Endurance · 4 min read
Once you know your sweat rate and your sweat sodium concentration, the question becomes what to do with them. The answer is rarely "replace everything you lose", and the reasoning behind that is worth understanding.
By Chris Gkoufas, DVOTION Labs ·
If you have been racing on 750 ml and 1,500 mg of sodium an hour and feeling good on it, that is evidence. It is evidence about your gut, your practical access to fluid, and what you can sustain under effort — none of which a calculation can produce.
Measured losses tell you how much of the gap that intake closes. At a sweat rate of 1.15 L/h it covers about 65% of fluid; at 2,187 mg/L it covers around 60% of sodium. Those are useful figures precisely because they contextualise something already working, rather than replacing it with a target nobody has tested on you.
Where measurement earns its place is when something is going wrong late in long sessions, or when conditions are about to change materially.
Replacing every millilitre is impractical over a long session and invites gastrointestinal trouble. Gastric emptying limits what can be absorbed while working hard, and exceeding it does not hydrate you, it sits in your stomach.
The standard position in the literature is to limit body-mass loss during exercise rather than eliminate it. Some deficit across a long session is normal. What matters is whether performance, concentration and comfort hold to the end.
Drinking substantially more than you are losing is not a safe error to make. Consuming hypotonic fluid in excess of losses is the principal behavioural risk factor for exercise-associated hyponatraemia, and body mass that is maintained or gained during prolonged exercise is a warning sign rather than a success.
Two things worth being clear about. Electrolytes in the drink do not remove the risk if total fluid intake is excessive. And the mechanism is not overdrinking alone — impaired water excretion, including inappropriate vasopressin secretion, is part of the physiology.
The practical conclusion is that both directions carry risk, and the aim over a long session is to avoid excessive dehydration and excessive intake, rather than to minimise one of them.
Body-mass change across a long session is a useful check in both directions. A large loss suggests intake was too low; gaining mass while exercising suggests it was too high.
Alongside it: thirst, gastrointestinal comfort, whether you can hold power or pace late on, and whether concentration is intact in the final third. If those hold, the strategy is working regardless of what proportion of your losses it replaced.
If they consistently do not, your measured figures give you a framework for what to change. In hotter conditions fluid intake may need to rise — and where sweat sodium is high, added fluid should carry meaningful sodium rather than being plain water.
A sweat rate measured indoors at 20 °C describes indoor exercise at 20 °C. Outdoors, airflow and evaporative cooling alter sweat production substantially, and heat raises it further.
How much it changes for you specifically is individual, and applying a fixed multiplier to one measurement produces a number with more apparent precision than the input supports. For a target event in materially different conditions, a second measurement in something closer to them is a better basis than any adjustment applied to the first.
For sessions under about 90 minutes in temperate conditions, thirst is a reasonable guide for most people. For longer or hotter events, thirst tends to lag behind losses, and a rehearsed plan informed by your own measurements is more reliable — provided it is practised in training rather than debuted on race day.
There is no single figure. It depends on your measured losses, the duration, the conditions, what you tolerate and what your normal diet provides. Measured losses set the upper bound of the question, not the answer to it.
Most commercial sports drinks carry considerably less sodium per litre than a salty sweater loses. Whether that matters depends on duration — over an hour it rarely does, and over five hours it can.
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