What Dehydration Actually Does to Your Training
The rule everyone repeats is that performance falls once you lose 2% of body mass in fluid, which is 1.6 kg for an 80 kg person. It comes from the American College of Sports Medicine’s 2007 position stand on exercise and fluid replacement, and it is the basis of most drinking advice you have ever read.
The rule has taken a beating since.
What the blinded studies found
A 2011 meta-analysis in the British Journal of Sports Medicine looked at cycling time trials and found that drinking to thirst performed as well as or better than drinking to replace losses. Athletes who deliberately overdrank did not go faster.
The sharper test came in 2015. Researchers delivered fluid through a nasogastric tube so subjects could not tell how much they were getting, then had them ride 25 km in the heat at 2% and 3% body mass deficits. Performance did not differ. When people cannot perceive their own hydration state, a large part of the effect disappears.
That does not mean dehydration is harmless. It means a chunk of what was measured in open-label studies was the athlete’s belief about being dehydrated, and that the practical threshold sits further out than 2% for most training.
What holds up regardless
- Heart rate climbs. Roughly 3 to 5 beats per minute for every 1% of body mass lost, because blood volume falls and the heart compensates.
- Core temperature rises. Around 0.2 to 0.3 °C per 1% lost, which matters in heat and barely registers in a cool gym.
- Perceived effort rises. The same pace feels harder, which is its own kind of real.
- Long duration endurance suffers most. Marathons, long rides and team sports in heat. Deficits beyond 3 to 4% impair these reliably.
Strength and short power hold up better than endurance does. Judelson’s 2007 review in Sports Medicine on whether fluid balance affects strength and power reported average reductions of roughly 2% in strength and 3% in power at hypohydration levels around 3 to 4%. Detectable, and small enough that a lifter who arrives slightly under-hydrated has not ruined the session.
Work out your own sweat rate
One session gives you a number that beats every general guideline. Weigh yourself unclothed immediately before training and again immediately after, towelled dry. Add whatever you drank, in litres, since a litre of water weighs a kilogram.
Sweat loss equals weight before, minus weight after, plus fluid drunk. Divide by session hours to get the rate.
Worked example. You start at 80.0 kg, finish at 79.1 kg and drank 500 ml over an hour. Sweat loss is 80.0 minus 79.1, which is 0.9, plus 0.5, giving 1.4 litres in an hour. Repeat it in different weather and you will find the range you sweat across, which is more useful than any single figure.
What to do with it
Under an hour, drink to thirst. Nothing more is needed and there is no benefit to forcing fluid. The daily baseline this sits on top of comes from how much water you actually need per day, and the session-by-session figure from the water intake calculator.
Over an hour, aim to replace 60 to 80% of what you lose during the session, then finish the job afterwards with about 1.25 to 1.5 litres per kilogram lost, since some of what you drink leaves as urine. Past 90 minutes, or in heat, add sodium, which is covered in the guide to electrolytes.
Do not aim to finish a long event heavier than you started. That is the pattern behind exercise-associated hyponatremia, which is far more dangerous than the dehydration it was meant to prevent.
Checking your baseline
Cheuvront and Kenefick’s 2014 review of dehydration assessment makes the point that no single marker is trustworthy on its own. Use two of the three: first-morning body weight against your usual, first-morning urine colour, and thirst.
Pale straw urine on waking is fine. Dark amber suggests you are behind. Vitamin B supplements turn urine bright yellow regardless, so ignore the colour for a few hours after taking them.
A 1 kg drop in first-morning weight against your seven day average usually means fluid rather than fat, which is one of the swings explained in why scale weight moves overnight. Two of those three markers agreeing is enough to act on. One on its own is not. The other calculator guides cover the daily target these markers are checking against.