A marathon can expose weak points that a 10K never reaches. You may have the fitness, pacing and gels sorted, then find that your stomach turns, your legs cramp or your energy drops late in the race. Electrolytes for marathon training are not a magic fix for every bad run, but getting your fluid and sodium plan right can make long sessions feel more controlled and race day far less of a gamble.
The aim is simple: replace enough fluid and sodium to support performance without drinking so much that you feel bloated or dilute your blood sodium. Your best strategy will depend on your sweat rate, the weather, your pace, the race course and the products your gut tolerates under load.
Why electrolytes matter in marathon training
Electrolytes are minerals that help regulate fluid balance, nerve signalling and muscle contraction. Sodium is the headline mineral for endurance runners because it is lost in sweat and helps the body retain the fluid you drink. Most sports drinks and hydration tablets also contain potassium, with smaller amounts of magnesium and calcium.
For marathon running, sodium deserves the attention. It does not directly stop every cramp, and cramps can also reflect fatigue, pacing, heat and conditioning. But heavy sodium losses combined with long hours of running and plain-water drinking can leave you feeling flat, thirsty, headachy or nauseous.
This becomes more relevant as your long runs build. A cool 90-minute easy run may need little more than water and your usual pre-run meal. A three-hour run in warm conditions is different. You are losing fluid, using carbohydrate stores and putting pressure on your gut, so a tested hydration and fuelling stack earns its place.
Electrolytes for marathon training: start with sweat rate
There is no single electrolyte target that fits every runner. Some runners finish a long run with salt marks on their kit, sting in their eyes from sweat and a big drop in body mass. Others sweat less, run in cooler conditions or simply need lower sodium intake during training.
A practical way to learn more is to weigh yourself, wearing minimal dry clothing, before and after a one-hour run. Record how much you drank during the session. Each kilogram lost is roughly one litre of fluid loss. Add the fluid you drank to the body mass lost, then allow for any toilet stops, to estimate your hourly sweat rate.
For example, if you are 0.6kg lighter after an hour and drank 500ml, your sweat loss was roughly 1.1 litres per hour. That does not mean you need to drink 1.1 litres every hour on the next run. Trying to replace every drop can be uncomfortable and, in some cases, unsafe. It gives you a useful starting point for practising a realistic intake.
Repeat the check in different conditions. A breezy March long run in the UK can produce a very different result from a humid July session or an overseas marathon. Pace matters too. Marathon-pace blocks usually generate more heat than a relaxed recovery run.
How much sodium and fluid should you take?
Many marathon runners start well with around 400-800mg of sodium per hour during long, sweaty runs. If you are a very salty sweater, racing in heat, or tend to finish with obvious salt residue on your clothes, you may sit towards the higher end. A lighter sweater running in cool weather may need less.
Fluid needs are equally individual, though many runners land somewhere around 400-800ml per hour. Use thirst, comfort and your sweat-rate testing to refine the number. Your goal is not to finish a long run at exactly your starting weight. It is to avoid excessive dehydration while staying comfortable enough to keep fuelling.
Do not treat electrolyte drinks as a licence to overdrink. Exercise-associated hyponatraemia, where blood sodium becomes dangerously diluted, is most often linked to taking in too much fluid over a long event. Symptoms can include worsening headache, nausea, confusion and unusual swelling. Seek urgent medical help if these occur, especially after prolonged exercise.
If you have kidney disease, heart failure, high blood pressure managed with medication, or have been advised to limit salt or fluid, get personal guidance from a clinician before following high-sodium endurance plans.
Choose a format that fits your run
Hydration products work only if you will actually use them. Powders are useful when you want to control the strength of your drink or carry enough servings for a long session. Tablets are tidy, easy to keep in a running belt and can make plain water more appealing. Ready-to-drink options are convenient before a session or when you are travelling to an event.
Look at the label rather than relying on front-of-pack claims. Check sodium per serving, then check how much water that serving is designed for. A product with 300mg of sodium may be ideal if you take it twice an hour, but too light if it is your only source across a hot four-hour run.
Carbohydrate content matters as well. Some electrolyte tablets are effectively calorie-free, which can suit shorter runs or runners using gels separately. Sports drinks can combine carbohydrates and electrolytes, reducing the number of items you carry. The trade-off is that strong drinks may become sickly or harder to digest when the pace rises.
A simple setup for many runners is water or a lighter electrolyte drink alongside gels. Others prefer a carbohydrate-electrolyte drink and fewer gels. Neither is automatically better. Build around the option that lets you consistently hit your carbohydrate target without gut trouble.
Build your long-run hydration and fuel plan
Your marathon plan should be rehearsed, not improvised. Use long runs to practise the exact bottles, soft flasks, cups, gels and timing you expect to use in the race. A product that tastes great on the sofa can feel very different after 25km.
Start drinking early in a long run rather than waiting until you are desperately thirsty. Small, regular sips are usually easier on the stomach than downing a large volume at once. Pair this with regular carbohydrate intake. Many runners work towards 60-90g of carbohydrate per hour for marathon efforts, although your ideal amount depends on body size, pace, previous fuelling experience and gut training.
Try a repeatable routine: take fuel every 20-30 minutes, then take a few mouthfuls of fluid around each serving. If your gels contain sodium, include that in your hourly total. If you use salt capsules, check the dose carefully and take them with adequate fluid. More is not better when your needs do not call for it.
Keep notes after key runs. Record the temperature, distance, pace, what you drank, sodium intake, carbohydrate intake and how your stomach felt. Patterns show up quickly. Perhaps 500ml per hour is fine until race-pace work begins, or a particular flavour becomes intolerable after two hours. That is useful information, not a failed session.
Adjust for weather, pace and race logistics
Warm weather raises both sweat loss and the practical difficulty of drinking enough. Start well hydrated by drinking normally through the day before a long run, then have fluids with your pre-run meal. Avoid forcing litres of water the night before. Pale-yellow urine is a useful general check, but it is not a performance score to chase all day.
In cold weather, thirst can be muted even when you are sweating under layers. Carrying a drink still matters for longer sessions, especially when you are taking gels. In wet conditions, do not assume you are not losing fluid simply because you do not feel hot.
Study your marathon's aid-station spacing before race day. If stations are every 5km, estimate what you can comfortably take at each one and whether that matches your needs. If it does not, carry a soft flask, plan a bottle drop where available, or use a vest. Make the decision during training, not while weaving through a crowded water station.
Common marathon hydration mistakes
The biggest error is leaving all testing until the taper. Your gut, taste preferences and carrying system need practice just as much as your pacing does. A second common mistake is relying only on water during very long, sweaty sessions while taking no meaningful sodium.
The opposite mistake is turning every easy run into a heavily dosed hydration protocol. For ordinary shorter runs, water and normal meals are often enough. Save the detailed strategy for long runs, quality sessions in heat and race day.
Finally, do not blame electrolytes for a pacing problem. If you go out faster than your fitness allows, no tablet will protect you from the cost at 35km. Hydration supports the work you have trained for. It cannot replace it.
Treat your next long run as a dress rehearsal, not just another mileage target. Choose one hydration approach, test it at the effort you plan to race, and make a clear note of the result. Small adjustments now can give you one less thing to worry about when the marathon gets hard.