Sodium for Runners: How Much You Lose, How to Replace
Of all the numbers in running nutrition, sodium is the one where the averages lie to you. Sodium for runners is a personal equation — your sweat rate multiplied by your sweat sodium concentration — and both variables differ so much between runners that copying a teammate's plan can leave you replacing half what you lose, or double. This page teaches the measurement, runs the worked examples, and gives replacement targets that respect the ranges instead of pretending they are points.
Of all the numbers in running nutrition, sodium is the one where the averages lie to you. Sodium for runners is a personal equation — your sweat rate multiplied by your sweat sodium concentration — and both variables differ so much between runners that copying a teammate’s plan can leave you replacing half what you lose, or double. This page teaches the measurement, runs the worked examples, and gives replacement targets that respect the ranges instead of pretending they are points.
What are the two variables?
| Variable | Typical range | How to get yours |
|---|---|---|
| Sweat rate | ~0.5–2.0 L/hour (occasionally higher) | Weigh-in method (below) |
| Sweat sodium concentration | ~20–80 mmol/L (~460–1,840 mg/L) | Sweat test, or estimate from salty-sweater signs |
| Resulting sodium loss | ~0.2 g to 3.5+ g per hour | Multiply the two |
Sweat sodium concentration is the quiet variable. Reviews of whole-body sweat measurements put most runners between 20 and 80 mmol/L, averaging around 40–50. Sodium converts at 23 mg per mmol, so 50 mmol/L ≈ 1,150 mg of sodium in every litre of sweat. Concentration also creeps up as sweat rate rises and falls somewhat as you acclimatise to heat over one to two weeks — another reason single numbers from a single test should steer, not dictate.
How do you run the worked example?
Take a 70 kg runner preparing for a marathon:
- Sweat rate test: after a one-hour run in race-like conditions, they are 1.2 kg lighter having drunk 500 ml. Sweat rate = 1.2 + 0.5 = 1.7 L/hour. (Heavy — this is a warm-day or hard-session number, and that is the point of testing in the conditions you will race in.)
- Concentration estimate: white crust on every shirt, stinging eyes — a salty sweater. Estimate 50 mmol/L ≈ 1.15 g sodium per litre.
- Hourly loss: 1.7 × 1.15 ≈ 2.0 g of sodium per hour. Over a 3.5-hour marathon: roughly 7 g of sodium and 6 L of sweat if nothing were replaced.
- The plan: drink ~0.75 L/hour (enough to finish about 1.3 kg down, inside the 2% guideline) at 0.7–1.0 g sodium per litre ≈ 0.6–0.75 g sodium per hour from drinks, plus pre-race sodium with breakfast and a properly salted recovery meal. They replace perhaps 40% during the race and the rest by dinner — a normal, evidence-consistent outcome.
Now run the same maths for a light sweater: 0.8 L/hour at 25 mmol/L (0.58 g/L) loses about 0.46 g of sodium an hour. A standard sports drink at 0.5 g/L nearly covers it. Same sport, same race — a fourfold difference in the plan. This is why electrolytes for runners is written in ranges, and why the thresholds on when do you need electrolytes depend on the runner, not just the run.
How much should you replace during the run?
The ACSM position stand’s drink target — 0.5–0.7 g of sodium per litre for exercise over an hour — is the sensible default for the middle of the distribution. Move up toward 1 g/L or beyond when two or more apply: salty sweater, heat, runs past two hours (see electrolytes for long runs), or a cramp history that tracks with long hot days. Move down for cool weather, short efforts, and low sweat rates.
Sodium also does a second job that pure replacement maths misses: it helps you keep the fluid you drink. Sodium in a drink maintains the drive to drink and reduces urine losses afterwards, which is why recovery guidance pairs 1.25–1.5 L per kilogram lost with sodium in the drink or meal. A homemade electrolyte drink at the right ratio does this job as well as a commercial mix — the ion does not know what it cost.
One honest caution about cramping: sodium is the most actionable lever, but exercise-associated cramping also tracks with fatigue and going out too hard. If cramps persist on a solid sodium plan, the answer is pacing and conditioning, not ever more salt.
How does heat acclimatisation change your numbers?
Your sweat is not a fixed fluid. Over roughly 10–14 days of repeated heat exposure, the body adapts: sweating starts earlier, sweat rate often rises (better cooling), and — the part that matters here — sweat sodium concentration falls as the glands get better at reabsorbing sodium before it reaches the skin. Reductions of a third or more in concentration are reported in acclimatisation studies, though the exact figure varies and we would rather you treat it as “meaningfully less salty” than as a number to plug in.
Practical consequences:
- Test late, not early. A sweat test in your first hot week overstates your season-long sodium losses. If you test once, test after two weeks of consistent heat training.
- Early-season heatwaves are the danger window. You are sweating at summer volumes with spring physiology — the highest sodium losses of your year, arriving when your habits are least prepared. This is when the salty-sweater plan should come out even for runners who normally sit mid-range.
- Travel flips it again. A runner arriving from a cool climate to a hot race has about a week of adaptation available at best. Arrive early if you can; if you cannot, plan sodium at the generous end and pace honestly — acclimatisation you do not have cannot be replaced by electrolytes, only respected.
Second worked example, to keep the ranges honest — a cool autumn day: same 70 kg runner, 12°C, sweat rate measured at 0.8 L/hour, concentration now estimated at 35 mmol/L (0.8 g/L) after a summer of adaptation. Hourly sodium loss ≈ 0.65 g. Across a two-hour run: 1.3 g — replaceable with a standard drink during and a normal lunch after, no capsules, no special protocol. The formula’s job is not to sell you sodium; it is to tell you when the day does not require heroics.
What about cramping — is sodium really the lever?
It would be tidy if cramps were simply a sodium meter hitting empty, and dishonest to claim they are. Exercise-associated muscle cramping clusters in fatigued muscles, at intensities above what training prepared, and in runners with a personal cramp history — which is why a rested, well-paced runner can finish “low” on sodium without cramping while another seizes up at mile 18 of a race they overcooked. The research picture is dual: altered neuromuscular control under fatigue lights the fuse, and significant fluid and sodium losses make the fuse shorter. That is why this page’s advice is sodium plus pacing, not sodium instead of it. What sodium planning demonstrably does: keep plasma volume up, keep you drinking appropriately, and remove the one cramp contributor you can fully control in advance. If you cramp in most long runs despite a tested sodium plan and honest pacing, the remaining conversation is about training load and, if cramps come with dark urine or confusion, a medical one. Salt is the first lever because it is cheap, safe within the ranges here, and frequently sufficient — not because it is the only story.
How do you track this without it taking over your training?
Keep a four-line log for your long runs, nothing more: conditions (temperature, humidity if you know it), pre/post weight with what you drank, sodium taken during, and a one-word verdict the next morning — fine, headache, cramps, or heavy legs. After five or six entries the patterns identify themselves: the headache that only follows hot days, the cramps that track the runs where the bottle came home half full. That log is worth more than a generic sweat test for most runners, because it measures the thing you actually care about — outcomes — in your actual conditions. Change one variable at a time between long runs (drink strength, pre-run sodium, total volume) so the log can attribute the difference. Six weeks of this and you own a personalised sodium protocol that no article, including this one, could have handed you. The formula starts the process; the log finishes it.
Who should not self-prescribe high sodium?
- People with high blood pressure, heart failure, or kidney disease. The plans here can add several grams of sodium on a long-run day. If your sodium intake is medically managed, your running plan must be too — talk to your doctor first.
- People on lithium or diuretics. Both interact with sodium and fluid balance in clinically important ways. Pharmacist or doctor review before any high-sodium fuelling.
- Anyone pregnant. Sodium and fluid regulation change in pregnancy; heat plus endurance exercise needs individual guidance.
- Under-18 runners. Young athletes lose less sodium in sweat than adults (their sweat is typically more dilute) and should follow food-and-water plans with adult supervision.
- Anyone tempted to take salt tablets dry. Concentrated salt without enough water irritates the stomach and can spike blood sodium. Always take sodium with fluid and food as the plan intends — and never chase symptoms of confusion or bloating with more salt; those are medical-help signs, covered on the electrolytes for long runs page.
Frequently asked questions
How do I calculate my sweat rate?
Weigh yourself (ideally nude, towelled dry) before a one-hour run and again after. Add the volume you drank, subtract any urine. Each kilogram of weight lost equals about one litre of sweat. Example: 1.2 kg down, 500 ml drunk, no bathroom break = 1.7 litres sweated in the hour. Repeat in race conditions, because heat and intensity move the number.
How much sodium is in a gram of salt?
Table salt is about 39% sodium by weight, so 1 g of salt provides roughly 390 mg of sodium — and 1 level teaspoon of fine table salt (about 6 g) provides around 2,300 mg. Product labels list sodium, not salt; multiply sodium by 2.5 to get the salt equivalent if you are comparing with food labels that list salt.
Do I need to replace all the sodium I lose during a run?
No. During the run, replacing a fraction is normal and sufficient — the ACSM drink target of 0.5–0.7 g per litre sits below most runners' sweat concentration on purpose. The remainder is a recovery job for salted meals and drinks across the following hours. Trying to match losses gram-for-gram mid-run upsets stomachs and is unnecessary for most events.
What are the signs I am under-replacing sodium?
A pattern, not a single symptom: post-run headaches that water does not fix, heavy salt crusting on kit and skin, cramps late in long runs, feeling washed out for hours after long efforts, and urinating large volumes of clear urine soon after drinking a lot post-run (the fluid is not being retained). Track two or three long runs with a simple log before changing everything.
Is a sweat test worth paying for?
For most runners, no — the weighing method plus a salty-sweater self-check gets you close enough. A lab or patch sweat test that measures your actual sodium concentration is worth it if you race ultras, cramp despite doing the basics right, or have failed hot races you cannot explain. Precision matters most at the extremes.