I’m glad to see that finally, some scientific investigation is being done on this old canard. And it turns out you don’t need to drink litres of water per day at all:
According to Heinz Valtin, a retired professor of physiology from Dartmouth Medical School who specialized in kidney research and spent 45 years studying the biological system that keeps the water in our bodies in balance, the answer is no.
The National Academy of Sciences’s Institute of Medicine also researched this in 2004:
Its panel on “dietary preference intakes for electrolytes and water” noted that women who appear adequately hydrated consume about 91 ounces (2.7 liters) of water a day and men about 125 ounces (3.7 liters). These seemingly large quantities come from a variety of sources—including coffee, tea, milk, soda, juice, fruits, vegetables and other foods. Instead of recommending how much extra water a person should drink to maintain health, the panel simply concluded that “the vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide.”
So there you go. Your body has a perfectly good way of telling you it needs a bit of topping up. Short of an illness that requires you to over-hydrate, or weather requiring the same, drink when you’re thirsty and stop worrying.