Hydration for training: how much to drink

Your daily baseline

The NHS Eatwell Guide recommends 6 to 8 cups or glasses of fluid a day for most adults. Water, lower fat milk, sugar-free drinks, tea and coffee all count. Food also provides a surprising amount, especially fruit, vegetables, soups and yoghurt.

Training adds to this. Rather than following a fixed number, use two simple checks through the day:

• Urine colour. Pale straw is good. Dark yellow or strong smelling means drink more. Completely clear all day may mean you are drinking more than you need.
• Thirst. For everyday life, thirst is a reasonable guide for healthy adults. It is less reliable during hard exercise and in older adults.

Some vitamins, especially B vitamins, turn urine bright yellow, so allow for that if you take a multivitamin.

Older adults are more likely to become dehydrated because the sense of thirst weakens with age. If you are over 60 and train regularly, drink with meals and after sessions as a habit rather than waiting until you feel thirsty.

Find your sweat rate

Sweat rates vary hugely between people. One person might lose half a litre an hour, while someone training next to them loses two litres. The only way to know yours is to measure it. A conditioning session such as an Engine class at Lycan Strength is a good one to test, because most people sweat heavily during that kind of work.

1. Go to the toilet, then weigh yourself in minimal clothing just before training.
2. Train as normal, noting how much you drink during the session.
3. Towel off and weigh yourself again in the same clothing straight afterwards.
4. Each 1 kg lost is roughly 1 litre of sweat. Add the amount you drank to get your total sweat loss.
5. Repeat in different conditions, such as a cool winter morning and a warm summer evening.

If you regularly lose more than 2% of your body weight in a session, drink more during training. If you gain weight during a session, you are drinking more than you need.

Before, during and after

Before: start training already hydrated. Drink normally through the day and have a glass or two of water in the couple of hours beforehand. Avoid downing a large bottle right before you start, which just sloshes around and sends you to the toilet.

During: for sessions under an hour, a few sips of water between efforts is usually plenty. For longer sessions, drink to roughly match your sweat rate without trying to replace every drop. Many people land somewhere around 400 to 800 ml an hour, but your sweat test is the better guide.

After: if you lost a lot of fluid, aim to drink around 1.25 to 1.5 litres for every kilogram lost over the next few hours, not all at once. The extra is needed because you keep losing fluid through urine while you rehydrate. A meal with some salt helps you retain it. Our guide to what to eat after a workout covers the food side.

Sessions longer than about 90 minutes, very sweaty conditions or salty sweat may call for more than plain water. That is where electrolytes come in.

When too much water is the problem

Overdrinking is less common than dehydration but more dangerous. Drinking far more than you sweat during long events can dilute the sodium in your blood, a condition called hyponatraemia. It has been seen in marathon runners and endurance athletes who drink at every opportunity out of fear of dehydration.

Warning signs include nausea, headache, confusion, bloating and swelling in the hands, and they can look like dehydration. Anyone who becomes confused or very unwell during or after a long event needs urgent medical help. Call 999.

The practical lesson: drink to your sweat rate and thirst, not to a target of 'as much as possible'.

Hot days and heat illness

UK summers are getting warmer, and indoor gyms can get very hot in July. On hot days, start sessions well hydrated, drink more during training and reduce intensity if you need to. Pre-cooling with a cold drink before a hard session can also help.

Know the signs of heat exhaustion: headache, dizziness, heavy sweating, cramps, nausea and a fast pulse. Stop, move somewhere cool, drink water and cool the skin. If someone is no better after 30 minutes, stops sweating, has hot skin, becomes confused, has a seizure or loses consciousness, it may be heatstroke. Call 999.

If you are preparing for a race, practise your drinking strategy in training. The HYROX race day checklist includes what to bring and when to drink on the day.