Almost every athlete can tell you their bodyweight, their max lifts and run times. Almost none can tell you how their sweat rate and how much sodium they lose in a session. They are numbers that can have a ten-fold variation between people, and they are numbers nobody measures.
How sweat is actually made
Sweat glands work in two stages. The secretory coil, deep in the skin, produces a precursor fluid that is the same as blood plasma. That fluid then travels up the duct toward the skin surface, and it's in the duct that things get interesting: epithelial sodium channels reabsorb sodium, with chloride following it.¹
What reaches your skin is therefore hypotonic: less concentrated in sodium than the plasma it came from. You lose proportionally more water than sodium, so plasma osmolality rises (ie the blood is more concentrated), and as your body notices this, you get thirsty.
Why sweating harder makes sweat saltier
The duct has a finite resorptive capacity. Push sweat through it faster and there's less time for sodium to be pulled back, so a greater proportion escapes. The relationship between sweat rate and sweat sodium concentration has been measured repeatedly and is close to linear across the usual range-the faster you sweat, the saltier your sweat becomes.² Another way of saying this is as sweat rate climbs, the sodium secretion rate increases proportionally more than the reabsorption rate.³
The practical consequence is that your sweat sodium concentration is not a fixed personal constant. It is higher on your hardest days. The session where you lose the most fluid is also the session where each litre carries more sodium out with it — the two multiply rather than simply add. This is also why athletes training and sweating hard (eg combat sports, hyrox…) have much higher sodium needs than those sweating at a lower rate (cycling, running).
The numbers
One review puts sweat sodium concentration across athletes at roughly 10–90 mmol/L, with mean values around 40 mmol/L. In milligrams, that's a range of about 230 to 2,070 mg of sodium per litre of sweat with an average of about 920 mg per litre. The same review finds that the differences between different athletes are substantially larger than the variation within a single athlete from session to session — which is what makes an individual measurement worth having.⁴
Barnes and colleagues' dataset of 1,303 athletes found the same broad picture and added sport-specific detail. Whole-body sweat rates were highest in American football (1.51 ± 0.70 L/hr) and endurance sports (1.28 ± 0.57 L/hr), and lowest in baseball (0.83 ± 0.34 L/hr), with wide spread inside every sport.⁵
Sweat rates typically run from 0.5 to 2.0 L/hr, and can exceed 2.5 L/hr in large athletes working hard in heat.⁴,⁶
Multiply those two distributions together and the spread is enormous:
|
Sweat rate |
Sweat [Na] |
Sodium lost per hour |
|
|---|---|---|---|
|
Low |
0.8 L/hr |
20 mmol/L |
~370 mg |
|
Typical |
1.2 L/hr |
40 mmol/L |
~1,100 mg |
|
High |
2.0 L/hr |
70 mmol/L |
~3,220 mg |
What moves your number
Heat acclimation lowers your sodium loss, and substantially. Allan and Wilson found that acclimatised subjects excreted roughly half as much sodium in their sweat as unacclimatised ones.⁷
The mechanism is increased ductal sodium reabsorption.⁸ The gland becomes more responsive to aldosterone (a compound that triggers sodium resorption), and absorbs sodium quicker.
Ten days of heat acclimation shifted the entire sweat sodium–sweat rate relationship downward.² An athlete who has just arrived somewhere hot loses far more sodium than the same athlete a fortnight later.
Genetics set your baseline sweat sodium. CFTR gene variation is the clearest driver of this, and cystic fibrosis gene carriers — around one in 25 people of northern European descent — tend to have more salty sweat without any clinical disease.
Diet has a modest effect. Habitual sodium intake shifts sweat concentration somewhat, but nothing like enough to override the individual baseline.
Sweat rate, as above, moves it session to session.
Notably, training status per se has less effect than people assume, once you separate it from heat acclimation.
Measuring it
Sweat rate you can do yourself easily. Weigh yourself nude and towel-dried before a session. Train for a known period. Towel dry and weigh nude again. Sweat loss in litres is the weight difference in kilograms, plus anything you drank, minus any urine passed. Divide by the hours trained. There is a calculator on this website to do the maths for you.
Sweat sodium is harder. The two established methods are regional absorbent patches and whole-body washdown, and they often give different answers. — Regional patches tending to overestimate sodium relative to whole-body washdown, and different anatomical sites giving different answers.⁴ Commercial sweat tests almost universally use the patch method. Treat the number as a guide rather than a precise value. These tests are often hard to find and a bit expensive to do.
Some visual tests can help: visible white residue on skin and dark kit after a dried-out session points toward the higher end. If sweat stings your eyes it's likely higher sodium.
Dose
The distinction that matters is during versus after.
During exercise, sodium's main role is limiting the fall in plasma sodium concentration when fluid intake is high. Full replacement of hydration losses during a session is neither necessary nor usually achievable.
After exercise, sodium determines whether the fluid you drink is retained or urinated out.
Shirreffs and colleagues varied the volume consumed after a fixed fluid loss and showed that drinking substantially more than the fluid loss is necessary if any of it is to be retained — replacing only what you lost leaves you short once obligatory urine losses are accounted for.⁹ Shirreffs and Maughan then held the volume fixed at 150% of the loss and varied only the sodium content of the drink, from none up to 100 mmol/L, and found urine output over the following six hours was inversely related to the sodium ingested.¹⁰
Volume drunk and sodium content have different effects and you need both. The working target is around 150% of the fluid deficit, taken with sodium.
Practical position:
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Measure your sweat rate this week. It's easy to do.
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Assume your sweat sodium content is average until you have evidence otherwise.
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Pre-load in the half hour before your session starts. Drink during exercise but not so much you feel bloated. Continue rehydration after your session until you have drunk 1.5 times what you estimate you have lost in sweat.
What's in DRVN Threshold
One serving mixed to a litre provides 1000 mg sodium (43 mmol/L), 400 mg potassium, 200 mg magnesium and 1062 mg chloride, alongside 30 g of carbohydrate.
References
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Baker LB. Physiology of sweat gland function: the roles of sweating and sweat composition in human health. Temperature (Austin). 2019;6(3):211–59.
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Buono MJ, Ball KD, Kolkhorst FW. Sodium ion concentration vs. sweat rate relationship in humans. J Appl Physiol. 2007;103(3):990–4.
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Buono MJ, Claros R, Deboer T, Wong J. Na+ secretion rate increases proportionally more than the Na+ reabsorption rate with increases in sweat rate. J Appl Physiol. 2008;105(4):1044–8.
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Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Med. 2017;47(Suppl 1):111–28.
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Barnes KA, Anderson ML, Stofan JR, Dalrymple KJ, Reimel AJ, Roberts TJ, et al. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: an update and analysis by sport. J Sports Sci. 2019;37(20):2356–66.
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Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377–90.
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Allan JR, Wilson CG. Influence of acclimatization on sweat sodium concentration. J Appl Physiol. 1971;30(5):708–12.
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Kirby CR, Convertino VA. Plasma aldosterone and sweat sodium concentrations after exercise and heat acclimation. J Appl Physiol. 1986;61(3):967–70.
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Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ. Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Med Sci Sports Exerc. 1996;28(10):1260–71.
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Shirreffs SM, Maughan RJ. Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses. Am J Physiol. 1998;274(5):F868–75.