CA Notice at Collection sweat test at the Skratch Lab Cafe in Boulder, Colorado, I was fully kitted out for a run. I don’t know exactly what I expected—I had never had my sweat content analyzed in-person—but I thought surely I would have to run enough to break a sweat.
Not the case: The appointment took about 30 minutes, and I sat through the entire thing. The dietitian placed a colorless pad on my arm and hooked it up to an electrical charge (you don’t even feel it!) to stimulate sweat in about five minutes, then strapped a patch to my arm to collect the sweat for another 10 or so minutes. When there was enough fluid in the patch, the dietitian ran the sample through a portable machine and voilà: I found out I lose just about 850 milligrams of sodium per liter of sweat. That’s pretty average for runners, who can lose between 200 and 2,000 milligrams per liter, according to Skratch.
Sodium is super important to pay attention to, because research shows it has a major effect on total hydration status.
The amount of sodium I lose in sweat won’t change for me; what will change is how much fluid How Under-Fueling Affects Performance and Health calculate on my own just by weighing myself pre- and postrun. (For an individual assessment, you simply convert your difference in weight before a run and after a run to ounces and add the amount of fluid you drank on the run. For example, if you lost a pound and drank 16 ounces of fluid, your total fluid loss is 32 ounces.) But fluid losses generally range between approximately 0.5 liters to 2 liters per hour, research shows.
Knowing how much sodium I’m losing per liter of sweat is more difficult to calculate, but it can help me dial in my electrolyte needs while marathon training in any conditions—and better prepare me for race day, especially as Shoes & Gear.
Calculating sodium loss via a sweat test is just one of the popular high-tech nutrition hacks marathoners can buy into—but not all of them are worth the price tag. While nutrition is crucial to Summer Running Gear (and is often overlooked by many runners as a path to better performance), you don’t have to spend a ton of money on it. Here’s how to decide what’s worth the spend, and what’s not, when it comes to fine-tuning your fueling.
The Benefits of Nutrition Tech and Where to Buy In
Worth the Price: Sweat Tests
Like my Skratch test, sweat testing is one of the growing trends in nutrition tech that sports dietitians can get behind. “Sweat testing can be a game-changer for runners who are heavy, salty sweaters or who struggle with GI issues and decreases in performance toward the end of hot races,” says Meghann Featherstone, CSSD, a Kent, Ohio-based sports dietitian and owner of Featherstone Nutrition.
It used to be that runners would have to go into a lab for this kind of test—if they could even afford it. Now, Skratch offers in-person sweat testing for $200, and Precision Hydration Advertisement - Continue Reading Below.
But you don’t even have to go that far; patches and wearables including Levelen, hDrop, and Nix are making at-home sweat testing accessible and (in some cases) more affordable. These devices stick to your forearm and bicep—areas that are proven by research to be most representative of a full-body sweat rate, says Featherstone—and collect your sweat for analysis. Prices for these devices range from $119 to $250.
“Runners need to be able to understand how much fluid and electrolytes they’re losing via sweat so that they’re able to replace that through their fuel and hydration during their runs,” says Amy Goblirsch, a registered dietitian and the nutrition coach of Running Dietitian. If you can mimic race day conditions in your training runs while wearing one of these, you can plan for a more personalized fuel and hydration strategy temperatures continue to rise.
Wait to Purchase: Continuous Glucose Monitors
Sweat testing tech is light years ahead of another wearable category that’s been growing in popularity: How to Train Your Gut Before Race Day (CGMs). These devices, which stick to the back of your arm and track blood glucose levels in real time, have long been used by diabetics and have been extensively studied for that purpose.
But a new breed of non-prescription CGMs—including Lingo, LEVELS, and Stelo—are being marketed to exercisers who want to dial in their nutrition and prevent bonking. With these monitors, you can input meals, workouts, and other habits (like sleep) and see spikes and drops in glucose levels, helping you uncover patterns and find what leads to these peaks and valleys. And while that sounds promising, it’s not always that informative.
“I don’t think CGMs are that helpful for runners because our blood sugar response is so dramatically different if we didn’t get enough sleep, depending on where we are in our menstrual cycle, how stressed we are, if we’re dehydrated—there are so many variables,” says Featherstone. “I think sometimes, it’s too much information for the average person.”
Goblirsch agrees: “If you’re someone who hasn’t been diagnosed with diabetes or pre-diabetes, it’s not going to be necessary. Plus, if you’re not working with a registered dietitian, you may not understand the data and it’s easy to freak out when you see these spikes and crashes. I worry that could cause fear around certain foods and habits.”
Blood sugar spikes are normal—it means you’re storing carbs very quickly, says Featherstone—and avoiding them would be like avoiding hydrating because you don’t want to use the bathroom.
What’s more important than seeing glucose level fluctuations? Actually paying attention to your body when you feel a dip in energy, because that’s a pretty good sign you need some carbs. how stressed we are, if we’re Chronometer or MyFitnessPal can also help you better track your macros to see if you’re getting enough carbs to keep energy steady.
Worth Considering: Athlete-Focused Blood Tests
Instead of tracking glucose, non-diabetic runners would be better served by identifying actual dietary problem areas via blood tests, which were popularized by Inside Tracker How to Build Fatigue Resistance ldquo;I don’t think CGMs are that helpful for runners because our and Quest’s Fitness Tests.
These services collect blood in the same way your doctor’s office would, but measure a number of biomarkers and genetic markers—like white blood cell count, which is linked to inflammation, and can therefore affect health and performance—to provide users with personalized, evidence-based daily action items. For example, if you’re low in iron, it might suggest you “bump up your bean intake” or if your running long distances are worrisome, it might encourage you to “continue to get more sunshine.”
“This is a great option to see where your levels are at and if there’s anything that can be optimized to support not just your running but your day-to-day life,” says Goblerisch. Deficiencies in nutrients like vitamin D, calcium, iron, and ferritin But you don’t even have to go that far; patches and wearables including.
“A healthy runner with no deficiencies is probably fine and could do this once a year,” says Featherstone. “If you have past deficiencies, I’d recommend doing it at the beginning of each training cycle, which is probably twice a year, and if something is really bad, even quarterly.” This can be especially helpful for athletes, she adds, because even if your numbers are okay for a “normal” person, they may not be normal for someone training for an endurance event.
Worth a Shot (and Free): AI-Backed Nutrition Advice
The biggest hang-up with nutrition tech right now is translating all the information. “In specific situations, certain devices might be very helpful in taking an athlete’s performance to the next level,” says Featherstone. “But all this data isn’t always intuitive or translative, and the application of that tech is lagging.”
Which brings up another potential nutrition tool for marathoners: AI. In a comparison of answers from AI to a small number of actual humans on the ULTRA-Q—a sports nutrition questionnaire adapted for ultra-endurance athletes—ChatGPT-4 answered with 93 percent accuracy, while registered sports nutritionists answered with 84 percent accuracy, registered dietitians 76 percent, ultra-endurance athletes 68 percent, and the general population only answered with 57 percent accuracy.
Obviously, AI isn’t a replacement for dietitians who can get to know you and your goals personally, but “AI models show high proficiency in sports nutrition knowledge, potentially serving as valuable tools for nutritional education and advice. AI-generated insights could be integrated with expert human judgment,” according to the 2024 study in trust is the one you’ve been working with the longest: your own body that compared the accuracy of nutrition info.
Most athletes would still benefit from working with a human specialist in sports dietetics, but AI can certainly be an accessible tool for enhancing nutritional education among athletes.
The Bottom Line on Nutrition Tech
Unfortunately, the biggest “hack” for optimal nutrition remains eating a healthy, well-balanced diet that consists of enough calories to support your marathon training needs.
Blood testing can help you ID any potential holes in your diet and sweat testing can take some of the guesswork out of your hydration strategy on race day, but don’t get swept up in thinking a certain device is going to totally overhaul your approach to nutrition.
This is still a new tech category, especially when it comes to serving athletes, and the only machine you should really trust is the one you’ve been working with the longest: your own body.
