A hydration plan for long training starts with two numbers an athlete can measure: sweat rate, which typically falls between about 0.5 and 2.5 liters per hour during hard exercise, and sweat sodium concentration, which commonly ranges from several hundred to well over 1,500 milligrams per liter. The American College of Sports Medicine's position stand on exercise and fluid replacement advises drinking enough to limit body-mass loss to about 2 percent, and notes that plans must be individualized because sweat losses vary enormously between people.
24 News Click publishes information, not medical advice. Fluid planning for athletes with heart, kidney, or blood-pressure conditions belongs with a clinician.
How much do athletes actually sweat?
Sweat rate is measurable without a laboratory. Athletes weigh themselves before and after a session, correct for fluid drunk, and convert the change to liters, since a kilogram of lost mass equals roughly a liter of sweat. Published values in the American College of Sports Medicine's guidance span about 0.5 to 2.5 liters per hour depending on intensity, weather, clothing, and the individual, with some athletes in hot-weather team sports documented above that range.
Because the spread is so wide, averages are close to useless for planning. Two training partners doing the same workout can differ by a liter per hour, which is the core argument sports dietitians make for measuring rather than estimating.
How much sodium is in sweat?
Sweat sodium concentration varies roughly between about 200 and 2,000 milligrams per liter between individuals, with most people somewhere near 900 to 1,000 milligrams per liter. The precise figure is partly genetic, and it falls somewhat as the body adapts to heat through acclimatization, a documented effect in heat-adaptation research.
Laboratory sweat testing, offered by some sports-science facilities, patches sensors on the skin during a controlled session and reports sodium concentration directly. Independent estimates can be roughed out from salty staining and cramping history, but those signs are weak proxies; a 2016 review by Baker and colleagues on sweat testing concluded that regional patch measurements, while useful, need correction factors to estimate whole-body losses.
Why does losing over 2 percent of body mass matter?
Dehydration reduces blood plasma volume, which raises heart rate and core temperature and makes aerobic performance fall. The American College of Sports Medicine's position stand concludes that performance impairment becomes measurable beyond about 2 percent body-mass loss, with larger losses producing larger deficits, particularly in the heat.
The 2 percent figure is a threshold, not a cliff. Evidence from field studies of elite marathoners shows some winners finishing races more dehydrated than that, and reviews debate how much dehydration elite competition tolerates. For training in heat, though, the conservative reading of the literature is to stay near fluid balance.
Can athletes drink too much?
Yes, and the result can be dangerous. Exercise-associated hyponatremia, a fall in blood sodium from overdrinking, was documented in marathon and ultramarathon fields in the early 2000s, with the highest risk among slower participants drinking ahead of thirst, mainly plain water. Fatal cases have been reported in endurance events, including a well-documented death at the 2002 Boston Marathon era of research on the problem.
The U.S. National Library of Medicine describes hyponatremia symptoms including nausea, headache, confusion, and in severe cases seizures. The prevention message from the research literature is simple: drink to a realistic plan matched to sweat rate, not to a maximal-volume rule, and include sodium-containing fluids during very long sessions.
What goes into a calculated plan?
The standard method taught in sports-nutrition courses runs in four steps. First, measure sweat rate from pre- and post-session weights across representative sessions. Second, estimate sodium losses by multiplying sweat rate by sweat sodium concentration, either from a test or an assumed mid-range value. Third, set a drinking rate that replaces most, not necessarily all, of the sweat loss during the session. Fourth, adjust for drink availability and gut comfort, since drinking more than roughly one liter per hour often causes stomach distress.
The result is a per-hour fluid and sodium target, tested in training. That is exactly what the position stand means by individualization: a starting calculation, revised from real observations.
Do sodium supplements prevent cramps?
The evidence here is weak. Exercise-associated muscle cramping has been studied for decades, and reviews, including work questioning the serum-electrolyte theory, find that sodium loss does not reliably distinguish crampers from non-crampers. The altered neuromuscular control hypothesis, associated with Schwellnus's research, currently explains the findings better than salt depletion.
Salty drinks still make sense for very long hot sessions on palatability and partial-replacement grounds. But the marketing claim that sodium tablets prevent cramps outruns what controlled trials show.
What does the evidence not show?
The evidence does not show a universal drinking rate, and it does not show that replacing 100 percent of sweat loss during exercise is better than replacing most of it. It also does not support replacing every milligram of sodium during a session; ordinary meals cover losses afterward for most athletes. What it supports is measured planning against a 2 percent body-mass-loss ceiling and strong caution against overdrinking.
Does heat change the plan?
Heat raises both sweat rate and total sodium losses, and it is where planning pays off most. In hot conditions, documented sweat rates at the top of the range — above two liters per hour — make full replacement during exercise impractical, since gut tolerance limits drinking to roughly one liter per hour for most athletes. The position stand's answer is pre-cooling, acclimatization, and accepting a managed body-mass deficit.
Heat acclimatization itself shifts the numbers. Over one to two weeks of training in the heat, athletes sweat earlier, sweat more, and their sweat sodium concentration falls, a documented adaptation that reduces total sodium loss for a given session. Hot-weather events like summer marathons therefore reward both a rehearsed drinking plan and a structured acclimatization block, not one or the other.
Cold weather flips the risk. Heavy clothing can produce sweat rates near summer levels while dampening thirst, and overdrinking remains possible in slow athletes at cool events, so the same measured approach applies in both directions.
FAQ
How do athletes calculate sweat rate?
Weigh in minimal clothing before and after a typical session, note any fluid consumed, and take the difference: one kilogram of body mass lost equals about one liter of sweat, divided by session hours. Repeating this across weather conditions gives the range a plan needs.
Do athletes need electrolyte drinks during long training?
For sessions beyond about an hour in heat, or very long sessions regardless of weather, drinks containing sodium are supported by sports-nutrition guidance, partly for fluid retention and partly for taste. For shorter easy sessions, plain water and normal meals are enough for most people.
Is drinking to thirst good enough?
For many recreational athletes in cool conditions, drinking to thirst tracks well against measured needs. For racing in heat, for athletes with very high sweat rates, and for slower endurance participants at hyponatremia risk, a rehearsed quantitative plan is the safer evidence-aligned option.
For more context, read Race-day fueling timeline for marathon runners.
For more context, read what to eat before morning training.
For more context, read making weight combat sports.