Electrolytes for Runners: Which Ones Actually Matter and How Much You Need

You have a shelf full of electrolyte products. Tablets, powders, capsules, sachets with names that sound like energy drinks from 2009. One says 1,000 mg of sodium. Another says 300 mg. A third focuses on magnesium and says nothing about sodium at all.

You have no idea which one to use, how much to take, or whether any of them are doing anything. You are not alone.

Electrolyte confusion is the single most common nutrition question in running communities right now. And most of the answers athletes get are either brand marketing dressed up as science, or advice so generic it could apply to anyone from a casual jogger to an Ironman finisher.

This article fixes that. We will break down which electrolytes actually matter during endurance exercise, what each one does, how much you lose in sweat, and — critically — why the right dose depends on your body, not a number printed on a packet.

What are electrolytes, and why do runners care?

Electrolytes are minerals that carry an electrical charge when dissolved in fluid. Your body uses them to regulate muscle contraction, nerve signalling, fluid balance, and blood pressure. During exercise, you lose electrolytes through sweat — and if you lose too much without replacing them, performance drops.

The electrolytes that matter most for endurance athletes are:

• Sodium — the big one
• Potassium — the supporting player
• Magnesium — the overlooked one
• Calcium — involved in muscle contraction, but rarely a concern during a single run

We will focus on the first three, because those are the ones you can actually control through what you drink and eat around your training.

Sodium: the electrolyte that matters most during a run

Sodium is the primary electrolyte lost in sweat. Not by a small margin — by a large one. A typical runner loses between 400 and 1,200 mg of sodium per litre of sweat, depending on genetics, fitness, heat acclimatisation, and diet.

Here is why that matters: sodium is the key driver of fluid absorption. When you drink plain water during a long run, a significant portion passes straight through without being absorbed efficiently. Add sodium, and your small intestine absorbs fluid faster. This is why oral rehydration solutions used in medicine always contain sodium — it is the transport mechanism.

How much sodium do you lose?

This varies dramatically between individuals. Two runners doing the same workout in the same conditions can lose vastly different amounts:

• Low sodium sweater: 200–400 mg per litre of sweat
• Average sodium sweater: 400–800 mg per litre
• High sodium sweater: 800–1,500+ mg per litre

If you have ever noticed white salt stains on your cap, shirt, or face after a long run, you are likely on the higher end. That is not a definitive test, but it is a useful signal.

How much sodium should you take per hour?

The general recommendation from sports science is 300–600 mg of sodium per hour during endurance exercise lasting longer than 60–90 minutes. But that number hides enormous individual variation:

• A lighter runner with a low sweat rate in cool conditions might need 200–300 mg per hour
• A heavier runner with a high sweat rate in warm conditions might need 800–1,200 mg per hour
• Ultra-endurance athletes in heat have been documented needing 1,500+ mg per hour

This is why "take one salt tablet every hour" is poor advice. It might be right for some runners and wildly wrong for others.

The sodium products runners actually use

The most popular sodium-focused products in running communities right now:

LMNT — 1,000 mg sodium per serving. High-sodium, zero sugar. Popular with low-carb and keto athletes, and with heavy sweaters who want a simple, high-dose option. The taste divides opinion.

Precision Hydration — comes in three strengths (250, 500, and 1,000 mg sodium), which makes it easier to dial in your dose. Used by professional triathlon and cycling teams. Science-forward branding.

Nuun — 300 mg sodium per tablet. Lower sodium, with added potassium, magnesium, and calcium. Good for moderate sweaters and shorter sessions. Convenient tablet format.

SaltStick — capsule form, 215 mg sodium per capsule. Easy to carry during races. Popular with marathoners and ultrarunners who want precise dosing without flavoured drinks.

None of these is universally "best." The right choice depends on your individual sweat rate and sodium concentration — which is exactly the problem most runners face.

Potassium: the electrolyte everyone forgets

Potassium works alongside sodium to regulate fluid balance and muscle contraction. When your sodium-potassium balance is disrupted, muscle function suffers. This is one of the mechanisms behind exercise-associated muscle cramps — though the science is more nuanced than "low potassium equals cramps."

How much potassium do you lose in sweat?

Less than sodium, but enough to matter over long efforts:

• Typical sweat potassium concentration: 150–300 mg per litre
• During a 3-hour marathon in warm conditions with a sweat rate of 1.2 litres per hour, you could lose 500–1,000 mg of potassium

For context, one medium banana contains roughly 420 mg of potassium. So the idea that a banana at an aid station solves your potassium deficit is not entirely wrong — but it is not enough on its own for a long race.

Do you need a potassium supplement during a run?

For most runners, no — if your electrolyte drink or gel already contains potassium (many do, in the 50–150 mg range per serving). Where potassium becomes more important is in ultra-endurance events (4+ hours) and in recovery nutrition after long sessions.

The bigger risk with potassium is chronic under-consumption in your daily diet, which leaves you starting a race already depleted. Most runners should focus on dietary potassium first: bananas, potatoes, sweet potatoes, avocados, spinach, and yoghurt.

Magnesium: the electrolyte that affects everything else

Magnesium is involved in over 300 enzymatic reactions in your body, including energy production, protein synthesis, and — critically — muscle and nerve function. It is the most commonly deficient mineral among endurance athletes, partly because it is lost in sweat and partly because modern diets tend to be low in it.

What does magnesium actually do during exercise?

• Helps convert glycogen to glucose for energy
• Supports muscle relaxation after contraction (the counterpart to calcium, which triggers contraction)
• Plays a role in electrolyte balance by influencing sodium and potassium transport
• Supports sleep quality and recovery — which matters for training adaptation

How much magnesium do you lose?

Sweat magnesium losses are relatively small — around 5–20 mg per litre — but the cumulative effect across weeks of heavy training is significant. Studies show that endurance athletes are frequently magnesium-depleted, even when they believe their diet is adequate.

Should you supplement magnesium?

For daily supplementation: probably yes, especially during heavy training blocks. Magnesium glycinate and magnesium citrate are the best-absorbed forms. 200–400 mg per day is the typical range for athletes.

For during-run supplementation: less critical than sodium, but some electrolyte products include small amounts (20–80 mg per serving), which can help over longer efforts.

Why the same dose does not work for every runner

Here is the fundamental problem with electrolyte advice: the right dose depends on variables that are unique to you.

Your sweat rate — how much total fluid you lose per hour. This ranges from 0.5 litres to 3.0+ litres per hour across the running population. A runner with a sweat rate of 0.8 L/hr needs a fundamentally different plan than one losing 2.0 L/hr.

Your sweat sodium concentration — how salty your sweat is. This is largely genetic and varies by a factor of five between individuals. Two runners with identical sweat rates can lose completely different amounts of sodium.

Environmental conditions — heat, humidity, wind, and sun exposure all increase sweat rate. A hydration plan built on cool-weather training runs will underperform in a warm race.

Exercise intensity — running at race pace produces more sweat than easy training. Your marathon hydration plan needs to account for race intensity, not training intensity.

Body size — a 60 kg runner and a 90 kg runner have different fluid compartment volumes and different total electrolyte needs.

This is why the manual sweat rate test (weigh yourself before and after a run, track fluid intake, calculate loss per hour) is the minimum starting point. But even that only tells you total fluid loss — not sodium concentration.

How to calculate your personal electrolyte needs

Step 1: Estimate your sweat rate

Before a training run, weigh yourself without clothes. Run for 60 minutes at your target race effort. Track exactly how much you drink during the run. Weigh yourself again after, towelled dry.

Formula: (Pre-run weight in kg - Post-run weight in kg) + fluid consumed in litres = sweat rate in litres per hour.

Repeat this in different conditions (cool, warm, humid) to build a range.

Step 2: Assess your sodium loss type

Until you get a professional sweat test or use a sweat analysis tool, use these proxies:

• White salt stains on your clothes, hat, or skin after running = likely a high sodium sweater
• History of cramping during long efforts despite adequate hydration = may indicate higher sodium needs
• Salty taste on your skin when you lick your forearm after a run = higher concentration

Step 3: Match your product to your profile

| Sweater Type | Sodium Target per Hour | Example Product Strategy | |---|---|---| | Low (light sweater, cool conditions) | 200–400 mg | Nuun tablet + water at aid stations | | Moderate (average sweater, mild conditions) | 400–700 mg | Precision Hydration 500 or 1 LMNT serving diluted | | High (heavy sweater, warm conditions) | 700–1,200 mg | Precision Hydration 1000 or LMNT + SaltStick capsule | | Very high (salty sweater, hot race) | 1,000–1,500+ mg | LMNT + SaltStick caps every 30–45 min |

Step 4: Test in training

Never race on a plan you have not practised. Use your long runs over the next 4–8 weeks to trial your sodium strategy at race intensity. Adjust based on how you feel, whether you cramp, and whether you finish runs feeling dehydrated or bloated.

Sweatr calculates your personal electrolyte needs automatically using your Apple Watch or Garmin data — your sweat rate, training load, and race-day conditions combined into a plan you can follow without spreadsheets or guesswork. If you want to skip the manual calculations and get a personalised plan in minutes, try Sweatr free.

The three electrolyte mistakes that ruin race day

Mistake 1: Starting too late

If you wait until you feel thirsty or crampy to take electrolytes, you are already behind. Sodium absorption takes 15–30 minutes. Start your electrolyte intake in the first 15–20 minutes of a race and maintain it consistently.

Mistake 2: Using a product you have never tried

Race expos are full of free samples. Do not try them on race day. Your gut needs to be trained on the specific products you will use. GI distress from an unfamiliar electrolyte drink can end your race faster than dehydration.

Mistake 3: Assuming more is always better

Over-supplementing sodium can cause bloating, nausea, and in extreme cases, contribute to fluid retention. More is not always better — the right amount for your body is what you are aiming for. This is where data beats guesswork.

Putting it all together: a practical electrolyte plan

Here is a framework for a runner training for a spring marathon:

Daily (training weeks):

• Eat potassium-rich foods at most meals (bananas, sweet potatoes, avocados)
• Consider 200–400 mg magnesium glycinate in the evening
• Stay on top of baseline hydration — half your body weight in pounds, in fluid ounces per day

Before a long run or race (2 hours prior):

• 300–500 mg sodium with 500 ml of fluid
• A carbohydrate-rich meal (this is not the article for carb-loading specifics, but pair your electrolytes with fuel)

During the run (per hour):

• Sodium based on your sweater type (see table above)
• Fluid based on your sweat rate — do not over-drink, do not under-drink
• If using gels, check whether they contain sodium (most contain 40–100 mg per gel — factor this in)

After the run (within 30 minutes):

• 500–750 ml of fluid with sodium
• A meal containing protein (20–30 g) and carbohydrate (1–1.2 g per kg body weight)
• Potassium-rich food to restore intracellular balance

Sweatr builds this entire plan for you — pulling your sweat rate from your wearable data, adjusting for race-day weather, and mapping your needs to specific products. No spreadsheets, no guessing. Download Sweatr and build your plan.

The bottom line

Electrolytes are not optional for endurance runners. But the specific mix and dose you need is personal — shaped by your genetics, sweat rate, the conditions you race in, and how hard you push.

Sodium is the priority. Potassium matters over longer efforts and in daily recovery. Magnesium is the silent contributor that most athletes under-consume.

Stop copying someone else's electrolyte strategy. Measure your sweat rate, assess your sodium type, match your products, and test in training. Or let your data do the work — your Apple Watch and Garmin already know more about your body than a generic label ever will.