It’s been brutally hot in the UK the last few weeks. I’m currently in Croatia, where the heat is on another level entirely, and I’m still running through it.

I brought this up with my team recently. One of them said people shouldn’t train in the heat, or wouldn’t want to. I disagreed. Training in all conditions is part of the point. I’ve run in the high 30s more times than I can count, and I’ve also trained in minus 20. Both have taught me something the other couldn’t.

Someone else on the team floated a different idea: training in the heat, sweating more, means you lose more weight. Look at boxers in sauna suits, they said.

That’s the claim worth settling first, because it’s the one that actually gets people making bad decisions.

The Sauna Suit Myth

Boxers wear sauna suits to cut water weight fast, usually in the final day or two before a weigh in, so they officially make weight for their category. The moment weigh in is done, most of them rehydrate and eat immediately, and a large chunk of that weight comes straight back on within hours. That’s not fat loss. That’s a short term water manipulation for a specific competitive rule, not a training strategy for anyone trying to actually get leaner over time.

The line does blur slightly in one genuine way. Your heart rate runs higher in heat for the same effort, because your body is working harder to cool itself down as well as move you. That means you can burn a modest amount more energy during the same session in a hot environment compared to a cool one. But the effect is small, and most researchers and physiologists who’ve looked at it agree it’s nowhere near enough to meaningfully change body composition on its own.

So here’s where I’d settle it. Almost all of the weight you see drop after a hot, sweaty session is water. It comes back as soon as you rehydrate. There is a small, genuine increase in calories burned due to the extra cardiovascular effort, but it’s a minor effect, not a fat loss strategy. If someone tells you sweating more means burning more fat, they’re repeating a myth that’s been sold to people for decades, usually alongside a sauna suit.

Where Heat Actually Does Make a Difference

This is the part that gets lost whenever the sweating myth gets debunked, because people swing too far the other way and assume heat does nothing useful. That’s not true either. There are real, well documented adaptations that come from training in heat, they’re just not about fat loss.

Plasma volume. Training in heat consistently increases blood plasma volume, in some studies within days. More plasma volume means your heart doesn’t have to work as hard to deliver oxygen and nutrients around the body, which improves endurance capacity. What’s genuinely interesting is that these adaptations don’t just help you in hot conditions, several studies on trained cyclists found that heat acclimation improved performance back in cool conditions too, not just in the heat itself.

Cardiovascular adaptation. Alongside the plasma volume increase, heat training has been linked to improved cardiac output and reduced heart rate for a given effort over time, essentially training your cardiovascular system under a heavier load than normal.

Joint and tissue benefits. Heat increases blood flow and makes connective tissue more pliable, which is part of why athletes warm up before training in the first place. Training in genuine heat effectively extends that warm up effect through the whole session, which can support range of motion and reduce the stiffness that often comes with cooler conditions.

Mental toughness. This one is harder to put a number on, but anyone who has trained through real heat or real cold knows it’s not just physical. Choosing to go out and train when the conditions are unpleasant, when it would be far easier to stay inside, builds a kind of resilience that a comfortable gym session simply doesn’t ask of you.

The Dangers, Because They’re Real

None of the above is permission to be reckless, and this is the part I’d never want to gloss over. Training in heat carries genuine risk, and it’s worth taking seriously.

Dehydration is the obvious one. You lose fluid and electrolytes fast in heat, and if you don’t replace them, performance drops sharply and risk rises just as fast. Heat exhaustion is the next stage up, symptoms include heavy sweating, weakness, dizziness, headache and a weak or rapid pulse. If you or someone you’re training with hits that point, the move is to stop, get to shade or somewhere cool, and rehydrate immediately. Left unchecked, heat exhaustion can progress into heat stroke, which is a genuine medical emergency, marked by a body temperature of 40°C or higher, hot dry skin, confusion, and in severe cases seizures. That needs emergency medical attention, not toughing it out.

A few practical rules I’d stand by. Build up gradually, your body needs time to acclimate to heat, don’t go from zero to a hard session in 35 degrees. Hydrate properly before, during and after, not just when you feel thirsty. Avoid the hottest part of the day where possible. And critically, think about where you’re training. Heading out for a long run in a remote mountain range in extreme heat with no phone signal and nobody knowing your route isn’t tough, it’s just a bad decision. Make sure someone knows where you are, carry a way to be contactable, and don’t put yourself somewhere help can’t reach you if something goes wrong.

Why I Still Choose to Train in the Extremes

Personally, I enjoy training in extremes. A dark, freezing, rainy winter morning in the UK in January. Minus 30 in Canada. 35 plus degrees like it’s been recently. I don’t do it because I think it’s the optimal way to train, I’ve made clear above that the science doesn’t back that up as a shortcut to results.

I do it because most people won’t. That’s the part of this that often gets missed entirely, the mental side, the discipline of doing the things you don’t want to do.

There’s real research behind this too. A part of the brain called the anterior mid cingulate cortex is closely linked to willpower and tenacity, our capacity to keep going through something difficult rather than pulling back from it. Some of the more interesting findings in this space suggest this region responds to being used, engaging repeatedly in things that are hard or uncomfortable appears to be associated with it functioning more effectively over time, in a similar way to how a muscle adapts to being trained. It’s an active and still developing area of research, but the underlying idea lines up with what most people who train hard already sense intuitively.

That doesn’t just show up in fitness. It shows up in how you handle a bad day at work, a hard conversation, a setback that would normally knock you off course. Training yourself to do the uncomfortable thing on purpose, whether that’s a run in the heat, a session in the cold, or anything else you’d rather skip, builds something that carries into every other part of your life.

That’s what this has always been about. Not the workout itself. The person it makes you into.