Training in the Heat — What Your Body Is Actually Doing

This Week — and What the Heat Does

London broke its all-time June temperature record this week. The Met Office recorded 35.8 degrees Celsius — the highest June temperature in the United Kingdom since 1976 — and issued a Red Extreme Heat Warning, its most serious level, for Wednesday and Thursday. In parts of southern England, temperatures reached 39 degrees. Overnight, the temperature did not drop below 20 degrees. There was no recovery period. The heat simply continued.

I have trained through this. Not every session, not without modification, but I have been out in it and felt what it does. It is not the same as training in normal conditions. The effort that would normally feel moderate feels enormous. The sun at this temperature is a different proposition entirely.

When you exercise in normal conditions, the body has one primary job — deliver oxygen and fuel to the working muscles. In extreme heat it has two. It must supply the muscles and simultaneously pump blood to the skin to release heat through sweating. The heart is doing twice the work for the same output. This is why a session that feels manageable at 15 degrees feels genuinely hard at 35. Your heart rate is higher, your perceived effort is higher, and your performance drops — not because you are less fit, but because the body is dealing with something your fitness alone cannot fully compensate for.

Core temperature rises faster than usual. Sweating increases significantly — in one session I went through four litres of fluid in forty minutes at 33 degrees and still wanted more. Joe, my training partner, was in the same position. Two people who train regularly and take their fitness seriously, both struggling in ways that had nothing to do with fitness level and everything to do with the temperature. We finished the session. We were glad we did. But we both knew that the heat had made it a different kind of session entirely.

Studies show that heart rate at the same pace or effort level can be ten to twenty beats per minute higher in extreme heat than in cooler conditions. That is not a small difference. Understanding what is actually happening is the difference between training sensibly and doing something genuinely dangerous.

Diabetes and Heat

For most people, training in extreme heat is uncomfortable and demanding. For someone with Type 1 diabetes, it carries an additional layer of complexity that cannot be ignored.

Heat affects blood glucose in unpredictable ways. It can cause levels to spike dramatically or drop without warning — sometimes within the same session. Insulin absorbs faster in heat because blood flow to the skin and subcutaneous tissue increases, which means a dose that would be entirely appropriate in normal conditions can act too quickly and too aggressively. Dehydration compounds this further. When fluid levels drop, blood glucose concentrates and readings become unreliable guides to what is actually happening in the body.

I monitor my levels carefully in this kind of weather. Not obsessively, but consistently. A session that goes well on a cool day can go very differently at 35 degrees, and the warning signs of a blood sugar problem — dizziness, unusual fatigue, a feeling that something is simply wrong — can be difficult to distinguish from the normal discomfort of training hard in heat. This is why I stop when something does not feel right. Not because I am being cautious for the sake of it, but because the consequences of ignoring those signals in extreme heat with Type 1 diabetes are serious.

If you have diabetes and you want to train in hot weather — and there is no reason why you should not — check your levels before, during and after. Carry glucose. Do not train alone if the conditions are severe. And be honest with yourself about how you feel. The session will still be there tomorrow.

The Body Adapts

In 2010 I summited Kilimanjaro and spent time travelling through southern and eastern Africa. I have also trekked through Morocco in temperatures that made this week's London heatwave feel familiar. Neither experience was planned as a training exercise. Both taught me more about what the human body is genuinely capable of than any session in a temperature-controlled gym.

The body adapts. That is not a motivational statement — it is a physiological fact. Given consistent exposure to heat, the cardiovascular system becomes more efficient, the sweat response improves and the perceived effort at any given temperature decreases over time. The body that struggled in the first week of a heatwave is measurably different from the body in the third week, if the exposure has been managed sensibly. The adaptations are real and they carry over — into cooler conditions, into competition, into everyday training. This is sometimes called free fitness: the gains made in heat transfer to performance everywhere else.

What that experience also taught me is that the body's signals are reliable when you learn to read them. Discomfort is not the same as danger. Fatigue is not the same as failure. But there are signals — dizziness, nausea, a sudden feeling of confusion or weakness — that mean stop immediately, find shade, take on fluid and do not push through. Those signals are non-negotiable regardless of fitness level, experience or how far you have come.

How to Handle It

Fluid is everything. Not sipping occasionally, not waiting until you are thirsty — drinking consistently throughout. Thirst is a late indicator and by the time you feel it you are already behind. At fifty, with Type 1 diabetes, I notice the difference immediately when I am properly hydrated. Energy levels, blood sugar stability, the ability to get through a session without it unravelling — water is the single cheapest and most effective performance tool available. Most people consistently underuse it.

Avoid training outdoors between eleven in the morning and three in the afternoon. Early morning or evening is a completely different experience. Rest longer between sets than you normally would — the body needs additional time to regulate core temperature between efforts. And do not overthink clothing. Something lightweight and breathable is sufficient. An old sports shirt does exactly the same job as an expensive technical fabric.

Adjust your expectations. A session in extreme heat is not the same session as one at fifteen degrees. Performance will be lower and perceived effort higher. That is not failure. That is the body working correctly.

Gym or Outdoors

The obvious answer in a heatwave is the gym. Air conditioning, controlled temperature, no direct sun. It is comfortable and it is accessible and there is nothing wrong with it. On the most severe days this week, training indoors was the sensible choice and I would not argue otherwise.

But outdoor training is the real deal. It always has been. The body working against genuine conditions — variable temperature, natural resistance, no climate control — responds differently from the body in a controlled room. There is a reason the greatest athletes in history have always included outdoor work in their training, regardless of what was available indoors.

The gym is a tool. Outdoors is a philosophy.

The One Rule

A workout is better than no workout. That has always been true and it remains true in a heatwave. Showing up matters. Consistency matters. The people who find a reason not to train when conditions are difficult are the same people who find a reason not to train when conditions are easy.

But there is a line. Heat exhaustion is a medical emergency. The signals the body sends — dizziness, nausea, sudden confusion, an overwhelming feeling that something is wrong — are not to be ignored, pushed through or dismissed as weakness. They are the body doing its job. Respect them.

Train in the heat. Adapt to it. Learn what your body does under those conditions and become better for it. Take on fluid, adjust the timing, rest properly between sets and be honest about how you feel. Then come back tomorrow and do it again. That is old school training. It has always been done in all conditions, in all weathers, without excuses and without drama.