Drinking Water During Exercise: When, How Much and Which Signs Matter

Hot summers in most Iranian cities, crowded gyms with poor ventilation and the habit of drinking too little during the day make dehydration a common problem for people who train. On the other hand, some people believe "the more water, the better" and drink several bottles each session. The truth lies in between. Drinking water during exercise should be adjusted to the length and intensity of your workout, the temperature around you and your personal sweat rate. Based on scientific recommendations, this article answers three key questions: when, how much and which signs to watch for.

Why water matters for performance

Water makes up roughly 50 to 60 percent of an adult's body weight. During exercise, muscles generate heat and the body releases it through sweat. Each litre of sweat carries away some sodium and other electrolytes as well as water. If lost fluid isn't replaced, blood volume falls, the heart works harder to supply muscles and skin, and body temperature rises.

Research shows that dehydration of 2 percent of body weight or more, about 1.5 kg for a 75 kg person, can noticeably reduce aerobic performance, focus and energy, especially in hot weather. The effect is smaller in short strength sessions, but it still brings earlier fatigue and headaches.

How much to drink before training

The goal is to start training well hydrated, not with a stomach full of water. Sports science guidelines suggest:

• About 2 to 4 hours before training: roughly 5 to 7 ml per kilogram of body weight; for a 70 kg person, about one and a half to two glasses.
• If you haven't urinated by then or your urine is dark, drink another glass about 2 hours before training.
• Drinking a large amount right before you start causes a heavy stomach and trips to the bathroom mid-session.

Your pre-workout meal helps with hydration too. Water-rich foods common on the Persian table, such as cucumber, watermelon, tomato, lettuce, soup and light ash, all contribute.

How much to drink during training

Sessions shorter than an hour

For most 45- to 60-minute strength sessions in a gym at normal temperature, a few sips of water between sets whenever you feel thirsty is enough. This usually adds up to a small to medium bottle (about 300 to 600 ml).

Longer sessions or hot weather

For activities lasting more than an hour, such as mountain hiking, cycling, football or group classes in a hot studio, needs are higher. Scientific recommendations commonly suggest a range of about 400 to 800 ml per hour, ideally in small, frequent amounts, for example a few sips every 15 to 20 minutes. Lighter people and light sweaters sit near the lower end, while heavier people and heavy sweaters in the heat sit near the upper end.

Electrolytes and sodium

In long, sweaty sessions you lose sodium as well. For most people, a regular meal with moderate salt, or foods such as doogh, bread and cheese or soup after training, replaces electrolytes. If you have high blood pressure, kidney or heart disease, set your salt and fluid intake with your doctor. Any decision about supplements, powders or electrolyte products is your own responsibility and should be made with a doctor.

How much to drink after training

After training, the goal is to replace the fluid you lost. If you weigh yourself before and after, drink about 1.25 to 1.5 litres of fluid for every kilogram lost over the following few hours; slightly more than the weight lost, because some fluid leaves through urine. Don't drink it all at once; spread it over several hours alongside your meals.

A simple weigh-in test to learn your sweat rate

Sweat rates vary widely; two people in the same class may sweat twice as much as each other. Use this simple test to find your personal needs:

1. Before training, after using the bathroom, weigh yourself in light clothing.
2. Do your usual workout and note how much water you drink.
3. After training, towel off the sweat and weigh yourself again in the same clothing.
4. Sweat loss = (weight before − weight after) + water drunk. For example, 0.8 kg of weight loss + 0.5 L of water = about 1.3 L of sweat in that session.

If you lost more than 2 percent of your body weight, drink a bit more in future sessions. If you weigh more after training, you drank more than you needed.

Recognise the signs of dehydration

• Strong thirst, dry mouth and lips
• Small amounts of dark urine, deep yellow or amber in colour
• Headache, early fatigue and a sudden drop in performance
• Dizziness, especially when standing up
• Muscle cramps (which can also have other causes)

Urine colour is a simple, practical indicator: pale yellow or straw-coloured urine usually means good hydration. Note that some foods and medications can change urine colour.

Drinking too much water is dangerous too

Few people know that drinking very large amounts of water, especially during long events, can dilute blood sodium too much. This condition, called hyponatraemia, can be dangerous in severe cases. Its symptoms may resemble dehydration: headache, nausea, confusion and swollen hands. The simple rule: don't drink more than you sweat, and treat weight gain after exercise as a warning sign. If you develop confusion, vomiting or severe neurological symptoms during or after prolonged exercise, go to the emergency department immediately.

Training in the Iranian summer heat

In cities such as Ahvaz, Bandar Abbas, Yazd and even Tehran in midsummer, heat and humidity raise the risk of heat illness. Follow these tips:

• Move outdoor training to early morning or after sunset.
• Wear light, pale, breathable clothing.
• In the first weeks of hot weather, reduce intensity and duration so your body can acclimatise.
• Bring your own water bottle rather than relying on the gym's crowded water cooler.
• If you stop sweating, your skin becomes hot and dry, or you feel confused or nauseous, stop immediately, move somewhere cool and get help; these can be signs of heat stroke.

What to drink: water, doogh or sugary drinks?

For most workouts under an hour, plain water is the best and cheapest choice. Soft drinks and packaged juices contain a lot of sugar and aren't needed for ordinary sessions. Low-salt doogh after training can provide fluid along with some protein and electrolytes, but people with high blood pressure should be careful with salt. Tea and coffee in usual amounts count toward daily fluid intake but are not a substitute for water during training.

Who needs to be extra careful?

Older adults, who have a weaker sense of thirst, children, pregnant women, people with diabetes, heart, kidney disease or high blood pressure, and anyone taking medication have different fluid needs. If you have a medical condition, get clearance from your doctor before starting an exercise program and agree on your fluid intake with them.

Summary

There is no one-size-fits-all rule for drinking water during exercise. Start training well hydrated, sip according to thirst in short sessions, follow a regular plan in long or hot sessions and replace the deficit afterwards. Use the simple weigh-in test to learn your personal needs and watch for the signs of both dehydration and overhydration. To calculate your daily energy needs, try the ProGym calorie calculator, and read more articles in the ProGym magazine.

Frequently asked questions