Do you know how much you should drink?
Many global guidelines recommend approximately 2 litres a day for women and 2.5 litres a day for men, including food.
But scientists say there is actually huge variation in how much each of us need as individuals.
Ruth Alexander speaks to Professor John Speakman at the University of Aberdeen in Scotland, who explains why your age, sex, weight, and other environmental factors such as air temperature, humidity and altitude all make a difference.
Dr Nidia Rodriguez-Sanchez, senior lecturer at the University of Stirling in Scotland, explains why our bodies need water and what happens if we drink too little, or too much.
That happened to Johanna Perry in the UK, she tells Ruth what happened when she drank too much water whilst running the London Marathon in 2018.
Presented by Ruth Alexander.
Produced by Beatrice Pickup.
(Image: a woman holding a plastic bottle of water. Credit: Getty Images/ BBC)