Heat
Sweat rates across individuals and conditions span a fivefold range. Generic fluid recommendations are therefore wrong for almost everyone, and wrong in both directions.
Hydration advice tends to arrive as a fixed number — a cup every fifteen minutes, or a litre an hour. The underlying quantity varies from about 0.3 to over 2.5 litres per hour depending on the person and the conditions, which makes any single recommendation useless at one end of that range and dangerous at the other.
Metabolic heat production is the primary driver. Roughly 75 to 80 percent of the energy used in muscular work is released as heat rather than mechanical output. Higher intensity means more heat and more sweat.
Body mass matters because a larger body doing the same work produces more total heat.
Environmental heat and humidity reduce the effectiveness of each unit of sweat, and the body compensates by producing more.
Acclimatisation raises sweat rate. This is counterintuitive but correct: acclimatised individuals begin sweating at a lower core temperature and sweat more copiously, which cools them more effectively. They also lose less sodium in the process.
Clothing that impedes evaporation raises the rate without raising the cooling, which is the worst combination.
Repeat across cool, warm and hot conditions and at different intensities. Three or four data points give you a personal table that is worth more than any published guideline.
| Scenario | Approximate sweat rate |
|---|---|
| Walking, mild conditions | 0.3–0.5 L/hr |
| Hiking with a pack, warm | 0.6–1.0 L/hr |
| Steady running, warm | 1.0–1.6 L/hr |
| Hard effort, hot and humid | 1.8–2.5 L/hr |
The upper end exceeds what the gut can absorb. Maximum gastric emptying and intestinal absorption of fluid during exercise is generally cited at 1.0 to 1.2 litres per hour, and lower for many people. Above that rate you are running a deficit no matter how much you drink, and attempting to drink past it produces gastrointestinal distress rather than hydration.
In genuinely hot conditions at high intensity, some degree of dehydration is unavoidable. The objective is to keep it modest, not to eliminate it.
A commonly repeated claim is that performance falls measurably at two percent body mass loss. The research base is more equivocal than the claim suggests: several field studies find no decrement at that level, and some find the fastest finishers in endurance events are among the most dehydrated, largely because they were working hardest.
What is better supported is that losses beyond three to four percent of body mass impair thermoregulation and cardiovascular function, particularly in heat, and that the impairment compounds with duration.
A practical target: keep losses under two to three percent of body mass. For an 80 kg person, that is 1.6 to 2.4 kg — a meaningful allowance, not a demand for minute-by-minute replacement.
Sweat sodium concentration varies widely, roughly 200 to 2,000 mg per litre, with acclimatised individuals at the lower end. Over a long hot session, total sodium loss can be substantial.
Sodium serves two functions in replacement. It supports fluid retention rather than rapid renal excretion of what you drink, and it is the electrolyte most directly implicated in exercise-associated hyponatraemia.
For sessions under about ninety minutes in moderate conditions, plain water is generally adequate. Beyond that, or in heat, sodium-containing fluid becomes more important — and beyond three or four hours in heat it is not optional.
Exercise-associated hyponatraemia occurs when fluid intake exceeds losses and dilutes serum sodium. Symptoms range from nausea and headache to confusion, seizure and death, and they overlap substantially with dehydration symptoms — which makes the failure mode dangerous, since the instinctive response is to drink more.
It is documented in marathons, ultramarathons, military training and long hikes. Risk factors include smaller body size, slower finishing times that allow more drinking opportunities, high availability of fluid, and the belief that more drinking is always better.
The distinguishing sign is weight. Someone who finishes an event heavier than they started has over-consumed, regardless of symptoms.
Start hydrated rather than trying to catch up. Pale yellow urine before setting out is a reasonable proxy.
Drink to a plan informed by your measured rate, and adjust for thirst rather than ignoring it. Thirst is a genuine signal that lags slightly but does not fail.
Include sodium for anything over ninety minutes or in heat. Weigh yourself before and after occasionally to calibrate, and correct the plan rather than the individual session.
The heat index and hydration calculator estimates sweat rate from body mass, metabolic intensity, apparent temperature and acclimatisation state, and returns hourly fluid and sodium figures alongside the heat index band.