Most folks lose about 1 gram of sodium per hour during sweaty activities. When sodium levels drop like that and aren’t replaced, you can feel it — brain fog, headaches, weakness, cramps, the whole gamut.
Personally I estimate that I lose over 2 grams of sodium while practicing my favorite sweaty sport, Brazilian jiu-jitsu. If I’m not aggressive about electrolyte replacement when I train, I feel like I’m moving through mud.
The amount of sodium you lose through sweat depends on two things:
- Your sweat rate (the liters of sweat you lose in an hour)
- Your sweat sodium concentration (the amount of sodium in each liter)
Both of these factors can vary a ton — both between and within individuals — so in this article I’ll explain which factors influence our sweat sodium concentration, which factors do not, and how salty sweat is on average. Then you’ll have the tools to estimate (or even measure) the amount of sodium you need to replace during exercise, work, sauna sessions, or excursions into the great outdoors.
Sweat Rate and Sweat Sodium
The more you sweat, the more sodium you lose. That’s obvious. But what most people don’t know is that higher sweat rates lead to higher sweat sodium concentrations. In other words, anything that boosts sweat rate will make your sweat saltier. That includes harder exercise, hotter temperatures, more humidity, less airflow, greater body mass, and even better aerobic fitness.
Why? Some sodium is reabsorbed while you sweat, and how much can be reabsorbed depends on how fast you’re sweating. Allow me to illustrate: As sweat begins to form in our sweat glands (before it reaches the surface of your skin), special ducts reroute some sodium back into the body. But the faster you’re sweating, the less sodium can be reabsorbed before it’s secreted as “final sweat.” That means the faster you sweat, the saltier your sweat will be.
This study of 10 healthy people demonstrates the effect clearly. While folks with a lower sweat rate reabsorbed 86% of pre-sweat sodium, the group with a high sweat rate only reabsorbed 65% of pre-sweat sodium. And by increasing exercise intensity from 50% to 90% of maximum heart rate, subjects experienced a 328% increase in sweat rate and a 311% increase in sweat sodium concentration. You’ll notice the rate of increase between sweat rate and sweat sodium is about equal — that’s because they rise and fall in lock-step.
One thing to note: this effect only begins at sweat rates of about 0.3 liters per hour, which is below the typical range of sweat rates during exercise (0.5–2.0 liters per hour). Once you sweat sufficiently past the 0.3 liter threshold, assume your sweat is getting saltier as you sweat harder.
Other Factors Influencing Sweat Sodium Concentrations
Next, I’ll run through a few other variables that affect how salty your sweat is. Keep in mind that I’m discussing these factors independently of sweat rate. Generally, anything that increases sweat rate will also increase sweat sodium concentrations. The only exception to that rule is heat acclimation, which I’ll cover last.
#1: Genetics
If Sue has a sweat sodium concentration of 600 mg/L (milligrams of sodium per liter of sweat) while exercising, and Larry has a concentration of 1400 mg/L in the same controlled experiment, much of this gap can probably be explained by genetics. Researchers haven’t untangled all the specifics here, but we know that a gene called cystic fibrosis transmembrane conductance regulator (CFTR) provides instructions for making a protein that helps regulate sweat sodium levels. Folks with lower CFTR activity seem to have saltier sweat.
#2: Aldosterone
The primary role of the hormone aldosterone is to regulate water and electrolytes in your body. As your baseline aldosterone levels rise, your body’s ability to reabsorb sodium in the kidneys and sweat glands increases along with it. Which means less salt in your sweat. Resting aldosterone levels are increased by both excessively salty and low-sodium diets, decreased by better aerobic fitness, decreased by heat acclimation, and influenced by genetics as well.
I should mention that aldosterone levels also rise acutely with exercise, but this is not the reason why vigorous efforts lead to saltier sweat. Aldosterone doesn’t work that quickly. Rather, it’s the effect I covered earlier: higher sweat rates lead to saltier sweat.
#3: Heat Acclimation
Heat acclimation entails exercising in hot conditions in order to enhance your body’s ability to regulate its temperature, among other adaptations. One such other adaptation is decreased sweat sodium concentration after a few days of training in the heat. After 10 days of heat acclimation, sweat sodium concentrations fall by 30–60%, depending on which study you look at. Check out my article on this to learn more.
Factors With No Direct Influence On Sweat Sodium
Many of the following factors influence sweat sodium concentrations indirectly because they affect sweat rate. But based on the available research, it’s unlikely that these factors have a significant and direct influence on how salty your sweat is.
- Age
- Aerobic fitness
- Body mass
- Biological sex
- Race
- Clothing
- Hydration status
- Ambient temperature
- Altitude
- Humidity
- Exercise intensity
- Exercise duration
What About Dietary Sodium?
Dietary sodium is a bit more debated. Some studies show a bump in sweat sodium concentration at sustained higher salt intakes, but others don’t. The explanation for this inconsistency may be that prolonged durations of increased sodium intake produce different results than more transient fluctuations.
Let me explain. When you increase dietary sodium, it takes 1–4 days for the sweat glands to recalibrate. When I say recalibrate, I mean your glands reabsorb less sodium so that your “final sweat” is saltier. The research showing that saltier diets lead to saltier sweat spanned several days and weeks. The research showing no effect typically spanned a maximum of 3 days.
The takeaway is that sweat sodium concentration varies with habitual (but not acute) salt intake. If you consume an extra gram of salt on a super sweaty day, your sweat glands probably won’t notice — but your energy levels will thank you.
Typical Sweat Sodium Concentrations
If you know both your sweat sodium concentration AND your sweat rate, you can determine sodium losses for a given workout.
Unfortunately, estimating sweat sodium concentration is like estimating blood pressure without a monitor. You can ballpark it based on published ranges, but the metric is highly variable both between and within individuals. Genes are probably the biggest driver of differences between individuals. Within individuals (you now vs. you later today), differences are mostly the result of your sweat rate changing.
With that in mind, what’s the average sweat sodium concentration during exercise? 826 mg/L is a decent estimate based on a review of 506 athletes sweating in various conditions. But the variability is dramatic from person to person. One comprehensive review put the range at 460–1840 mg/L. In other words, the saltiest sweaters have 4–5x more sodium in their sweat than the least salty.
Determining Your Sweat Sodium Concentration
Unlike calculating sweat rate, which entails simple body weight math before and after a workout, measuring sweat sodium concentration requires laboratory work or equipment.
The most common technique is to use sweat absorption patches on various body regions. The issue is that different body parts can have significantly different sweat rates. Remember the 506 athletes with an average whole-body sweat sodium concentration of 826 mg/L? The average sodium concentration on their forearms was over 1000 mg/L. Some body regions (forearm, back, chest) tend to overestimate whole-body sweat rate, while others (foot, calf, thigh) tend to underestimate it.
Thankfully, scientists have created some pretty nifty regression equations to adjust for these shortcomings. They simply apply patches to a person for 30 minutes of exercise, and then send the patches to a lab for analysis. You can also experiment with wearable sweat biosensors such as from Nix (no affiliation) that calculate sweat composition in real-time. I have yet to try this tech, but I’m keen to.
The “whole body washdown” is the holy grail of sweat collection tests because it captures all sweat — not just arm, back, or chest sweat. Unfortunately, this modality requires a controlled laboratory setup and requires subjects to be hosed down with ammonium sulfate, a chemical that reeks of rotten eggs. Not exactly enticing if you’ve got a date that night, so most folks go for regional sweat testing.
Calibrating Your Sodium Intake
Ultimately, your sodium intake goal is to replace the sodium you lose through sweat. You’ll never replace your sodium losses perfectly, but that’s okay. Your kidneys and sweat glands work hard to maintain equilibrium so that you don’t have to be a mad hydration scientist. If you’re in the right ballpark, you’ll avoid symptoms of sodium deficiency so you can feel and perform your best.
What’s next? Try out our Sodium Intake Calculator to estimate your sodium needs. Then, play around with your diet and see how your body reacts. Keep tinkering until you find an intake that works for you. You’ll feel the difference when you get it right.
