Electrolytes for Endurance Sport: What You Need to Know
Electrolytes are probably the most over-marketed and least understood part of endurance nutrition. Walk into any bike shop or running store and you'll find a wall of tablets, powders and sachets promising to stop cramp, prevent bonking and generally save your race. Some of that is fair. A lot of it isn't. The honest version is narrower and more useful: electrolytes matter enormously in specific conditions, and barely at all in others, and knowing which situation you're in is worth more than any product on that shelf.
I've spent enough long days on the bike in the Scottish hills — and enough summer marathons where the forecast lied — to have got this wrong in both directions. I've finished rides with white salt crust on my shorts and a headache that lasted until bedtime, and I've also drunk so much electrolyte drink on a cool 60-miler that I spent the last hour looking for hedges. This article is the practical version of what I wish someone had explained to me a decade ago: what electrolytes actually do, roughly how much you lose, when to bother replacing them, and where they sit relative to the thing that actually determines whether you finish strong, which is carbohydrate.
What Electrolytes Actually Do
Electrolytes are minerals that carry an electrical charge when dissolved in body fluid: sodium, chloride, potassium, magnesium and calcium. Sodium and chloride are the two that matter most in sport, because they're the ones you lose in meaningful quantities through sweat.
Their main job isn't muscular — it's fluid management. Sodium is the primary determinant of how much water sits in your blood plasma and where that water goes. That has two knock-on effects an endurance athlete cares about. First, plasma volume affects cardiovascular efficiency: when it drops, your heart has less blood to push per beat, so heart rate climbs for the same power or pace. Second, sodium drives thirst and drinking behaviour, and it enables water absorption in the small intestine — sodium and glucose are co-transported across the gut wall, which is why plain water is actually absorbed more slowly than a lightly salted, lightly sweetened drink.
What electrolytes almost certainly don't do is prevent cramp. The old sodium-depletion theory of exercise-associated muscle cramping has held up badly. Studies comparing crampers and non-crampers in the same race have generally found no meaningful difference in blood sodium or hydration status. Current thinking points to neuromuscular fatigue — you cramp because that muscle is working beyond what it's trained for, often late in a race you paced optimistically. Salt tablets are not a substitute for the long ride you skipped.
How Much Do You Lose? Electrolytes for Endurance Sport in Real Numbers
This is where generic advice falls apart, because the variation between athletes is enormous. Two useful numbers:
- Sweat rate: typically 0.5–2.0 litres per hour. A cool, steady ride might be 0.5 L/hr. A hard summer half marathon can exceed 1.8 L/hr.
- Sweat sodium concentration: roughly 200–2,000 mg per litre, with most athletes clustering around 800–1,000 mg/L. This is largely genetic and doesn't change much with training or heat acclimatisation.
Multiply those out and the range is stark. A low-sweating, low-sodium athlete might lose 100 mg of sodium an hour. A heavy, salty sweater in the heat can lose over 3,000 mg an hour. That's a thirty-fold difference, which is precisely why a single "take one tablet per hour" instruction can't be right for everyone.
You don't need lab testing to work out which end you're at. Two cheap diagnostics: weigh yourself naked before and after a long session (1 kg lost ≈ 1 litre of sweat, adding back anything you drank) and check whether you get visible white salt residue on your kit, cap or sunglasses. Gritty white deposits and a persistent taste for crisps afterwards mean you're a salty sweater and should take sodium seriously. No residue on a hot day means you probably don't need to think about it much.
How Much to Take, and When to Bother
A reasonable working range for most athletes in sessions over 90 minutes is 300–600 mg of sodium per hour, rising towards 800–1,000 mg/hr for heavy salty sweaters in genuine heat. Note that's sodium, not salt — table salt is only about 39% sodium by weight, so 1 g of salt gives roughly 390 mg of sodium. Product labels move between the two units, sometimes unhelpfully.
The honest caveat is that for a lot of the training we all actually do, none of this is the limiting factor. Under 60–75 minutes, in temperate conditions, you can drink water to thirst, eat normally afterwards, and be entirely fine. Electrolytes start to matter when at least two of the following are true: the session is over two hours, it's warm or humid, you're a demonstrable salty sweater, or you're going again tomorrow.
Sequence matters too. Fluid deficits of around 2% of body mass — 1.4 kg for a 70 kg runner — are where performance measurably suffers in the heat. But carbohydrate is still the bigger lever for anything over an hour: 60 g of carbohydrate per hour is the baseline, 90 g/hr for long races if your gut is trained for it. Get the carbs and fluid right first, then layer sodium on top. Doing it the other way round is a common and expensive mistake.
The Risk of Overdoing It
Exercise-associated hyponatraemia — blood sodium below 135 mmol/L — is caused by drinking too much fluid relative to sodium losses, not by insufficient salt. It shows up most in slower marathon and Ironman finishers who drink at every station out of diligence rather than thirst, and it's genuinely dangerous. Confusingly, the early symptoms (headache, nausea, bloating, muddled thinking) look a lot like dehydration, which tempts people to drink even more.
The practical protection is simple: drink largely to thirst rather than to a schedule, take sodium alongside fluid rather than water alone on long hot efforts, and be suspicious if you finish a race heavier than you started or with rings that no longer fit.
What We Use at FTP Endurance Fuel
Our own approach is to keep the jobs separate rather than pretend one product does everything. Carbohydrate and a modest amount of sodium come from what we're chewing; the bulk of sodium replacement, if the day demands it, comes from what's in the bottles.
Our Mixed Fruit Energy Pastilles are built around a glucose–fructose blend, which lets you absorb more carbohydrate per hour than glucose alone by using two separate intestinal transport pathways. Practically, that means the difference between capping out around 60 g/hr and getting closer to 90 g/hr on a long day. The Tart Cherry Pastilles do the same fuelling job with the addition of anthocyanins — the polyphenols that give tart cherry its colour — which have reasonable evidence behind them for reducing oxidative stress and easing next-day soreness. They're the ones I reach for during a hard training block with back-to-back days.
Electrolytes aren't magic and they aren't a cramp cure, but on a long, hot, salty day they're the difference between finishing in control and finishing in survival mode. Work out your own sweat and sodium profile with a set of scales and a bit of honest observation, fix your carbohydrate intake first, then add sodium to match the day in front of you. If you'd like something to build that fuelling plan around, the Taster Pack is the easiest way to find out which flavour you can still stomach at hour four.
