A few weeks ago, I was flat out in a 31°C dojang grading for my Blue Stripe belt in Kuk Sool Won (KSW) - a Korean martial art ("Aaaa-rrrrrrr" 👊).

Two full hours of forms, techniques, breakfalls, strikes, and board-breaking … plus some laps of the dojang and running on the spot…

All while wearing full black KSW uniform: thick jacket and trousers, my yellow belt, and a black T-shirt underneath.

Just two wall-mounted fans. No air con.
Still - nowhere near as brutal as cooking in a roasting hot kitchen for several thousand troops including Australian and American Special Forces while on Ops in Jordan!

I didn’t flake, cramp, or quit.

Why? Because I didn’t wing it.

I weighed myself before and after grading.

I hydrated beforehand (based on current sports nutrition guidelines – no guessing).

I brought the right fluids to have during the session (although we had to wait to be told when we could drink). I didn’t overdo it either – I’ll cover this in a bit.

And I tracked my sweat rate so I could rehydrate properly afterwards – not just drink and hope for the best!

This wasn’t luck. It was a strategy. And it made all the difference.
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Sweating is crucial for regulating body temperature.

When the body heats up from exercise or hot conditions, you sweat! And that’s how your body gets rid of excess heat.

So being able to predict your sweat rate is important, because it helps you create a tailored hydration plan, avoid overheating, and stay ahead of dehydration before it wrecks your health, performance, or overall wellbeing (de Korver et al., 2025).

Sweat rate is how much fluid you lose through sweat – usually measured in litres per hour (L/Hr).

It varies from person to person and with the conditions. Some guys sweat like a leaky radiator and lose ~0.5L/Hr. Others sweat buckets - over 2.5L/Hr, especially in hot, intense conditions (Gonzalez et al., 2022).

It depends on things like body size, intensity, temperature, and clothing (like my thick KSW jacket and trousers) (Jay et al., 2024).

And this isn’t just about martial arts or marathons.

You don’t have to be smashing out workouts to sweat buckets.

If you’re wearing thick clothing, PPE, or working in hot environments, like grafting in a warehouse, factory floor, commercial kitchen, a construction site, or even a stuffy office - you’re sweating.

Maybe you're waking up drenched because you've been sweating like 10 men through the night...

All that sweat loss adds up, especially in summer.

Most guys don’t realise how it’s affecting them.

In fact, even the military have flagged it.

UK military guidance shows that in extreme conditions, soldiers can lose up to 8–12L a day! That’s NOT your target - but it gives you a sense of just how much fluid the body can burn through when heat, physical effort, and heavy clothing stack up.

So if you’re only sipping a couple of brews and wondering why you feel foggy, sluggish, irritable, or just off your game... hydration might be the missing piece.

Because being under-hydrated can quietly chip away at how you feel and function - mentally, physically, and gut-wise. 

Now add IBS into the mix.

If you’ve got IBS with diarrhoea, you’re losing more fluids. If you’re constipated, being dehydrated can make it worse. Either way, your gut’s not going to thank you for running on empty.

Hydration won’t fix your IBS. But being dehydrated will definitely add to your load.

So yeah, sweat rate isn’t just a number for elite athletes.

It’s a useful tool for anyone who wants to feel sharper, train smarter, and avoid making IBS worse, and stop dehydration becoming yet another thing your body’s battling.

Bottom line: “one-size-fits-all” doesn’t cut it – especially when IBS is in the mix.

Yeah, there are general guidelines, like the UK Government recommends 6–8 glasses (~1.5-2L) of fluid a day (UK Government, 2016), but that’s just baseline. It doesn’t cover extra fluid losses from sweat, diarrhoea (common in people with IBS), or vomiting.

Because hydration affects:

👉 Physical performance

👉 Mental focus

👉 Gut function

Short-term symptoms of dehydration include feeling thirsty, having dark pee, headaches, feeling dizzy, and tired. And when you’ve got IBS, you can already struggle with head fog, and fatigue.

But here’s the thing, by the time you feel thirsty it’s already too late (Adams et al., 2019). So relying on thirst as your only cue to drink isn’t enough, versus staying ahead of the game.

Be proactive, not reactive.

Studies have shown even mild dehydration (as little as 2% of body weight lost) can mess with:

•  Coordination, focus, mood,
•  Thermoregulation and endurance
•  Gut motility and digestion – things that IBS loves to hijack
(Jardine et al., 2023; Nuccio et al., 2021; Pugh et al., 2021).

You’re also looking at increased heart rate, reduced blood volume, and a higher core body temperature. Plus, it makes you feel like you’re working harder than you actually are (Thomas et al., 2016).

All bad news for performance and for your gut.


By the way, dehydrating yourself to “make weight” can be fatal (Amado, 2025; Barley et al., 2019).

Let me be clear: This isn’t just risky – it’s killed people.

Dehydration should never be used as a weight-cutting strategy for combat sports, rowing, weightlifting or any sport with weight divisions. It’s a dangerous practice, and sadly, not rare. One high profile case was an MMA fighter who died in 2015. And he’s not the only one – weight-cut deaths have been reported in many sports.

Hydration isn’t just about muscle cramps or performance – it also affects your gut.

Simon doesn’t train less because he’s not motivated. It’s because every hard session triggers IBS …and frustration. So he avoids it. Then the weight creeps on, the stress ramps up, and symptoms spiral, and the stress gets even worse.

And dehydration only pours fuel on that fire.

Here’s how:

(Al-Beltagi et al., 2025; Costa et al., 2025; Jeukendrup & Gleeson, 2025)

IBS loves chaos. And dehydration creates chaos.

But it goes beyond this, because chronic dehydration can lead to constipation, urinary tract infections, kidney stones and even chronic kidney disease.
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Strenuous exercise and endurance sports increases your chance of gut symptoms (Ribichini et al., 2023) ...even when you don’t have IBS.
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​Symptoms can include stomach pains, bloating, burping, farting, diarrhoea. Similar to IBS (and other gut conditions).

Exercise-induced gut symptoms can occur in an estimated 30-90% of endurance athletes during or after intense sessions (Ribichini et al., 2023). And they’re more likely when:

IBS is a gut–brain condition. Stress turns up the volume on symptoms. It activates your fight-or-flight response (sympathetic nervous system), which slows digestion, and increases gut sensitivity.

Add stress + dehydration + exercise = a perfect IBS storm.

Try to train → gut flares up → avoid training → feel worse → repeat.

Sound familiar?

You don’t need a lab. Here’s a really simple way to work out your sweat rate:

​(Jeukendrup & Gleeson, 2025)

Example: Lost 1.2Kg + drank 0.5L over 2 hours → Sweat rate = 0.85 L/Hr.
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There are plenty of recommendations out there about how much to drink and when. But there's lots of factors to consider, like your tolerances, age, fitness level, clothing, exercise intensity, and whether you’re acclimatised to the heat all play a role.

That's why I'd rather give you specifics when I'm working with you and done assessments and got some data to work with. But for now, here's a general plan...

Before Training:
It’s always wise to start your session well hydrated. A quick pee check can help - if it’s pale or clear, you’re good to go. If it’s dark yellow? You’re already on the back foot.

During Training:
For sessions longer than an hour, aim to sip regularly, unless your sport, event, or job doesn’t allow (like my KSW grading, where I could only get a drink when we were told to).

The goal? Minimise fluid losses, but don’t overdo it.

You don’t want that horrible sloshing belly feeling, and guzzling too much too fast can backfire, especially if your gut’s sensitive.

As I mentioned earlier - you can't rely on thirst to let you know when to drink, because you've already got a level of dehydration by the time your body's telling you you're thirsty!

Use drinks with sodium and carbs if you’re a heavy or salty sweater, working in the heat, or doing prolonged sessions over an hour.

And if you train or compete regularly (which might be hit-and-miss if IBS gets in the way), don’t trial a new hydration strategy on competition day - test it first.

After Training:
To rapidly rehydrate, it’s recommended to replace 125–150% of your fluid losses within 4-6 hours (Bonilla et al., 2021). Adding salt and carbohydrates helps your body retain more of that fluid. This can be done through drinks, or simply by having your drink alongside your next meal or planned snack.

And don’t forget, some foods count towards hydration too - think watermelon, tomatoes, cucumber, yoghurt, and soups.

💪 For tailored advice (which is always the best option), especially if you’ve got IBS in the mix, invest in a sports Dietitian who specialises in IBS, so you’re not bouncing between two pros and getting conflicting advice.
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White crusty marks on your kit after working out? That’s salt.

Sweat is mostly water and salt (electrolytes). And your body needs these to function properly, so it’s crucial to replace these.

Heavy sweaters (no, not jumpers!) and salty sweaters need higher sodium drinks. Just plain water won’t cut it. Having a salty snack with your drink can also help stimulate thirst and improve fluid retention (Racinais et al., 2023).
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It’s not about guzzling litres blindly.

Overhydration (hyponatraemia – too little sodium in the blood) is real.

And can be fatal.

It’s often caused by drinking too much plain water in a short space of time without replacing sodium.

I’ve known of soldiers who’ve had a heavy PT (Physical Training) session or been out on the hills for Special Forces training, and piled in because they’ve drunk too much water (obviously there could’ve been other reasons as well). And they’ve only had water – no sports drinks or snacks to have with it either. So they were tanning (guzzling down) the wrong type of fluids and too much.

And research has shown it’s happened in military training, charity runs, and elite sport. A recent study of 919 ultra-cyclists found that overdrinking was linked to swelling, confusion, and gut symptoms (Schwellnus et al., 2023).

If your weight goes up after training – you’ve drunk too much! Plan your intake. Don’t wing it.

So it’s not just about “drinking plenty of water” – more is NOT more.

It’s about drinking smart. Planning. Tracking. Adjusting.

Don’t wait for heat, stress or symptoms to hit before you take it seriously.

• Jeukendrup, A. & Gleeson, M. (2025). Sport nutrition. 4th Edition. Human Kinetics.

• Jay O. et al. (2024). Whole-body sweat rate prediction in outdoor running and cycling exercise.

• IOC Consensus Statement. Racinais S. et al. (2023). Br J Sports Med, 57(8), 8–25.

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• ISSN Position Stand. (2023). Nutrition and Weight Cut Strategies for MMA and Combat Sports.

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• Heaney S et al. (2022). Systematic Review of Athletes’ Nutrition Knowledge and Intake. Nutrients, 14(2), 439.

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• Amado, K. J. M. (2025). Rapid weight loss in combative sports: Systematic literature review. Scientific Journal of Sport and Performance, 4(3), 466-469.

• Adams, W. M., Vandermark, L. W., Belval, L. N., & Casa, D. J. (2019). The utility of thirst as a measure of hydration status following exercise-induced dehydration. Nutrients, 11(11), 2689.

• Kerksick, C. M., Wilborn, C. D., Roberts, M. D., Smith-Ryan, A., Kleiner, S. M., Jäger, R., ... & Kreider, R. B. (2018). ISSN exercise & sports nutrition review update: research & recommendations. Journal of the international society of sports nutrition, 15(1), 38.

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• Al-Beltagi, M., Saeed, N. K., Bediwy, A. S., El-Sawaf, Y., Elbatarny, A., & Elbeltagi, R. (2025). Exploring the gut-exercise link: A systematic review of gastrointestinal disorders in physical activity. World Journal of Gastroenterology, 31(22), 106835.

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