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Why Thirst Is a Late Signal

By the time you feel properly thirsty during a workout, you are usually already mildly dehydrated. Thirst lags behind fluid loss, especially during hard training, when your attention is on the exercise and not on your body. Mild dehydration — losing about 2 percent of your body weight in sweat — is enough to raise your heart rate, make the same effort feel harder, and reduce performance. You do not need to be dizzy or desperate for water to be affected. You just need to be a little behind on fluids, and most beginners are, because they start training already under-hydrated.

Sweat is also more than water. It contains sodium, and to a lesser extent potassium and other minerals. In short sessions this hardly matters, because your meals replace what you lose. In long or very sweaty sessions it matters a lot: replacing litres of sweat with plain water alone dilutes the sodium left in your blood, which is why long endurance events carry a real risk of hyponatraemia, a dangerous drop in blood sodium. The amounts in this guide are designed to keep you on the safe side of both problems — too little and too much — which is why they are ranges, not single numbers.

The Exact Plan: Before, During, and After

Before: 2–3 hours ahead, then a top-up

Drink 500–600 ml (about 2–3 cups) of water in the two to three hours before training. This gives your body time to absorb the fluid and pass any excess, so you are not running to the bathroom mid-session. Then drink another 200–300 ml (about one cup) in the 10–20 minutes before you start. If you train first thing in the morning, drink a full glass as soon as you wake up and another smaller glass on the way to training, because you will have lost fluid overnight and had nothing to drink for hours. A practical check: your urine should be pale yellow before you start. Dark yellow means drink more and, if you can, delay the hard part of the session by 20 minutes.

During: small sips, on a schedule

Drink 120–240 ml every 15–20 minutes during training. That is about half a cup to a full cup at each break, or roughly 400–800 ml per hour. Do not wait for a water break to be offered — in a home workout or a kickboxing session, build the sips into your rest periods. Drink between sets, between rounds, and whenever the coach or the timer gives you a pause. Small, frequent sips are absorbed better than one huge gulp at the end, which is more likely to slosh uncomfortably in your stomach and send you to the bathroom.

After: replace what you lost, with salt

After training, drink about 1.25–1.5 litres for every kilogram of body weight you lost during the session, spread over the next few hours. In pounds, that is roughly 600–700 ml (about 2–3 cups) per pound lost. You do not need to drink it all at once — in fact, you should not. Drink a large glass immediately after training, then keep drinking with your next meal. Include some salt in that meal, or use an electrolyte drink, because sodium helps your body hold on to the fluid instead of passing it straight through. A normal meal with some salty food is usually enough after a typical gym session. After a very long or sweaty session, an electrolyte drink or a pinch of salt in your food is worth adding deliberately.

How Much by Session Type

Not every workout sweats the same. A gentle 30-minute strength session in an air-conditioned room loses far less fluid than an hour of kickboxing rounds in a warm garage. Use this table to set your starting amount, then adjust it with the sweat-rate test below.

Session	Before	During (per hour)	What to drink
30–45 min strength, moderate effort	500 ml over 2–3 hours + 200 ml top-up	300–500 ml, sipped between sets	Plain water
60 min gym session or class	500–600 ml + 200–300 ml top-up	400–800 ml	Plain water; electrolytes optional in heat
60 min beginner kickboxing	500–600 ml + 200–300 ml top-up	500–800 ml, sipped every round break	Water; electrolyte drink if very sweaty
90+ min training, or training in heat	500–600 ml + 200–300 ml top-up	600–1,000 ml	Electrolyte drink with 4–8% carbohydrate
Easy walk or gentle mobility, 30 min	Normal daily drinking	A few sips as needed	Plain water, only if thirsty

Kickboxing deserves a special note, because beginners often drink nothing during rounds. They do not want to break rhythm, they do not want to feel full, and nobody reminds them. But kickboxing rounds are intense, and a 60-minute session with rounds, footwork, and pad work can easily cost 500–800 ml of sweat, sometimes more. Mah's rule in his own sessions is simple: the rest minute between rounds is for breathing and drinking, in that order. Three or four mouthfuls, bottle down, breathe, next round. Nobody's technique improves by being dehydrated.

Find Your Own Sweat Rate in 10 Minutes

Ranges are a starting point. Your own sweat rate is the real answer, and you can measure it with a bathroom scale. Do this once, on a typical training day, and you will never have to guess again.

1. Weigh yourself before training, in light clothes, after using the bathroom. Write the number down.
2. Train normally for 60 minutes, drinking a measured amount — for example, one 500 ml bottle, finished by the end.
3. Weigh yourself again afterwards, in the same clothes, before eating or drinking more. Write that number down too.
4. Your sweat loss is: (starting weight − finishing weight) + the fluid you drank. One kilogram lost equals roughly one litre of sweat.

Example: you start at 80.0 kg, drink 500 ml during training, and finish at 79.4 kg. Your loss is 0.6 kg plus 0.5 litres drunk, which is 1.1 litres in one hour — so your sweat rate is about 1.1 litres per hour, and you should aim to drink close to that amount per hour in similar conditions. If you finish heavier than you started, you drank more than you sweated, and you should drink less next time. The goal is to finish within about 1–2 percent of your starting weight: for an 80 kg person, that means losing no more than about 1.6 kg during the session.

Water vs. Sports Drinks: When to Switch

For sessions under 60 minutes, plain water is enough for almost everyone, provided you eat normal meals during the day. Your food replaces the small amount of sodium and carbohydrate you lose, and a sports drink adds sugar you do not need. Save your money and your teeth, and drink water.

For sessions over 60–90 minutes, or for very sweaty sessions in heat, switch to a drink that contains both electrolytes and some carbohydrate. The useful range is a 4–8 percent carbohydrate drink — that is roughly 40–80 grams of carbohydrate per litre — with sodium. This combination does two jobs: the sodium helps you retain fluid, and the carbohydrate provides a little fuel and helps absorption. A standard commercial sports drink fits this range. A homemade version — water with a pinch of salt and a splash of juice — can also work, but measure it rather than guessing, because too much sugar slows stomach emptying and causes cramps.

Heat, Humidity, and Clothing

Heat raises your sweat rate sharply, and humidity makes it worse: sweat evaporates less efficiently in humid air, so you feel hotter while losing just as much fluid. In hot conditions, drink at the top of the ranges in the table above, sip on schedule even before you feel thirsty, and schedule hard sessions for morning or evening where you can. Wear light, breathable clothing — heavy layers worn to "sweat more" only lose water without improving the workout.

Hydration Mistakes Beginners Make

The first mistake is drinking nothing until the session ends. By then you are already 1–2 percent down, your performance has dropped, and one big drink afterwards cannot undo an hour of training while dehydrated. The second mistake is the opposite: drinking a litre immediately before training, then spending the session uncomfortable and visiting the bathroom. Spread your pre-training fluid over two to three hours, with only a small top-up just before you start.

The third mistake is relying on thirst alone. Thirst is a lagging signal, as explained above, and it is especially unreliable in cold weather, where you may sweat a surprising amount without feeling thirsty. Drink on schedule, not on feeling, during any session longer than 30 minutes. The fourth mistake is drinking only water during very long, sweaty sessions and ignoring salt. After 90 minutes in heat, water alone is an incomplete replacement, and your post-training meal needs salt in it.

The fifth mistake is confusing hydration with energy. A sports drink does not replace a meal, and water does not replace food. If you train fasted in the morning and feel weak, the problem is usually fuel, not fluid — read what to eat before and after a workout for the food side of the same question. Hydration and nutrition work together: fluid carries nutrients, food replaces salt and fuel, and neither does the other's job.

A Practical Drinking Routine That Sticks

Here is the whole plan on one line, for a typical 60-minute evening session. At lunch and through the afternoon, drink normally with meals. Two to three hours before training, drink two to three cups of water. Twenty minutes before, drink one more cup. During training, sip half a cup to a cup every 15–20 minutes — keep the bottle beside you and drink at every rest. Afterwards, drink a large glass straight away, then keep drinking with your evening meal, which should contain some salt. That is it. No app, no calculation, no special product required.

For a morning session, shift the plan: a full glass on waking, another glass before you leave, sips during training, and a large glass plus breakfast afterwards. For a long weekend session over 90 minutes, add an electrolyte drink during training and a salty post-training meal. If you ever want precision, do the sweat-rate test above once and replace the ranges with your own number. Precision is nice, but consistency beats precision: a rough plan followed every session outperforms a perfect plan followed once.

If you want a coach to build the whole picture — training, food, sleep, and recovery — around your life, explore the coaching options on the programs page. Good coaching removes the guesswork, including the small habits like hydration that quietly decide whether a session feels strong or feels like a struggle. For the food side, read how much protein you need for fat loss and how sleep affects fat loss, because water, food, and sleep recover you together.