My Sweat Test: 40 Ounces an Hour, Sodium at 1,500 mg a Liter

Some numbers you can guess at. Some you have to measure. The two that run my hydration plan are the second kind. I lose about forty ounces of fluid an hour. In heat that is close to three pounds an hour off my body weight. My sweat sodium concentration is roughly 1,500 milligrams per liter, which is high. Neither of those came off my watch.

Here is what changed. In any race over 80 degrees, or any time I hold zone 3, I drink up to fifty ounces an hour with electrolytes matched to that concentration. I am not sipping when I feel like it. I am not copying the guy next to me.

Two runners can drop the same two pounds on an hour run. One loses 500 milligrams of sodium. The other loses 2,000. If the second one follows the first one's plan, he is cramping three hours in. I broke this down on TikTok.

The Test Is Two Numbers, Not One

It is two measurements and almost everyone smashes them together. Sweat rate is how much fluid you lose per hour, from weighing yourself before and after. No lab. Sweat sodium concentration is how salty that fluid is, and that part needs a lab, a patch, or a sensor. The clinic test measures only the second one, at rest. Nobody leads with that.

The PF&H Advanced Sweat Test takes 30 to 45 minutes and you do not run. A tester cleans one forearm, places two gel discs, one holding pilocarpine, and runs about 1.5 milliamps through it to wake the local sweat glands. Warm tingle. No needles. A coil collector sits there 5 to 15 minutes, the sample goes through a conductivity analyzer, and you leave with one number and an hourly target. It is worth knowing what you bought. A resting sample. A forearm-only sample. A chemically stimulated one. Forearm values overestimate whole-body sodium by 25 to 100 percent before correction, and concentration rises with sweat rate itself. It is not a decimal. It is a band.

To measure sodium while I am actually sweating, Levelen mails a patch kit. It goes on a cleaned outer forearm 20 to 30 minutes in, once steady sweating has started. It collects for 30 to 60 minutes. Then the gauze comes off with tweezers, because fingers salt the sample. The nude weigh-in happens in that same session. Results come about 48 hours after they receive it.

Wearables like Nix, Flowbio and hDrop estimate both numbers live, but Nix's own validation reports roughly 7.8 percent underprediction of sweat rate. A university lab runs the real protocol. Multi-site patches. 60 to 90 minutes in a heat chamber. Sometimes a whole-body washdown. Three hours on site, and the only version that measures whole-body sodium.

A Number Alone Is Not a Plan

Sodium lost per hour equals sweat rate times sweat sodium concentration. Neither number alone is actionable. Two conversions matter. One millimole per liter of sodium is 23 milligrams per liter. One gram of sodium is 2.5 grams of salt. Marketing quotes salt. Physiology uses sodium.

The bands run on the regional scale your test reports. Under 600 mg/L is dilute sweat, and sodium is a non-issue under three hours. 600 to 1,000 is average, where a standard sports drink covers it. 1,000 to 1,500 is above average, where normal drinks under-deliver in long hot events. Over 1,500 is the top few percent. That is where I sit. Data on 506 athletes puts forearm sodium near 1,000 mg/L and sweat rate at 1.21 liters an hour. My forty ounces is about 1.2 liters. My fluid loss is dead average. My salt loss is not.

A heavy sweater is not automatically a salty sweater.

Then the plan gets built, and it does not replace everything. It replaces 60 to 80 percent of fluid losses and holds body mass loss under about 2 percent. The gut caps out near 1.0 to 1.2 liters an hour regardless. Under 90 minutes, sodium barely matters. From 90 minutes to three hours at average concentration, 300 to 600 mg an hour. Past three hours, or above 1,000 mg/L, or in real heat, 500 to 1,000. Only a very high concentration athlete in a long hot race has a case for 800 to 1,500. That one gets rehearsed in training. Never on race day.

Two things this test will not do. It will not fix your cramps. Cramping tracks neuromuscular fatigue, not electrolyte status, and the ultramarathon paper is titled almost word for word: sodium intake does not prevent cramping. Pacing fixes it. Specificity fixes it. Heat adaptation fixes it. It also will not license you to drink more. Exercise-associated hyponatremia comes from drinking past your losses, and salt tabs do not protect you. Thirst is the governor. Your sweat rate is a ceiling. Lose 1.0 liter an hour and drinking 1.5 is not diligent. It is dangerous. Meanwhile the cheapest lever is the one nobody sells you, because heat acclimation lowers sweat sodium for free. After that, the smartest move is to leave your numbers alone.

Half of This Costs Nothing

I did this before I spent a dollar on any of it. Weigh yourself. Run an hour at race pace. Do not eat, do not drink, not once. Weigh yourself again. Every pound you lost is 16 ounces you have to put back. That is your sweat rate.

Do it in the climate you are going to race in. That is the part everyone skips. If you live in New York and you are racing in Florida, your numbers from March are worthless to you in July, and humidity can push the same effort two to three times higher than dry heat. I test again every time I travel to race.

For sodium, the free version is symptom scoring. White crystalline lines on a dark shirt. Sweat that stings your eyes. Salt cravings. Gritty hat brims. Three or four of those and you are probably above average. It will not tell 900 from 1,400, and that gap is what changes a plan.

Buying the Number Without Buying the Pitch

It starts with the PF&H test center map. Sites are bike shops, tri stores, PT clinics and coaching studios, and you book with the partner, not the brand. The price runs 150 to 200 dollars, usually 175. CTS in Colorado Springs is 200, or 175 if they coach you. Skratch in Boulder is similar. Independent shops swing from 99 to 225, and a race expo pop-up is 100 to 150 for the identical test.

At home, Levelen's kit is 179 dollars, 129 for a repeat. Nix is 189 plus 25 a patch pack. Flowbio is around 405 with no consumables. hDrop is 150 to 200. A university performance lab runs a sweat protocol for 150 to 400, or 250 to 600 bundled with VO2 max or lactate, which is the better buy since they already have you on a treadmill.

Then comes the phone work. A call before the drive matters, because the map goes stale and sites run out of kit. The two questions are whether they have kit in stock and what they charge. A metro search with the right nouns works, like sweat test sodium plus your city. The local university is worth a search for community performance testing, because kinesiology departments sell testing to fund the lab. A board-certified sports dietitian is findable through the Academy's finder or CPSDA. The tri club is worth asking too, because clubs organize group testing days at a group rate.

One warning. Almost every in-person tester sells the product they are about to recommend. That does not make your number wrong. It makes the conclusion built on it something other than neutral. I take the number, I walk out, and I build my own plan. I keep the providers, prices and search terms for every test I have done in the KEEP test finder at /testing. The free weigh-in is worth doing this week. The sodium number is worth buying once, and only if you race long in the heat.

And that brings me back to where this started. Some numbers you can guess at. Some you have to measure. The two that run my hydration plan are the second kind. But more than that, they are mine. Not the average. Not the watch. Not the guy running next to me. Forty ounces an hour and 1,500 milligrams a liter, measured once, honored in every hot race since.