see the range of sweat rates that we need.

[00:04:24] Taryn: Yeah, and doing it indoor and outdoor in both cycling and running.

[00:04:28] Lindsey: Yeah,

[00:04:29] Taryn: Did one person do all four of those things or did somebody just stick with running and say a cyclist just stuck with cycling?

[00:04:36] Lindsey: the same person would have only done cycling outdoors or cycling indoors. I don't think we had anyone come and cross over between running and cycling. Yeah. This,

[00:04:50] Taryn: need to do to get the good data that you're looking for?

[00:04:53] Lindsey: all that people really needed to do was hold a steady power output for the 60 minutes and people ranged in their ability to hold a steady power output indoors, this is much easier because we could just set a fixed workload when they were running at the steady pace or cycling at, you know, 150 watts or something like that.

[00:05:12] Lindsey: Whereas outdoors, of course, you've got the natural terrain variation, even for running. It's it was reasonably flat, but there was some up and down, so it wasn't as steady as you'd hope, and there's natural variation in how much people, how fast people run and ride. And so there's some variation in their intensity there, but for the most part it was steady state, and they did this for around 60 minutes.

[00:05:34] Taryn: Sounds relatively easy.

[00:05:37] Lindsey: Yes, so people could come and do relatively easy sessions so they could select their own intensity. So someone could come along and say, I want to smash 300 watts today for 60 minutes. Is that okay? And I'd be like, great, 

[00:05:48] Lindsey: let's do it. And then other people would come and do a recovery ride.

[00:05:52] Lindsey: So we've got a nice big range of sweat rates there, but I think the main aim of the study was to sort of see how well it works at lower sweat rates, but also where it becomes more important is at higher sweat rates. So if you're only losing a litre, half a litre per hour, you'll probably, be able to replace that relatively easy, like as you drink to thirst and whatnot, and you won't necessarily have issues of dehydration at that sweat rate for extended durations.

[00:06:19] Lindsey: But as soon as you start going above one and a half, two and a half, three liters an hour, then that starts getting quite unsustainable and also becomes like important to forecast so you can know that, oh, okay, in these conditions, I'm going to sweat two liters an hour. So I need to bring at least. 1.

[00:06:36] Lindsey: 75 or 2 litres an hour with me.

[00:06:38] Taryn: what's the highest fluid intake rate you've ever seen in somebody in an hour?

[00:06:43] Lindsey: Uh, it's just over 2 litres an hour I've seen in the lab. Whether or not that's sustainable over more than one hour, I'm not sure.

[00:06:50] Lindsey: Especially, the need to urinate as well, but was kind of at a sweat rate of like 3. 3 litres an hour as well, so they were still in a pretty massive deficit there after just one hour.

[00:07:02] Taryn: Yeah, solid. That happens a lot here where I live in Brisbane, Queensland. It's very common for large men to have sweat rates that high. And just the ability to keep pace with that by drinking fluid is so difficult. So they end up in a very big dehydration hole, no matter what they do. Yeah.

[00:07:20] Lindsey: Definitely, definitely, it's a real issue. And I mean, if they were more aware of how much they were dehydrating, potentially, it would help them drink a little bit more.

[00:07:29] Taryn: Amen to that. So thankfully you had some women participants and males and like a range of calibre of athletes too, is that correct? It wasn't just a study in only elite athletes or only what's the right word? I was gonna say something like not particularly friendly.

[00:07:45] Lindsey: bottom end of the age group is, no, um,

[00:07:48] Taryn: the Packers.

[00:07:49] Lindsey: Back of the package. I mean, we, we definitely had a range of people come and do the study. Like, I mean, I ran all of the outdoor cycling trials personally and more within my own PhD research. So I'm familiar with the fitness level of all of those guys and girls.

[00:08:03] Lindsey: We didn't do like proper VO2 max testing and profiling beforehand because it, doesn't really matter how fit you are in terms of what your sweat rate will end up being. So your VO2 max doesn't necessarily matter. All that matters is your actual metabolic rate at that power output or running speed.

[00:08:19] Lindsey: And so people would come along and do a range of exercise sessions. The lowest power output for an hour we had in indoor cycling was 75 Watts for an hour, which is, quite low. And then all the way up to 340 Watts in indoors for an hour, which is.

[00:08:33] Lindsey: Pretty substantial.

[00:08:34] Taryn: And what did you find? So you've got so much data, I would hate to be the person that had to go through that. It's like my worst nightmare. But what are some of the key things that you found from testing so many people with their sweat rates across all those modalities and temperatures and humidities?

[00:08:51] Lindsey: Yeah, for sure. I think one of the things we talked about before was does whole body sweat rate increase linearly or close to linearly with ambient temperature? I wouldn't say it, it does. It doesn't really have a massive increase until you get to around degrees Celsius when you're cycling outdoors.

[00:09:09] Lindsey: It does more or less increase linearly with power output and so that means, yeah, if you're going from a hundred watts in sort of normal conditions, 20 22 degrees you might be sweating half a litre an hour depending on your body mass and then by the time you're going up to 200 watts you're kind of sitting at around litre per hour So that's something that's quite interesting.

[00:09:30] Lindsey: It is more dependent on your external workload or running speed than kind of anything else.

[00:09:36] Taryn: And can you see the relationship between body size? Or male versus female, is there anything there?

[00:09:43] Lindsey: We didn't test exactly for male versus female. There's no, reason that we're aware of where women would tend to sweat more or less given the same The only reason that women tend to sweat less is because of body size. And so they, on average, have a lower body mass, and so their metabolic rate, in absolute terms, in terms of watts, not watts per kilo or watts per meter squared, is, generally lower.

[00:10:11] Lindsey: And so, for the same power output at 200 watts, if you're a 60 kilo female versus a 70 kilo male, you will be sweating less because your metabolic rate is lower, because you're using less skeletal muscle to produce that power, if that makes sense. Yeah, back to that one, it is definitely related to body mass, and so if you're a larger individual, you're more likely to be able to produce more power in an absolute term, and so, 200 watts is often easier for someone who's larger, and so they'll be able to produce that more power, and more metabolic heat is associated with that, and so they end up sweating more, 

[00:10:51] Taryn: that makes sense. That makes total sense.

[00:10:53] Taryn: Is there anything else that you found doing the research that you're like, Oh, we didn't know that before? Or that's really new and novel?

[00:10:59] Lindsey: , in my data from outdoor cycling, we didn't see a massive influence of relative humidity or humidity on sweat rates. So there was kind of no relationship there, 

[00:11:10] Taryn: Oh, that's so interesting. 

[00:11:12] Lindsey: is kind of surprising, but I would say that that was related to the humidities that we observed.

[00:11:16] Lindsey: So they were relatively low humidities if we're talking about absolute humidities in the, climate of Sydney in our summers. So we didn't actually experience anything too humid. And so when you're cycling at 30, an hour, which most people were doing, because it was a relatively flat loop you're not really having issues of sweating efficiency.

[00:11:36] Lindsey: So pretty much every gram of sweat that you produce on the skin surface gets evaporated by the airflow. And so, the only way humidity actually impacts heat transfer is by reducing the amount of sweat that can evaporate. And so if you don't have that limitation, which I don't think we did it makes sense that there would be no impact of humidity. on sweat loss. But if you were running, for example, in the same conditions, you have much less airflow going over the skin surface, then I think you'd probably, we probably would have seen an impact of humidity in those conditions. But in these studies in particular, we didn't actually assess these relationships.

[00:12:13] Lindsey: We just put together the model and assessed how well the model did. And so in my PhD I kind of went through these things in more detail to try and assess some of these relationships, but you won't see that in the papers just yet. If that makes sense.

[00:12:27] Taryn: Yeah. You need to come and do some testing up in Queensland where our humidity is crazy high.

[00:12:33] Lindsey: Yeah, I've been working with an athlete right now who's saying he's losing just over three liters an hour just sitting in his garage heat acclimating for Kona. it's a lot of sweat and you just hope that that doesn't happen when he's out in Kona.

[00:12:46] Taryn: Fingers crossed.

[00:12:47] Lindsey: Yeah.

[00:12:49] Taryn: That is the joy of Kona though. High humidity, all the black surface on the road from riding and running. Like, yeah, , it's Ironman World Championships for a reason, I think.

[00:12:59] Lindsey: That's it. And one of the things we're planning on doing when we get out to Kona is bringing a environmental monitoring little setup so we can actually check, or at least try and check what the conditions are on those lava fields on those hot, highways and see how well those temperatures and conditions match what nearby weather stations say because in Kona, it might be saying it's 27 degrees, but maybe On the ground, you're feeling , and it's actually 30 where you're actually racing which has a pretty big impact on predicted fluid losses.

[00:13:28] Taryn: And we were talking offline the other day and you said that that's where a lot of sports are going is their own little weather station to really finesse and dial in their recommendations or, plans for people. Because we've always relied on, weather data from a weather app or the bureau of Meteorology, but each, like all the sports that have money anyway, , I can't see triathlon getting that anytime soon, but having their own little weather station, which is pretty cool.

[00:13:53] Lindsey: For sure, , it makes perfect sense. So if you're a sporting organization, even like the Tour de France or the Vuelta and you're conducting a race, out there, a stage, and it's maybe you start, you often don't start in the mountains, but often you finish in the mountains and your conditions, your terrain changes across, 180k, 200k stage. How are you going to know what the conditions are at the worst point where people are climbing up the hill or something like that and they're actually doing 450 watts for 20 30 minutes like these guys are? Is that safe? I don't know. If you know the conditions at that point then you'll, you'll be able to know.

[00:14:29] Taryn: I don't think they take safety into consideration in the Tour de France, do they?

[00:14:33] Lindsey: I won't comment on that, but potentially they could do a bit more.

[00:14:38] Taryn: So very PC of you, I love it. Okay, so you've done a buttload of research, got a whole heap of data. How do you then take that data and develop this sweat rate calculator tool? Like, talk me through the development of that because it is such a cool piece of tech and I'm sure a lot of people listening will love to go and have a play with it.

[00:14:57] Taryn: I'm not going to give you the link just yet, but how did you get to the developing of that tool?

[00:15:02] Lindsey: So our partners at Coca Cola actually put the function of that calculator together online. So I wasn't involved directly in that, but In terms of the calculator, it's different in the sense that from a lot of other prediction equations in the past, where they're statistical models of sweat loss, where you, say your power output is roughly this, your body mass is roughly this, you're probably going to sweat.

[00:15:25] Lindsey: around about this based on previous data. That's not what we've done here. We've actually put together a calculation of heat exchange based on assuming you're in heat balance across 60 minutes or 90 minutes or whatever length of exercise you're doing. And so if you assume the body ends up in a state of relative heat balance and you know how much heat is being produced, and then the heat exchanged between the human and the environment, you can calculate what the sweating requirement would be to maintain heat balance.

[00:15:55] Lindsey: it's simply some mathematics that goes through a couple of equations and it spits out a value sweat loss.

[00:16:02] Taryn: Yeah, it's very cool. I was playing with it the other day to create something like this, you obviously have to test it and retest it and look at the actual data for somebody's sweat rate versus what is predicted.

[00:16:12] Taryn: How close are you getting across cycling and running to being accurate with somebody's sweat rate prediction?

[00:16:19] Lindsey: So, in the indoor studies, our predictions were the best. And so,, mean absolute error for predicted sweating rate cycling was pretty good. 0. 01 and then 0. 02 for treadmill just means there's no systematic error. One way or the other. And so we had modeling and validation groups for both indoor running, indoor cycling, outdoor running, outdoor cycling.

[00:16:44] Lindsey: So we split those groups up to train the model or at least develop the model on that initial data set and then test it on the validation group so that we didn't have any issues of overfitting the data.

[00:16:57] Taryn: Yeah, that's pretty close. And what about outdoor?

[00:17:00] Lindsey: the mean absolute errors were pretty similar in outdoors, so 0. 02 in cycling and 0. 03 in running. But the variability in those was a little bit higher. And so, the variability explained indoors cycling at 92 percent and running was 78%. 

[00:17:18] Taryn: Does that mean? For somebody listening going, What are you talking about, Lindsay? What does that mean?

[00:17:24] Lindsey: it just means that the predicted sweat rate explains the variability 95 percent of the time. So it's a tough metric to explain in terms of real numbers. So the kind of better way to describe it is with confidence intervals. So 95 percent of the time you'll be within.

[00:17:42] Lindsey: 400 mils an hour for cycling and running out, outdoors, if that makes sense.

[00:17:48] Taryn: Yep, that's 5 percent of uncertainty.

[00:17:50] Lindsey: Yeah, that's right.

[00:17:51] Taryn: Yep. stats.

[00:17:54] Lindsey: easy,

[00:17:55] Taryn: Okay, so it's relatively close. I did mine and I thought it overestimated what my sweat rate is or would be. But like you said earlier, there is variability and, you know, somebody's sweat rate can vary, like on a daily basis, depending on what's going on in their body. Did you notice any of that in your research did anybody ever come back and do subsequent days and their sweat rates changed?

[00:18:17] Lindsey: They definitely did, but they were under different conditions almost exclusively. So we, we weren't able to test the. replication or the repeatability of a sweat rate in a certain condition. If all variables were the same, I'd expect that things would be very, very similar and 

[00:18:37] Taryn: you were struggling on day two and your power output was either way higher or way less because you're fatigued.

[00:18:43] Lindsey: Exactly. Exactly. So other things like that change, or perhaps you get slightly more acclimatized or acclimated, then your sweat rate might come down or go up depending on the type of conditions you're exercising in. So there are things that can change it, but the biophysics doesn't change. I guess it's our ability to measure the biophysical interactions, changes and create measurement.

[00:19:05] Lindsey: variability, let's say. So one of the research studies that this study was based off happened in 2013, where they looked at whole body sweat rate indoors in a whole body calorimeter. So it's basically just a small metal tin can where they increase the airflow and increase the temperature and reduce the humidity so that all sweat can just freely evaporate. And they saw that in that kind of perfect environment, this kind of modeling explained up to 95 percent of the variability in sweat rate. And so that's kind of what we saw here, where we, we saw up to 92% in this one. And that was still indoors, but indoor cycling and still a good situation for this kind of modeling.

[00:19:49] Taryn: That's so cool, and this research is very much needed. There's not a lot of work in this space, so it's really good and really exciting to see such a huge study with so many participants to help guide this space in the future. Can you talk me through the sweat rate calculator and all of the, I guess, metrics that you need to plug into it so that if somebody does it, they go in it and go, OK, I understand how to work this thing to get the best results out of it.

[00:20:14] Lindsey: Yeah, for sure. So we'll start with indoors, I guess, because it's the most simple. It doesn't matter if you're cycling and running, but just the variables that you input would change. So if you're running, I think you just put in your running time and running,

[00:20:28] Lindsey: And then if you're cycling, you just put in your, your power output as well as your duration of exercise, just so that you can get that total fluid loss value.

[00:20:38] Lindsey: You put in your height and weight. And cause this helps determine your, some of the biophysical interactions that happen between the skin surface and the environment. And then you put in your temperature, your wind speed. and your humidity. And then if you're in an outdoor environment, you actually can include something called black globe temperature.

[00:20:56] Taryn: And what is that?

[00:20:57] Lindsey: It's a way to measure environmental radiation, essentially. So it would go up if it's sitting in the sun and it would be basically the same as air temperature if it was in the shade. And so it's measured via kind of a 15 centimetre diameter black globe with a thermometer stuck inside it, and it just absorbs all forms of radiation, but mainly solar radiation.

[00:21:19] Lindsey: And sometimes outdoors, I mean, in Sydney and across Australia, we've seen if air temperature is around 35, 40 degrees, you can easily see this black globe thermometer be close to 50 degrees sometimes. 50, 52, 53 degrees, which is pretty uncomfortable to be in, that's for sure.

[00:21:37] Taryn: And how do you get that data? Like if somebody's looking at putting that in, is there any way you can get that from a weather website or anywhere, or you need, need one of these cool black globes with a thermometer inside?

[00:21:48] Lindsey: Unfortunately, that one in particular, you need that particular measurement device. weather stations don't typically report this bit of information. And it is highly variable depending on the wind speed and the amount of solar radiation it's affected by. So if a bit of cloud comes over, it drops pretty quickly.

[00:22:05] Lindsey: And so one of the things we've done in this calculator is that you can just provide a little estimate of how sunny you think it is. So if it's outdoors and you don't have That black globe temperature information, you can click a little button in the calculator and just say, is it sunny or not sunny or very sunny, for example.

[00:22:23] Taryn: Okay. So we can kind of estimate based on what data you have without having to have this sexy piece of equipment because I work with triathletes and they're like, how do I get me one of these things? How to cut?

[00:22:34] Lindsey: Yeah, you don't need to buy one just yet. They're reasonably expensive. 

[00:22:38] Taryn: so you plug all these things in and it gives you a number. It gives you your sweat rate in mils per hour, and your total sweat volume for the duration of the session, which is pretty cool. And you're saying that we're 95 percent confident that your number, depending on whether it's indoor, outdoor, cycling or running, is about right, give or take a few hundred mils. So if you want to go and play with it, the URL is sweatratecalculator.com. It's a pretty sexy little tool that is free to use, and it can just help understand, like, ballpark what you might be aiming for. So, check it out. what does somebody do with that information?

[00:23:19] Taryn: That is the most, like I'm very much a practitioner, that is the most important thing. You get this number or you do your own sweat testing, what on earth do you then do with that?

[00:23:26] Lindsey: It really depends on the event that you're doing, and where that event is. So if you're doing an event that's hours and hours and hours, you often want to increase the amount or the percentage of fluid replacement that you go for. So if you're doing a 10k run, for example, then it's depending on where it is.

[00:23:45] Lindsey: If it's hot, you might want to replace some fluid, but you might not need to replace much at all to support performance or health but if you're doing a half marathon or longer then you start needing to replace quite a decent proportion of those fluids especially if it is warm and so then your percentage of replacement goes up and up and up as the intensity as the duration and as the thermal load goes up

[00:24:08] Taryn: Do you recommend a set percentage of replacement depending on what's going on?

[00:24:13] Lindsey: Not necessarily, just because we don't have guides on that for a specific event. For example, there hasn't been any research done on that. The research that has been done, sort of settled on this 2 percent of your body mass is where you start to see performance losses. And that's sort of also the point at which physiologically we start to feel quite thirsty.

[00:24:36] Lindsey: And so it's, it's kind of at that point where if you get to two percent, you start to feel thirsty and you've already started to see performance losses. And often that kind of happens a little bit sooner when it's hotter because we've got less blood flow available , to the working muscles.

[00:24:50] Lindsey: So when it's hotter there's more blood flow going to the skin surface to facilitate evaporative cooling. And so if we've got less blood flow available, that performance decrement happens sooner and or happens more substantially. So, in some of the modeling that I'm doing for my athletes is making sure that I provide fluid recommendations.

[00:25:11] Lindsey: to keep them within 1, 1. 5%, 2 percent at the worst, at the end of the event in the last couple of Ks, if it's a triathlon, for example, so that when you're doing the bike course at Kona, for example, you might only be sitting at half a percent body mass loss for the whole thing. And then when you're starting the full marathon, you're not starting that in a state of one and a half percent because when you start running you're going to start sweating more and not be able to replace two and a half liters an hour when you're running. you want to get to that point at the marathon as hydrated as possible let's say.

[00:25:46] Taryn: Yes. What do you do with somebody though that sweats three litres an hour on the bike? 

[00:25:51] Lindsey: I'd say outdoors it probably won't happen. Unless you have Taddei Pogacar doing 370 watts for an hour or two hours

[00:26:01] Lindsey: in 35 degrees, which he doesn't usually do. In the hot stages, he might sweat that much for, you know, 20 minutes up a climb, 30 minutes up a climb, and then do that again, but there's usually breaks in that.

[00:26:16] Lindsey: The professional guys out in Kona, they might be getting up there, but they're going to be so well trained and tolerate that amount of fluid that they probably can get that high in terms of fluid volumes getting back on board. So, 

[00:26:30] Taryn: Yeah, it is hard. This is a question you may not know the answer to, because I don't think we've done the research, but what do you think happens to your sweat rate over time? Like we don't know what happens to somebody's sweat rate in a full distance Ironman as the bike extends or the run extends and they're out there for, 10 plus hours.

[00:26:49] Lindsey: Mm. 

[00:26:50] Taryn: Lindsay Hunt think happens to our sweat rate?

[00:26:53] Lindsey: There's a few competing thoughts. I think the first one is that. As we start to fatigue over time, the relationship between power and metabolic rate changes, especially when we're working at higher intensities. And so if you start out doing 250, 300 watts at a high level, maybe you're still doing that power output two, three, four hours later you're probably going to be producing more sweat or need, at least need to produce more sweat to maintain heat balance.

[00:27:21] Lindsey: And therefore your sweat rate might go up. But the other thing is that your power output probably will come down depending on how well you pace that effort. And so it might come back down and be the same. The other thought is that if someone does get dehydrated, if someone does extend beyond 2%, then you do start to see reductions in sweat rate. And then body temperature will start to climb. , that's something you definitely want to avoid. So if you start noticing your sweat rate coming down, not that you'd necessarily notice but certainly you'd start to notice you warming up. And so that's something you definitely want to avoid.

[00:27:58] Taryn: Yeah, good tip. Thank you. I like to pick brains on all the things that I deliberate on over all the time also.

[00:28:05] Lindsey: Yeah, that's alright. It's an interesting one, because, you've got lots of different options there. I mean, if all things remain the same, The only thing that's going to increase your sweat rate is like fatigue, but then of course, as I said, dehydration will reduce your sweat rate at a certain point.

[00:28:19] Lindsey: So yeah, it's competing ideas.

[00:28:21] Taryn: so if somebody wants to go play with it, the URL again is sweatratecalculator.com. Are you collecting the data from people actually? Are you looking at what people put in and, and collecting any of that?

[00:28:30] Lindsey: I am not aware of that. I don't think so.

[00:28:34] Taryn: what do you want to do to change it or make it better for 2. 0 if there's going to be a 2. 0?

[00:28:40] Lindsey: The tricky thing with this is there's of course more things we can put in to the calculator. So There's things that we know affect, sweat rate that aren't in there, for example, clothing. But for one thing, this is meant to be practical for people to use. And so when you start adding in additional things that we know affect things, there's already quite a lot of numbers and bits of information that you need to pop in.

[00:29:02] Lindsey: And so one of the pushbacks is, how do we add more things in there to make it better? without adding in everything

[00:29:08] Taryn: not that hard to do though. It probably takes 20 to 30 seconds to input the data. So don't be scared off that it is an overload. It is very simple. Like, choose your modality, put your height and weight in, the environmental conditions that you know.

[00:29:22] Taryn: If not, there's some good, like, options to estimate what the environmental conditions are if you don't know for sure. And then put your metrics in. Power output, duration distance, done. Like, it's very, very quick and simple.

[00:29:35] Lindsey: definitely, definitely. And that, that's partly why we tried to keep it that way. As soon as we start adding in options, more options that don't make it easier it just makes it harder for people to use.

[00:29:45] Taryn: Yep, like what color clothing have you got on? Have you got a speed suit? Have you got, you know, whatever?

[00:29:50] Lindsey: it certainly would, would matter. I think when I, when I modeled the difference between a fully black piece of kit versus fully white out in Kona for a female cycling at kind of 200 watts in 30 degrees and decent humidity, the sweat rate difference or the required sweat rate difference is around 150 200 mls an hour.

[00:30:10] Lindsey: difference is going to be amplified in those conditions because it is kind of more stressful, but it's certainly a large amount

[00:30:17] Taryn: Yeah, good tip. So if you are doing Kona, maybe look at investing in some white cycling slash triathlon kit 

[00:30:24] Lindsey: the first thing it does is reduce the sweat or the fluid you need to take on. That's just huge. If you don't need to bring an extra almost, , 700 mils maybe, then that's, that's huge

[00:30:34] Taryn: which is what 24 ounces ish or just under 24 ounces for the US people

[00:30:38] Lindsey: for sure.

[00:30:39] Taryn: Well, thank you so much for letting me pick your brain, Lindsay. I know that a lot of people will love to go and play with that calculator. So, sweatratecalculator.com, it should be really easy to find. Uh, and I guess, like, what you do with that data is, you know, the age old question.

[00:30:54] Taryn: Ha, 

[00:30:55] Taryn: ha. 

[00:30:55] Lindsey: For sure.

[00:30:56] Taryn: n equals one, do your own experimenting and go and do your own sweat rate testing too, to understand what your numbers are compared to what the calculator is spitting out and see if you are in that tiny little range where it should be relatively accurate.

[00:31:09] Lindsey: For sure. I think the one thing I will say is, I wouldn't aim to replace a hundred percent of your losses, particularly if you're not drinking any sodium in those fluids because you'll certainly increase the risk of issues such as hyponatremia. That's one thing to certainly avoid with this calculator.

[00:31:26] Lindsey: It's not a fluid replacement calculator. 

[00:31:29] Taryn: Yes. Good advice. Thank you. And yeah, thank you so much for joining me.

[00:31:32] Lindsey: no worries. Thanks, Tony. 

[00:31:33] Taryn: Woo!

Thanks for joining me for this episode of the Triathlon Nutrition Academy podcast. I would love to hear from you. If you have any questions or want to share with me what you've learned, email me at [email protected]. You can also spread the word by leaving me a review and taking a screenshot of you listening to the show. Don't forget to tag me on social media, @dietitian.approved, so I can give you a shout out, too. If you want to learn more about what we do, head to dietitianapproved.com. And if you want to learn more about the Triathlon Nutrition Academy program, head to dietitianapproved.com/academy. Thanks for joining me and I look forward to helping you smashed in the fourth leg - nutrition!