Jul 23, 2018
Drops of Data
Lindsay Langford

The best way to individualize hydration is to know what athletes lose in each bead of perspiration. Sweat testing analysis makes this possible.

“I know I need to stay hydrated, so I drink a gallon of water each day,” say many athletes during their nutrition consultations with me. Yet they’ll follow that up by telling me they suffer muscle cramps, headaches, and fatigue during athletic activity. Little do they know, the clues for curing these problems lie in the beads of sweat dripping right under their noses.

Most athletes understand the importance of hydration, but a vast majority are uneducated about the volume and composition of the fluids they lose during exercise and how to successfully rehydrate. At St. Vincent Sports Performance (SVSP) in Indianapolis, Ind., we’ve found that sweat testing allows us to address both of these issues. We utilize patches to collect and analyze athletes’ sweat and then use that information to create individualized hydration plans for them.

Once educated on what they lose in their sweat and how to replenish it, the athletes soon reap the rewards. These can include feeling more energized, enhanced sleep quality, decreased recovery time, mental clarity, fewer muscle cramps and headaches, and overall improved performance. Despite some of the challenges that can accompany sweat analysis, a comprehensive process for testing and interpreting the data enables athletes to hydrate better than ever before.


The overall goal with sweat analysis is to find out what’s in each athlete’s sweat. In general, sweat is made up of water, electrolytes, and other substrates that aid in thermoregulation of the body’s core temperature and fluid balance.

Although sweat contains numerous electrolytes, the greatest loss in volume comes with sodium. Potassium, magnesium, calcium, and chloride are excreted, as well, but at much lower rates, and a balanced diet will typically restore their levels. With sodium, however, the reported normal range lost can vary drastically from 20 to 80 millimoles per liter (mmol/L) or 400 to 2,000 milligrams per liter-mg/L-and that’s just in one liter. Many athletes can easily produce two to three times that amount of perspiration in a single workout.

In addition, athletes rarely replenish sodium sufficiently by drinking water and sports drinks alone. Often, this can leave them 100 to 200 milligrams per eight ounces short of their recommended sodium intake. In an endurance event, or extended play like a football game, this could equate to being sodium deficient by 2,000 to 3,000 milligrams. That’s more than the entire daily intake recommended by the American Heart Association.

This has a tremendous effect on the body. Out of all the electrolytes, studies have shown that sodium has the greatest impact on hydration status and overall performance. Low sodium repletion also decreases plasma blood volume and can increase risk of hyponatremia.

While sodium intake is a key component to building an optimal hydration plan, another factor to consider is carbohydrate loss. For exercise exceeding 90 minutes, maintaining blood glucose is an important factor for mental clarity, energy production, and preventing injuries and muscle cramps. Therefore, consuming carbohydrates at a rate of 30 to 60 grams per hour during activity is a standard recommendation by the Academy of Nutrition and Dietetics. For ultra-endurance events, that number may increase to as much as 90 grams of carbohydrates per hour.

A final piece of an optimal hydration plan is sweat rate. This component can vary drastically depending on environmental factors, but collecting a good average for the athlete is key. I typically have athletes conduct three to five sweat rate tests on their own in a variety of settings to get an average of sweat lost during workouts. SVSP is even launching an app called My Sweat App in late summer/early fall to help athletes track their sweat rate and provide pre-, during, and post-activity hydration recommendations. To calculate a sweat rate, follow these steps:

• Weigh the athlete in minimal clothing (ideally compression shorts only) prior to working out.

• Give them a bottle of their desired drink to hydrate during training, but be sure to measure the amount they drink.

• Send the athlete to work out or practice.

• Weigh the athlete as soon as possible after their workout/practice in the same minimal clothing.

• Document how many ounces they lost.

• Add in how many ounces they drank.

• Divide by the number of hours they trained.

For example, if an athlete weighed 150 pounds prior to a two-hour practice and 148 pounds after, the difference is two pounds or 32 ounces. Add the 10 ounces of sport drink they consumed during the practice for 42 total ounces. Then, divide that by the length of the practice (two hours) for a sweat rate of 21 ounces per hour.

Once we know the athlete’s sweat rate, we have a good idea of how much they need to drink. We don’t expect them to completely match their sweat rate with hydration, but consuming a minimum of 50 percent of fluids lost is a starting point.


At SVSP, while all clients are candidates for sweat analysis, we’ve found that the testing proves most helpful for athletes who experience frequent muscle cramps and premature fatigue or compete in events lasting longer than 60 minutes. Many of the best results we’ve seen have been with endurance athletes, specifically triathletes and marathon runners.

We prefer for athletes to undergo testing twice a year, but most will only do it once, as their values generally don’t change much. Cost can be a factor in determining frequency of testing, as well. Our retail price for a sweat analysis is $150 for a basic test (athletes receive their sodium/potassium values lost in sweat, their sweat rate, and a general hydration plan) or $299 for a premium test (athletes receive the same information as the basic test, but the plan is customized based on their product preferences and includes a complete fueling plan).

The technique we use to conduct sweat testing is called the regional absorbent patch method. (See “Protocol to Follow,” below, for a step-by-step look at the process.) We make our own patches with gauze and an adhesive.

Studies have shown that patch locations are important. The sites most people request to place patches are on their foreheads or chests. Although these spots are the first to perspire, their corresponding analyses have been found to overestimate full-body sodium loss by as much as 100 percent. This could lead to an invalid hydration plan and put the athlete at risk.

Instead, averaging measurements from patches on the forearm, thigh, and back seems to provide a reasonable estimate of full-body sodium loss. Using the forearm only is also an acceptable method and likely the most popular, the quickest, and the most cost-effective. We’ve done both at SVSP.


Once the sweat has been collected from the patches, we send it to a lab for analysis. It takes about a week for us to receive the results.

Analyzing them requires first converting the mmol/L values provided for electrolytes into a usable U.S. measurement, such as mg/L. To do so, multiply the electrolyte values by their molecular weight. For instance, sodium’s is 23, while potassium’s is 39. The sum of those equations represents the volume of each electrolyte excreted in one liter of the athlete’s sweat.

From here, I find that breaking down the per-liter volume into eight-ounce increments is helpful for athletes. This is because most sports drink labels provide nutritional data for an eight-ounce serving. If athletes know how many electrolytes they lose per eight ounces of sweat, they can easily compare their sodium concentration to most sports drinks. To convert mg/L to mg per eight ounces (mg/8oz), simply divide your mg/L value by four, since eight ounces goes into a liter roughly four times.

Obtaining these values is one thing, but understanding what to do with them is another. You would assume that knowing how many electrolytes a certain athlete sweats out means you could simply instruct them to replace that amount, right? Yes and no. Yes, we want to replace the electrolytes, but no, we don’t want to replace all of them. This would be an unrealistic amount to consume and absorb. The athlete would likely need to be pulled from their event due to major gastrointestinal discomfort, excessive urination, or other serious medical conditions.

Standard protocols for implementing a hydration plan based on sweat-testing results have yet to be determined. However, most practitioners have found that 50 percent of the excretion rate is a safe and effective starting point. So if an athlete loses 350 mg/8oz of sodium and 60 mg/8oz of potassium during a workout, they should look for a sports drink that offers roughly 175 mg of sodium and 30 mg of potassium in each eight-ounce serving.

That being said, individual gut tolerance and the athlete’s previous hydration/fueling plans must be taken into account before providing a concrete plan. We should also consider environmental factors, such as temperature and humidity, when providing hydration recommendations, as these can affect an athlete’s sweat rate and electrolyte loss. (See “Outside Influence” below for more on how the weather can influence a hydration plan.)

With the data from the sweat-testing analysis, we can build a comprehensive hydration plan for the athlete using sports drinks, electrolyte supplementation, sport gels or gummies, and food. We keep in mind that we want the plan to be effective yet realistic for the athlete, so it helps to give them different replenishment options. (See “Plan it Out” below for a sample hydration plan that we built.)


As perfect as we wish sweat analysis was, there are still challenges. One obstacle with our testing method is analyzing the samples in an accurate manner. Further research needs to be conducted to determine variance in electrolyte loss in reference to sample storage length, sample storage temperature, and patch saturation duration.

Another challenge pertains to putting a hydration protocol in place. Practitioners often find this level of individualized hydration unrealistic for team sports. One reason is because these athletes are typically not able to watch a clock or refuel with whole foods during competitions. Plus, there are many more bodies to account for.

When using sweat analysis with team sports, one protocol we recommend is to categorize players into high-risk, moderate-risk, and low-risk categories. High-risk athletes have electrolyte losses, most specifically sodium, that rank within the top 33 percent of the team. They are encouraged to drink a high-sodium beverage during competitions or training. The moderate-risk players are lumped into the middle 33 percent of electrolyte loss values and should consume a beverage with moderately high electrolytes. The low-risk players rank in the bottom 33 percent and could likely hydrate with a standard sports drink and water.

Finally, the last potential obstacle with sweat testing is getting athletes accustomed to their individualized hydration plan. They may need an adaptation period to train their gastrointestinal systems to hold more than they have before. This can occasionally be unsettling, and you may experience pushback.

If you do, take baby steps. Work with products athletes are willing to try, and gradually add fluids and fuel. There’s no one-size-fits-all approach, so meet each athlete where they are for the most success.

Despite any issues with buy-in, an individualized hydration plan will only increase fluid balance and prevent negative hypohydration effects. In time, most athletes will appreciate having a structured plan and feel more confident in their athletic ability knowing their hydration is customized to their needs. Sometimes sweating the small stuff pays off in big ways.

This article appeared in the July/August 2018 issue of Training & Conditioning.

To view the references for this article, go to Training-Conditioning.com/References.


To evaluate athletes’ sweat at St. Vincent Sports Performance in Indianapolis, Ind., we use the regional absorbent patch method to collect sweat and ion chromatography to analyze it. However, there are numerous other options available.

For one, if you don’t have access to ion chromatography, you can use a portable ion-selective electrode. This has been found to be a reliable and effective tool in the field.

Another testing method requires a post-activity, full-body, wash-down technique, followed by laboratory-based chromatography electrolyte analysis. This is likely the most accurate test. But it’s also the most impractical because it can only be conducted in a lab.

Other methods include using pilocarpine iontophoresis to induce sweat during a rested state and then collecting a sample on the forearm with a macroduct sweat collection device. Lastly, there are patches in production to provide real-time sweat analysis measurements using Bluetooth technology.

I’m sure there will be more options to consider in the future. As sweat testing becomes more mainstream, the research is quickly evolving to keep up.


Here’s how we modify our hydration recommendations based on the weather. The electrolyte and carb values are per liter.


Per hour in 70 degrees or lower

Sodium: 1027 mg

Potassium: 199 mg

Carbs: 30 to 60 g

Fluid: 13 oz


Per hour in 70 to 85 degrees

Sodium: 1438 mg

Potassium: 279 mg

Carbs: 30 to 60 g

Fluid: 18 oz


Per hour in 85 degrees or higher

Sodium: 1849 mg

Potassium: 359 mg

Carbs: 30 to 60 g

Fluid: 23 oz

Information extracted from SVSP Sweat Analysis Report.

Sidebar 3:


At St. Vincent Sports Performance in Indianapolis, Ind., we take the following steps to conduct a sweat test using the regional absorbent patch method:

• Clean each site the patch will be placed on with alcohol and deionized water and dry it with a sterile, electrolyte-free gauze pad.

• Place patch on the site(s) chosen.

• Allow the athlete to train for 20 to 30 minutes or just enough for the patch to collect some sweat without becoming fully saturated.

• Pull the patch off with sterile tweezers and place it in a 10 milliliter (mL) syringe.

• Squeeze the syringe to extract sweat into a one mL vial.

• Send the vials of sweat to a lab to be analyzed.

Sidebar 4:

Lindsay Langford, MS, RD, CSSD, is a Sports Dietitian with St. Vincent Sports Performance in Indianapolis, Ind. She can be reached at: [email protected].

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