Endurance · 3 min read
Sweat rate tells you how much fluid leaves you. Sodium concentration tells you what is in it. Of the two, concentration is the more stable, the more individual, and the one more likely to change what you actually do.
By Chris Gkoufas, DVOTION Labs ·
Sweat is not simply water. It carries sodium, chloride, potassium and smaller amounts of other minerals, and sodium is the one that matters most for performance and for how you feel late in a long session.
Concentration is reported either as milligrams of sodium per litre of sweat, or as millimoles per litre, which is the unit the research literature uses. One millimole is 23 mg, so 1,150 mg/L and 50 mmol/L are the same measurement expressed twice.
Whole-body sweat sodium across athletes spans roughly 10 to 90 mmol/L. That is close to a tenfold difference, and it is far larger than the variation within any one person between sessions.
This is what makes concentration the more portable of the two figures. Your sweat rate moves substantially with heat, intensity and airflow. Your sodium concentration moves far less, so it travels between sessions and conditions much more safely — which is why a plan built on it holds up better than one built on a rate measured in a room you will never race in.
It also separates two things that are constantly confused. A heavy sweater is not a salty sweater. Someone losing 2 litres an hour of dilute sweat and someone losing 1 litre an hour of concentrated sweat can lose almost identical sodium, and their fluid plans are nothing alike.
Concentration is not uniform across the body. A reading taken at one site — the forearm, the upper back, wherever the sensor or patch sits — is a regional measurement, and regional values do not equal what a whole-body washdown would give.
This is a real limitation and it is why a regional figure should be reported as an estimate of your whole-body value rather than as a measurement of it. What it is not is a reason to dismiss the result: the differences between athletes are considerably larger than the uncertainty introduced by the method, so a regional reading still tells you which end of that tenfold range you sit at.
The practical rule is that the site has to be recorded and kept the same. Two tests are only comparable with each other if the reading came from the same place both times.
Multiply concentration by sweat rate and you have sodium loss per hour. At 1.15 L/h and 2,187 mg/L that is around 2,515 mg an hour, which over a three-hour ride is roughly 7.5 g of sodium.
What it does not mean is that 2,515 mg an hour has to be consumed. Full replacement during exercise is not required, and an intake that is already well tolerated is evidence a calculation cannot overrule. Sodium intake should be set against losses, duration, conditions, tolerance and what the rest of the diet already provides.
The genuinely useful conclusion from a high figure is narrower and more actionable: sodium is worth being deliberate about, rather than assumed away, on anything long or hot.
Acclimatisation tends to reduce sweat sodium concentration, as the sweat glands reabsorb sodium more effectively. The effect is real but modest against the size of the differences between individuals.
Habitual sodium intake has some influence, but it does not move an athlete from one end of the population range to the other. Concentration is largely individual.
It is not a defect and it is not a diagnosis. It is a characteristic that makes sodium a more important part of your plan than it is for someone at the other end of the range, particularly over long durations.
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