How much water should I drink when training?

Start with a daily baseline of around 35 mL per kg of body weight, then add fluid to cover each session. During exercise, 400-800 mL per hour covers most athletes in temperate conditions. Heavy sweaters in heat can need 1000 mL or more. The sweat-rate test gives you a real number for your body.

Do I need sports drinks for a 2 hour run?

For a 2 hour steady run in cool weather, plain water with a pinch of salt in a bottle is usually enough. Once duration climbs past 90 minutes at intensity, or the session is in heat, a sports drink with 30-60 g carbs per hour and 300-700 mg sodium per litre becomes useful. Below that, water does the job.

What are the signs of dehydration during exercise?

Pace drifting up at the same heart rate, rising perceived effort without matching output, a dry mouth that does not resolve with a sip, and urine turning dark after the session. A 2 percent drop in body weight from before to after training is the threshold where performance and thermoregulation start to degrade.

When do endurance athletes actually need salt tablets?

Salty sweaters doing sessions over two hours, or anything in heat, are the clear case. If your kit shows white salt marks after rides, if you cramp late in long sessions, or if you lose over 1 g of sodium per litre of sweat, targeted sodium helps. For a 90 minute tempo run in mild weather, they are not needed.

What is hyponatremia and how do I avoid it?

Hyponatremia is low blood sodium, caused by drinking more fluid than you lose while also losing sodium through sweat. Symptoms include nausea, headache, confusion, and in severe cases seizures. Avoid it by not overdrinking plain water on long sessions, matching intake to sweat losses, and adding sodium when sessions run past two hours or happen in heat.

Hydration for Endurance Athletes: Fluids, Sodium, and Sweat Rate

Why hydration gets oversold and undercooked

Most endurance athletes are either drinking too much of the wrong thing or too little of the right thing. The fitness industry has built a fluid marketing machine around this confusion. The actual science is narrower and more useful than the marketing suggests: fluid loss over 2 percent of body weight degrades performance and thermoregulation, sodium losses vary enormously between athletes, and the only way to know your own numbers is to measure them once.

This guide covers the daily baseline, what to drink during sessions, when electrolytes matter, how to run a sweat-rate test, and how to avoid the two failure modes at either end: dehydration and hyponatremia.

Daily baseline before you even train

The working figure for active adults sits around 35 mL per kg of body weight per day as a baseline, before factoring in training. A 75 kg athlete needs roughly 2.6 L just to stay on top of day-to-day losses. Training adds to that, and hot climates push it further.

Two quick checks tell you if the baseline is covered:

Morning urine colour. Pale straw is the target. Dark yellow means yesterday’s intake missed. Clear and frequent means you are overdoing it.
Morning body weight. Weigh yourself at the same time daily for a week. A sudden drop of more than 1 kg overnight is usually fluid, not fat.

Chronic low-grade dehydration shows up as morning stiffness, poor sleep, and sessions that feel harder than the numbers suggest. Fix the baseline first; session hydration is a much smaller problem once daily intake is dialled.

Intra-session fluid: the 400-800 mL/hour range

For steady endurance work in temperate conditions, 400 to 800 mL per hour covers most athletes. That range is wide on purpose. A 60 kg runner in 15C weather sits at the low end. An 85 kg cyclist on a long ride in 28C sits at the top, and in extreme heat can push past a litre per hour.

Two practical notes:

Gastric tolerance caps intake. The gut absorbs fluid at roughly 800-1200 mL per hour under good conditions, less when running hard. Chugging more than that just sits in the stomach.
Intensity matters. Running redirects blood from the gut to working muscle more aggressively than cycling. You can drink more on the bike than on foot, which is why hydration for cyclists on a long ride looks different from the same athlete on a long run.

For sessions under 60 minutes at moderate intensity in cool weather, you can skip in-session fluid with no real performance cost. Drink before, drink after, move on.

The sweat-rate test: do it once, use it for years

Knowing how to calculate sweat rate turns hydration from guesswork into a number. The test takes one session:

Weigh yourself nude before the session.
Train for exactly 60 minutes at a representative intensity.
Track fluid intake during the session (mL).
Weigh yourself nude after, towelled dry.

Sweat rate (mL/hour) = (pre-weight minus post-weight in grams) + fluid consumed in mL.

A 75 kg runner who drops to 74.2 kg after an hour and drank 500 mL has a sweat rate of 800 + 500 = 1300 mL/hour. That is the target replacement for similar conditions. Run the test twice, once in cool weather and once in heat, and you have bookends for the year.

Sodium and electrolytes: when they actually matter

Sweat sodium concentration varies from about 200 mg/L in low sweaters up to 2000 mg/L in salty sweaters. That is a tenfold range. Generic electrolyte advice misses this. The useful rule:

Under 90 minutes, cool weather: sodium is rarely limiting. Water is fine.
Over 90 minutes, or in heat: aim for 300-700 mg sodium per litre of fluid. Heavy sweaters with visible salt marks on their kit push towards 500-1000 mg/L.
Over 3 hours, or multi-session days: sodium becomes non-negotiable. Cramps, flat legs, and late-session fade often trace back here.

Salt tablets for endurance athletes are a tool, not a default. They are useful when you know your sweat sodium is high, when carrying fluid with dissolved electrolytes is impractical, or when the session runs long enough that baseline sodium in food is not enough. For most sessions under two hours, a sports drink or a pinch of salt in a bottle is simpler and works.

Electrolytes for runners are often sold as a cure for cramps. Cramps in late-session endurance work are usually a combination of fatigue, electrolyte loss, and neuromuscular drive. Sodium helps the electrolyte piece, but training the distance matters just as much.

Hyponatremia: the other failure mode

Hyponatremia symptoms in athletes look like the opposite of what you would expect: nausea, headache, bloating, confusion, and in severe cases seizures. It is caused by overdrinking plain water during long events while losing sodium in sweat. Marathon runners at the back of the field, slower Ironman finishers, and ultra-endurance athletes are the classic cases.

Two rules prevent it:

Do not drink past thirst on long sessions. Thirst is a reasonable regulator when you are not badly dehydrated going in.
Add sodium once sessions pass 2 hours. Plain water plus high sweat losses over time is the setup that creates trouble.

Pre-hydration before hot sessions

Starting a hot session already under-hydrated is the most avoidable mistake in summer training. The fix is simple: 500 mL with 500-700 mg sodium about 2 hours before the session. That gives time to absorb and pee off the excess, leaving plasma volume topped up at the start.

Hyper-hydrating beyond that with glycerol or extreme fluid loads has some evidence in heat, but it is a specialist move with real downsides (weight gain, GI discomfort) and rarely worth it for training.

How Pelaris fits in

Pelaris does not sell you electrolytes or track every sip. It does something more useful: the AI coach grounds hydration advice in the session you are actually about to do.

Session-specific prompts from the AI coach. When a session is long, hot, or at high intensity, the coach surfaces targeted fuelling and hydration notes before you start. A 30 minute Zone 2 run in autumn gets nothing. A 3 hour ride in 30C gets a specific intake target based on duration and intensity. The logic is methodology-aware, not generic.

Engine Room content matched to your phase. The Engine Room surfaces fuel-category guides based on where you are in your training block. Build phase with long rides on the calendar pulls up sweat-rate testing and sodium content. Taper week pulls up carbohydrate loading instead. You see what is relevant to the next two weeks, not a generic library dump.

Strava-informed recommendations. When Strava is connected, session duration and intensity feed into the next session’s hydration prompt. A 4 hour ride yesterday shifts today’s recovery and fluid guidance without you logging a thing. The workout tracker pulls this together so hydration, fuelling, and training load sit in one place rather than three apps.

Methodology as the anchor. Pelaris treats hydration the same way it treats every other variable: evidence-based, individualised, and subordinate to the actual training plan. The methodology page covers how session prescriptions are built, and the endurance pages go deeper on how fuelling and fluid work inside a proper periodised block.

The numbers worth remembering

Three figures cover 80 percent of the decisions:

35 mL per kg per day as a daily baseline.
400-800 mL per hour during sessions in temperate conditions.
2 percent body weight loss as the threshold where things start to go wrong.

Measure your sweat rate once, know your sodium roughly, and stop overthinking every bottle. The athletes who get this right are not the ones with the most supplements. They are the ones who set a baseline, check it occasionally, and let the training do the work.