Personally I tell my clients to aim for 4-5 litres per day.
I also advise them to drink an electrolyte drink during the workout to replace the vital salts lost during exercise.
The best product would be Herbalife CR7 Drive which is formulated by leading scientists in the health and nutrition field.
This is a hypotonic sports drink with no nasty sugars.
On average between two to three litres. The people I usually advise are training, so I want them to operate at an optimum, which means staying adequately hydrated. Think of it as putting fuel in the tank of a car. And when it comes to working out how much water you should be drinking, your weight is the best barometer – a simple guideline would be 50ml per kg of body weight. But with exercise, you perspire as your internal heat rises, and a lack of water reduces perspiration and leads to fatigue. A 70kg, 2 hour 30 minute marathon runner can lose up to five litres of bodily fluid in that time, so there isn't a set amount of how much to drink, as it's relative to the amount (and style) of the exercise you're performing.
Overhydrating is rare but don't try and drink gallons of water, and drink regularly in suitable quantities rather than tons at one time. To keep track, fill up a two-litre bottle in the morning and keep an eye on your intake. If you’re exercising, increase that amount. The key to sufficient intake depends on your pee colour; you want it to be clear, so drink until it is. The colour is basically waste being removed by the body and it's always there – the more water that’s in your body, the lighter it is, and less you notice it. Imagine your pee as orange squash. If it's too concentrated, it will be too dark (and too sweet!), so add water till it's diluted enough.
You might notice that you’re peeing a lot more than normal at first, but this is where bladder control comes into effect. Some see the bladder like the lungs, a muscle you can control; this is your chance to exercise it.
If you don’t drink enough, you’ll dehydrate. That means you won’t be able to perform effectively – mentally or physically. Dehydration, fatigue, even hypothermia can kick in. But it's not about looking at the effects, it's about operating at your optimum levels. Performance deficit is noticeable after as little as a three per cent decrease in hydration. If you’re exercising, and therefore requiring three litres a day, missing just 90ml means you’ll be affected.