Electrolytes for Long Runs: Marathon Fuel and Fluids

A 45-minute run forgives almost any hydration plan. A three-hour run does not. Electrolytes for long runs stop being about a product and become a system — fluid, sodium, and carbohydrate, dosed by the hour, built from your own sweat rate, and rehearsed until race day holds no surprises. This page is the marathon-and-beyond protocol: what to do before, during, and after, with the numbers the sports-medicine position stands actually give.

If your long run is under 90 minutes, most of this is overkill — the when do you need electrolytes thresholds will route you correctly. Come back here when the long run passes two hours.

What does a long-run plan look like by phase?

Phase	Fluid	Sodium	Carbohydrate
Night before / morning of	Drink normally; pale-yellow urine	A normally salted meal; extra sodium if hot	Normal carbohydrate-rich meals
During, per hour	~0.4–0.8 L, adjusted by sweat test	Drink at 0.5–0.7 g sodium per litre (up to ~1 g+ for salty sweaters)	30–60 g/hour (up to ~90 g trained, very long events)
First hours after	1.25–1.5 L per kg of body weight lost	Sodium in drink or recovery meal	A meal within a couple of hours; keep eating normally

The during-run fluid range is deliberately wide. The ACSM’s guidance is a principle, not a prescription: drink enough to keep body-mass loss under roughly 2%, and do not drink so much that you gain weight. For a 70 kg runner, 2% is 1.4 kg — over a three-hour run sweating 1.2 L an hour, that means drinking roughly 0.7–0.8 L an hour, not the full sweat rate. Find your own sweat rate with the weighing method on our sodium for runners page; it takes one training run and ends the guessing.

Why does carbohydrate share the plan with electrolytes?

Because on long runs the two are physiologically entangled. Glucose and sodium are absorbed together in the small intestine through a co-transport system — each helps pull the other (and water) into the body. That is why classic sports drinks pair them, and why the sugar-free electrolytes trade-off is real: a zero-sugar tablet delivers sodium but leaves both the co-transport advantage and, more importantly, the fuel on the table.

The fuel maths is unforgiving. Muscle and liver glycogen hold roughly 1,600–2,000 kcal of carbohydrate for most runners — gone in 90–120 minutes of marathon-pace running. The 30–60 g per hour target (up to ~90 g per hour for trained guts using multiple carbohydrate sources on very long events) does not replace expenditure; it stretches the tank across the finish line. Start in the first half hour, dose little and often, and practise the exact products in training. An untested gel at mile 20 is an experiment conducted on your race.

How do the pieces change as the run gets longer?

Two to three hours (marathon long run): the table above, unmodified, covers most runners. Sodium through the drink, carbohydrate through drink plus gels or chews.

Three to five hours (ultra build): palates revolt against sweetness; add savoury real food, keep sodium toward the top of your range, and plan aid-station refills. Total sodium losses on a hot five-hour run can pass 8–10 g for a salty sweater — replacing a reasonable fraction during the event is what keeps the last third runnable.

Back-to-back days and two-a-days: the recovery column matters most. Under-replaced sodium after day one quietly sabotages day two, because fluid without sodium is poorly retained. This is the scenario where the homemade electrolyte drink earns its place in the fridge — cheap, mixed strong, and always available. The general hub plan is on electrolytes for runners.

How do you rehearse all of this before race day?

A long-run plan is a skill, and it is trained like one. Sports dietitians call it gut training: the stomach and intestine adapt to handling more fluid and carbohydrate per hour when they are repeatedly asked to. The progression that works for most runners:

• Weeks 1–2 of the build: run your current long run with the drink only — target sodium and fluid, no carbohydrate goal. Learn what concentration your stomach accepts at marathon effort (easy-pace tolerance lies to you; test at pace).
• Weeks 3–5: add carbohydrate at 30 g per hour, in the exact products you intend to race with. Note any sloshing, bloating, or urgency with times and doses in your training log — patterns, not single incidents, drive changes.
• Weeks 6–8: step toward your race target (45–60 g per hour for most marathoners). If the gut objects, hold the dose for another week rather than forcing the schedule; adaptation is gradual and largely trainable, but it is not negotiable on a deadline.
• Final long run (2–3 weeks out): full dress rehearsal — same breakfast timing, same start-of-run sodium, same hourly plan, same recovery protocol. The goal is a boring log entry. Race day should be a photocopy.

Two rehearsal details runners chronically skip: practise drinking at race pace from the actual bottles or cups you will use (aid-station cups are a skill), and weigh yourself across the dress rehearsal to confirm the plan kept you inside the 2% window. If it did not, adjust volumes — not hopes — for race day. Ultras add one more layer: flavour fatigue. Plan a savoury rotation (broth, salted potatoes, pretzels at aid stations) from half-way, because the runner who cannot face another sweet gel stops fuelling entirely, and the plan collapses from the palate inward.

What goes wrong most often — a failure catalogue?

Long-run fuelling fails in a handful of repeating patterns, and naming them is half the prevention:

The heroic start. Drinking and eating aggressively in hour one “to get ahead of it,” then fighting a sloshing, nauseated stomach for three hours. Gut capacity per hour is a ceiling, not a challenge; little and often beats front-loading every time.

The aid-station lottery. No plan, grabbing whatever each station offers — cola here, a random gel there, water everywhere — and discovering at kilometre 30 that the totals never added up to the targets. Aid stations are a supply chain; your plan tells you what to take from them.

The sweet collapse. Five hours of exclusively sweet products until the palate revolts and intake stops entirely. Build the savoury rotation in from the start of ultra training, not as an emergency measure.

The silent under-salt. Hitting every carbohydrate target with gels and water while sodium quietly runs a multi-gram deficit — the runner who fades with a headache, blames the gels, and repeats it next race. Gels carry little sodium by design; the salt plan is separate homework, set out on the sodium for runners page.

The perfect plan, untested. Flawless spreadsheet, first outing on race day, new product at kilometre 25. Every item in this catalogue is forgivable in training and expensive in a race. That asymmetry is the entire argument for the rehearsal section above: the long run is where plans earn the right to exist.

What about the day before — and the night after?

Long-run outcomes are partly decided off the course. The day before: eat normally with normal-to-generous salt, drink to thirst rather than forcing volume, and skip the folklore of aggressive “water loading,” which mostly buys bathroom trips and occasionally dilutes the sodium you will need. A runner who starts a long run over-hydrated has made the first fuelling error before the first step. Sleep matters here too — a short night measurably raises perceived effort and heat strain, and no electrolyte plan compensates for it.

The night after: the finish-line meal deserves the same respect as the race plan. Replace the measured deficit gradually across the evening — the 125–150% rule with sodium in food and drink — and expect body weight to normalise by morning rather than demanding it by dinner. Alcohol that evening works directly against rehydration (it suppresses water retention), so the celebratory drinks are a genuine trade-off, not a moral judgement: enjoy them knowing the recovery account settles tomorrow. Runners who wake the next day with a lingering headache and dark urine under-replaced, almost always on sodium plus total fluid, and the fix was available at dinner. In back-to-back training blocks, that single evening is where the second run is quietly won or lost.

Who should not follow this protocol as written?

• People with high blood pressure, heart failure, or kidney disease. This protocol deliberately adds grams of sodium. If your sodium is medically restricted, build your long-run plan with your clinician.
• Anyone pregnant. Heat tolerance, fluid needs, and blood volume all change; long efforts in heat need your care team’s agreement.
• Under-18 runners. Adult fuelling schedules are not scaled-down-safe by default; young endurance athletes need adult supervision and food-first plans.
• People on diuretics, lithium, or blood-pressure medication. Sodium and fluid plans interact with these drugs directly — pharmacist or doctor review first.
• Anyone who feels confused, bloated, or develops a worsening headache while drinking. Those are hyponatraemia warning signs. Stop drinking, seek medical help. More fluid — even electrolyte fluid — is not the fix at that point.