Feeling thirsty is your body's way of saying you are already dehydrated. The trick is that you want to keep yourself hydrated throughout the day so that you don't get to this point.
"When the normal water content of your body is reduced, it upsets the balance of minerals (salts and sugar) in your body, which affects the way that it functions," says the NHS.
"Water makes up over two-thirds of the healthy human body. It lubricates the joints and eyes, aids digestion, flushes out waste and toxins and keeps the skin healthy."
Why dehydration doesn't sound like quite a big deal, if you consistently keep your body dipping in and out of hydration, you may be putting it under unnecessary stress. And, although it may not seem like it is related to how often you get sick, or your efforts to handle your weight, a feature by Natural Cures Not Medicine says that it may do.
On weight gain, they say: "When cells are dehydrated within the body they become depleted of energy. This sends signals to the brain which the majority of people confuse with hunger signals. This means we often eat more to restore energy whereas we should in fact simply reach for a glass of water first."
A good test is when you feel inexplicably hungry an hour after eating a meal, have a drink of water or a herbal tea (not tea or coffee as it doesn't hydrate you in the same way) and see if the feeling passes.
As for illness, they mention a number of ailments dehydration exacerbates in the original article, but two of the most relevant are waste disposal and asthma.
"In a dehydrated body there is a build up of toxins and acid waste as the kidneys are unable to flush these out effectively. This in turn provides an ideal environment for bacteria to thrive and frequent bladder or kidney infections are the result.
"Our bodies will automatically constrict our airways to conserve water if we become dehydrated. Studies have shown that the rate of allergy causing histamines produced within our bodies actually increases exponentially as the body loses more and more water."
The best thing to drink is water, but if you aren't used to drinking a lot (on average you need to take in about 1.5 litres a day) you can start off with squash. Alternatively there are foods that hold a lot of water and have vitamins and minerals - see below: