What's the difference between hydration and water retention?

Your body is about 60% or more water and that's because we're made of cells and cells are literally bags of water. Cells have an oily membrane that encloses water and also on the outside of that membrane washing around our tissues and cells is water.

So the average 70kg person has at least 40kg of water. That water exists in an equilibrium between how much is in the blood vessels, how much is surrounding our cells and how much is in the cells.

Your body knows how much water it's got on board because it measures things like how much salt it's got in the blood stream.

Your kidneys work out whether you have too much or too little water and then secrete various hormones to regulate how much water is lost.

When you get water retention, something encourages your kidneys to keep more water back when you would normally put it into the urine and this increases the total amount of water in the body.

When you get a little bit too much water in the body, it spreads among the tissues and starts to surround the cells in your legs in what's called dependent oedema. It goes to the lowest point in your body under gravity, and that's why people get puffy legs.

Water retention can be caused by kidney problems and heart problems, but just in normal hot weather people who take on a lot of water to rehydrate and it sections out into various parts of the body.