Hydration and performance

What dehydration actually costs

Hydration advice tends to swing between "you are chronically dehydrated and it is ruining everything" and "just drink when thirsty". The measured effects sit in between and are worth knowing precisely.

A meta-analysis of hypohydration and muscular performance found that being dehydrated consistently attenuated strength by about 2%, power by about 3%, and high-intensity endurance by around 10%. At roughly 3% body mass loss, muscle endurance fell 8.3%, strength 5.5% and anaerobic power 5.8%.

The pattern that emerges:

• Endurance suffers far more than maximal strength. A single heavy triple is barely affected; a set of twenty is.
• Later sets suffer more than early ones. Studies found significant decrements in sets two to three at 2.4% body mass loss, and sets two to five at 4.8%.
• The threshold is around 2%. Below that, effects are small or absent.

How much to drink

The eight-glasses-a-day rule has no scientific origin. A more useful framework:

• Baseline: roughly 30–35 ml per kg of body weight per day from all sources — 2.4–2.8 litres for an 80 kg person, including what comes from food and other drinks.
• Add 500–750 ml per hour of hard training, more in heat.
• Drink 400–600 ml in the two hours before training. Enough to start topped up, early enough not to be uncomfortable.
• Sip during the session rather than drinking a large volume at once.
• After training, replace roughly 1.25–1.5 litres per kg of body mass lost if you are training again soon. Otherwise normal drinking over the rest of the day is fine.

The simplest check that actually works: urine colour. Pale straw is right. Dark yellow means drink more. Completely clear all day means you are probably over-drinking, which is not beneficial and dilutes electrolytes.

Electrolytes: when they matter

Electrolyte products are marketed as though every session requires one. The genuine cases are narrower.

Worth it:

• Sessions over about 90 minutes, especially in heat.
• Heavy sweaters — if your clothes are soaked and you see salt marks on them.
• Hot or humid environments, or outdoor summer training.
• Multiple sessions in a day.
• Endurance events.

Not worth it:

• A 60-minute indoor lifting session.
• Daily use "for hydration" when you eat a normal diet containing salt.
• As a replacement for drinking water.

Sodium is the electrolyte lost in meaningful quantities in sweat. Potassium, magnesium and calcium losses are small and easily covered by food. A pinch of salt in a bottle of water is functionally equivalent to most commercial products at a fraction of the price.

Hydration and the scale

A practical connection worth understanding, because it causes a lot of unnecessary distress.

• A litre of water weighs a kilogram. Drinking or sweating shifts scale weight immediately and tells you nothing about fat.
• High salt intake retains water for a day or two, which looks like fat gain and is not.
• Carbohydrate stores water. Each gram of glycogen holds around three grams of water, which is why low-carbohydrate diets produce a fast initial drop.
• Hard training retains water through inflammation and repair, particularly after a novel session.

All of which is why weekly averages are the only sensible way to read the scale — see why the scale lies day to day.

Over-drinking, and the limits of more

The idea that more water is always better is wrong in both directions.

• Excess water is simply excreted. There is no benefit to being over-hydrated, and no stored reserve.
• Hyponatraemia is a real, if uncommon, risk. Drinking large volumes of plain water while losing sodium in sweat — most often in endurance events — dilutes blood sodium dangerously.
• Constant clear urine is a signal to drink less, not a sign of doing well.
• Water does not "flush toxins" or accelerate fat loss. Kidneys do the first job; a calorie deficit does the second.

The one modest exception: drinking water before a meal has small evidence for reducing intake, which is a satiety effect rather than a metabolic one.

Common questions

Does coffee dehydrate me?

No. Caffeine has a mild diuretic effect that is more than offset by the fluid in the drink. Coffee and tea count toward your intake.

How do I know if I am dehydrated?

Urine colour is the practical check. Thirst is a reasonable but lagging signal.

Should I weigh myself before and after training?

Useful once, to learn your sweat rate. Not something to do routinely.

Do I need electrolytes for a normal gym session?

No, unless it is long, hot, or you sweat heavily.

Does dehydration cause cramp?

The evidence is weaker than the folklore. Fatigue and unaccustomed intensity are better predictors, though fluid and sodium loss may contribute.

Is drinking during a set bad?

No. Between sets is more comfortable.

Do I need more water on high protein?

Somewhat, since nitrogen excretion requires fluid. Drinking to thirst covers it.

The practical version

• 2% dehydration costs about 2% strength, 3% power and around 10% high-intensity endurance.
• Endurance and later sets suffer far more than a single heavy effort.
• Baseline 30–35 ml per kg per day, plus 500–750 ml per hour of hard training.
• Pale straw urine is the check that matters; constantly clear means too much.
• Electrolytes matter for long, hot or heavy-sweat sessions — not a normal 60-minute lift.
• Over-drinking has no benefit and, in extreme cases, real risk.