A clinic above Lac Léman. September, mid-afternoon.
The sun comes in from the west, off a lake flat and silver all the way to the French side, and I watch it through a window while being taken apart on a bicycle.
Electrodes on my chest and back. A cuff on my arm every few minutes. A cardiologist behind a screen, watching lines I cannot see, raising the resistance, smiling. He is irritatingly fit.
I am there because I have had atrial fibrillation, and been treated for it.
The test runs to maximum. Nothing left. The lines settle, the wattage drops away, and he tells me my heart is perfect.
He hands me half a ream of paper towel. Sweat on the bike, the floor and me.
In the cool-down I ask the question that started this. What should I pay attention to when I train, to protect the heart?
Stay well hydrated, he says. Make sure you take electrolytes.
I had asked how to protect my heart. The answer was about blood volume — which turns out to be an answer about everything.
His words are as close as I can remember them a fortnight on, not a transcript. I am a rower with a search engine, not a doctor. If your own heart is the question, that conversation belongs in his office.
Start with what leaves the body. Barnes and colleagues measured 1,303 athletes: the endurance group sweated 1.28 litres an hour, losing sodium at 51.7 millimoles an hour. About three grams of table salt. A ninety-minute row: four and a half grams gone.
The World Health Organization says an adult should eat under five grams of salt a day. Ninety minutes on the water and you are almost there. That is what leaves you in a session, not an argument for salting your dinner.
Now look at what goes back. Water, and a gel if it is long.
Water alone does not hold. Sodium keeps what you drink in circulation instead of running it straight through. Without it you refill the bottle and pass most of it an hour later — which is how you drink all morning and still finish down two kilos.
Then the cascade starts, and none of it is disputed.
Blood volume falls. Stroke volume falls with it, because there is less to pump. Heart rate climbs, because that is the only way left to hold cardiac output. Your heart is doing more work than the session asked for. The boat is no faster.
It does not stop at the heart. Blood flow to the working legs drops by two litres a minute in dehydrated cyclists — less oxygen just as you ask for more. Sweating and skin blood flow are impaired by the dehydration they caused, so you run hotter. And once core temperature climbs, blood flow to the brain falls away too, with dehydration accelerating the drop — sobering, for the organ steering your boat down a head race. The brain copes. Strain, not emergency. But strain in four places at once, from one cause.
You have seen the first part of this. You filed it under getting older.
The steady-state piece where the rate creeps ten beats in forty minutes and the split never moves. González-Alonso and colleagues took it to the extreme: in hyperthermic endurance athletes at three to four per cent body mass loss, stroke volume fell by about a third. That is not your Sunday outing.
Atrial fibrillation sits inside this too, and here I will be careful. I have skin in the game. Four electrophysiologists, writing in HeartRhythm Case Reports this year, put dehydration on one endurance athlete’s list of modifiable risk factors. Adequate hydration, they write, is imperative for AF patients, to prevent the fluid shifts and electrolyte depletion that can trigger arrhythmias. Now read what that is. Clinical judgement, on a single case. Not a trial.
Why did the salt go missing in a sport obsessed with fuelling? A compound noun nobody reads. The American College of Sports Medicine’s finding is that carbohydrate-electrolyte drinks help beyond an hour. We heard the first half, built a culture of gels to replace the glucose, and left the other half in the small print with its job: holding blood volume up.
And when we buy the sachet we buy the wrong thing off the front. An electrolyte is anything that dissolves into charged particles. Seven do real work in you; sodium and chloride are what leave in sweat in quantities worth counting. And yes — those sweat numbers come from Gatorade’s own lab: peer-reviewed, and funded by people who sell sports drinks. Notice where their measurements land. On salt, which nobody can charge much for. Magnesium is on the front because it sells. It is also the one electrolyte put through randomised trials. They found nothing.
So, plainly. Nobody has shown that a sachet protects your heart. What is not in doubt is the other direction: sweat for ninety minutes, put back water and sugar alone, and every system that runs on blood volume pays at once. Not a rhythm problem. A whole-body one, of which rhythm is part.
He tested me in a cool room, wired to a screen, with a full tank — the only conditions under which a stress test works. It cannot tell me what I ask of that same heart ninety minutes into a hot Sunday, two kilos down. That question is not answered in a clinic. It is answered in the bottle.
So next time you are sweating hard and the rate creeps and the split does not, do not blame your birthday.
I use electrolytes. I will go on using them.
Faster Membership starts the same way every time: measure, then decide. The lactate protocol, the training plans, the five supplements with evidence behind them. Electrolytes are not on that sheet and never will be. They will not make you faster. They stop the session costing more than it should.