What Is a Sweat Test — and Do You Actually Need One?

If you've ever finished a long run with white streaks on your kit, suffered leg cramps despite drinking plenty, or bonked in a race where you thought your nutrition was dialed — there's a good chance your hydration strategy is built on guesswork. A sweat test changes that.

What a Sweat Test Actually Measures

A sweat test measures one thing that is entirely unique to your body: your sweat sodium composition — specifically, how much sodium is in each liter of your sweat (measured in mg/L).

That number is the piece of your hydration puzzle that you cannot estimate, guess, or calculate on your own. Sweat sodium concentration varies up to 10-fold between individuals, and there's no way to know where you fall without testing.

Sweat rate — how much fluid you lose per hour — is a related concept that athletes can estimate themselves by weighing before and after a workout. When you combine your sodium concentration with your sweat rate, you can calculate exactly how much sodium you lose per hour of training. That's the number your electrolyte strategy needs to match.

Why Generic Advice Fails So Many Athletes

Most hydration recommendations — "drink 16–24 oz per hour," "take one electrolyte tab every 45 minutes" — are built on population averages. The problem is that sweat sodium concentration varies up to 10-fold between individuals. One athlete might lose 300 mg of sodium per liter of sweat. Another might lose 2,000 mg.

If you're at either extreme and following average advice, you're either under-fueling your sweat losses (leading to cramping, fatigue, and performance decline) or over-supplementing (wasting money and potentially masking real hydration issues).

Signs You Probably Need a Sweat Test

• You cramp regularly despite staying hydrated
• You see white residue on your skin or clothing after workouts
• You feel flat or fatigued late in long efforts even when you've "done everything right"
• You've had GI issues during races from sports drinks
• You train in heat, at altitude, or for endurance events longer than 60 minutes

What the Test Involves

At Run on Fuel, the sweat test is quick, non-invasive, and doesn't require you to exercise. We use a localized sweat stimulation method that produces a measurable sample from your forearm in minutes. Results are analyzed on the spot, and you leave with a written, personalized hydration protocol the same day.

There's no running on a treadmill in a sauna. No guesswork. Just data — and a clear plan for what to do with it.

What You Walk Away With

After your sweat test, you'll know:

• Your hourly sodium loss target for training and racing
• Which electrolyte products actually match your needs (and which don't)
• How to adjust your protocol for heat, altitude, and different effort levels
• Whether you're a light, moderate, or heavy sodium sweater — and what that means for race day

Most athletes who get tested describe it as one of the most clarifying things they've done for their performance. Not because the test is complicated — but because finally having real numbers makes every other decision easier.

Ready to stop guessing? Book your sweat test at Run on Fuel in Sandy, Utah →