Running in the Heat: Why Slowing Down Today Can Make You Faster Tomorrow

As we’re now getting more used to having more sunshine and good weather than usual, we have to start thinking about how we get the most from this weather to both maintain and improve our training. I’ll be the first to admit it, I have said “its too hot to go for a run, I’ll go when it cools down”, but avoiding this heat is also missing a big opportunity to train as well.

Before we get into the hows, whys, what you should and shouldn’t do, also consider this, both Manchester and London Marathons took place with peak temperatures around 26 degrees. Sure, you trained all winter, you did the dark mornings, the cold long runs, the sessions in the rain. Then race day rolls around, the sun is out, it is 26 degrees, and suddenly your legs feel like they belong to someone else. Your watch says your heart rate is through the roof and you are barely holding a pace that felt easy back in February.

Here’s the thing, that is not you being unfit. That is heat. And once you understand what your body is actually doing as temperatures soar, it gets a lot easier to run smart instead of blowing up.

Expect a Higher Heart Rate and a Slower Pace

The first rule of hot running is simple. The same effort is going to cost you more.

When it is warm, your blood has two jobs instead of one. It still has to carry oxygen to your working muscles, but now it also has to shuttle heat out to your skin so you can cool down. Same amount of blood, twice the workload. On top of that you are sweating, and a good chunk of that fluid comes straight out of your blood plasma. Less fluid in the system means each heartbeat pumps a little less, so your heart just beats faster to keep up.

The result is what coaches call cardiac drift. Same pace, climbing heart rate. As a rough guide, for every 1% of body weight you lose through sweat, your heart rate ticks up by around 3 beats a minute. Lose a few per cent on a hot long run and you can easily be 10 to 20 beats higher at a pace that normally feels comfortable.

So do not panic when the numbers look ugly. On a hot day, run to effort, not to the pace on your watch. Accept that you will be slower, and that being slower is the correct response, not a failure. The runners who ignore this and chase their usual splits are the ones you see walking at 30k.

Hydration, and the Bit Everyone Forgets: Salt

Everyone knows to drink in the heat. Fewer people get the salt part right.

You can lose anywhere from half a litre to two litres of sweat an hour when it is hot, and sweat is not just water. It is loaded with sodium and other electrolytes. If you are out for a long, hot effort and you only replace the water, you are effectively watering down what is left in the tank, and that brings its own problems.

For anything under an hour, plain water is usually grand. For longer and hotter, you want electrolytes going in alongside the fluid, whether that is a tab in your bottle, a proper sports drink, or salt in your gels. On long runs, aim for somewhere in the region of 400 to 600ml an hour, and drink to thirst rather than forcing it down or ignoring it completely.

Here is a simple way to learn your own sweat rate. Weigh yourself before and after a hot run, with as little on as possible. Every kilo you have lost is roughly a litre of fluid you did not replace. On race day the aim is to finish having lost no more than about 2 to 3% of your body weight. Go beyond that and performance falls off a cliff, and you start drifting into genuinely risky territory.

What is Actually Happening to Your Blood

This is where it gets interesting, and where the real "why" behind that higher heart rate lives.

When you train in the heat over a couple of weeks, your body adapts by expanding your blood plasma volume, sometimes by around 10%. More total blood is a good thing for cooling and for keeping each heartbeat strong. But there is a catch in the short term. Adding all that plasma thins the blood out, so the concentration of haemoglobin, the stuff that actually carries the oxygen, drops. Your blood is more watered down, so litre for litre it is shifting a bit less oxygen. That is a big part of why those first hot runs feel so rough.

Now the good news, and it is genuinely good news. That drop in concentration seems to act as a signal to your body to start building more red blood cells. Stick with heat training for three to five weeks and you can raise your total haemoglobin mass by somewhere around 2 to 4%, which nudges your VO2 max up along with it. In other words, the suffering is not wasted. Heat training does a lot of the same work as altitude training, without the plane ticket or the mountain. People in the endurance world literally call it "poor man's altitude".

So those brutal early-summer runs where you feel like you have lost all your fitness? You have not. You are quietly laying down adaptations that pay off when the weather cools, and on every race after that.