1st Revision: Tamoghna Das 
How much water is enough water? Is there such a thing as too much water? Why is water important? These are seemingly simple questions without simple answers.
The average adult human body is about 60% water by weight, so an adult human weighing 100 kg would contain 60 litres (13 gallons) of water.1 Most of this water is found inside the body’s cells.
In order to survive, this water must be replenished regularly, since it is lost in sweat, breath, urine, and faeces every day. The rate of loss will increase if the weather is hot, if you’re physically active, if you take water tablets or other diuretics (substances that make you pee more), or if you have a fever. As you age, your kidneys become less efficient at conserving water, so you still need to drink plenty even though you might be less physically active.
It is recommended that the average woman requires about 1.7-2.2 litres of water, and the average man about 2.4-3 litres (USGS, Patient.info) to replace lost water. Most of this comes from drinking, but the food we eat contains water too (about 30% of our total intake), so you need to drink about 6-9 glasses per day. Failure to take in adequate water will lead to dehydration, which has a number of consequences. On average, in the UK we take in only 53% of the water we need (Waterlogic), compared with only 43% in the USA and a tiny 4% in China.
The amount we need to drink each day will vary from person to person and from climate to climate. For example, you’d need to drink more if you live in the tropics than if you live in a temperate region. The amount you need to drink also depends on age, gender, body weight. and your overall level of physical activity.
What functions does water have in the body that make it so vital to our survival?
Water is a vital component of every chemical reaction in the body, and almost everything your body does will involve a chemical reaction, from thinking and moving to digest food.
In addition, water helps rid the body of waste, particularly in urine, and lubricates the joints, acting as a shock absorber for the brain and spinal cord. Water helps regulate body temperature, deliver oxygen to the tissues as a major component of blood, and keeps mucosal membranes moist, such as in the lung and the mouth, making processes such as breathing and tasting possible.
Many non-alcoholic fluids, including tea, coffee, and fruit juice, all count towards your water intake. It is often mistakenly believed that, because caffeine is a mild diuretic, tea and coffee cannot count towards your total fluid intake. However, you would have to drink eight cups of coffee or ten cups of tea in a day for the diuretic effect to be significant. On the other hand, alcohol is a potent diuretic and will make you lose more fluid than you take in, so excessive alcohol leads to dehydration. So that pint of beer, while refreshing, doesn’t count toward your total water intake.
Can you Drink Too Much Water? ─ Overhydration
As a general rule, if you’re rarely thirsty and your urine is almost colourless or pale yellow, you’re getting enough water. But can you drink too much water? The kidneys are very good at compensating for increased water intake - you just pee more.
However, there is such a thing as overhydration, when the kidneys can no longer keep up. Overhydration can lead to a condition called hyponatremia when the sodium in the blood becomes too diluted by excess water.
Sodium is critical to maintaining the fluid balance of your cells. Hyponatremia causes this balance to be disrupted, allowing too much water into the cells and leading to cell swelling. You may experience water intoxication or poisoning, or even a disruption of brain function, caused by cell swelling. If brain cells swell, this may lead to confusion, drowsiness, and headaches, and in extreme cases bradycardia (slow heartbeat) and hypertension (high blood pressure).
The signs of overhydration include overly pale urine and urinating much more often than useful. On average, you will urinate six to eight times a day, and up to ten times if you are a regular caffeine or alcohol drinker. Other symptoms can include nausea and vomiting, throbbing headaches, swelling, and discolouration of the hands, feet, and lips. You may also experience weak muscles that cramp easily and tiredness or fatigue.
Avoid overhydration by not drinking a lot of water when you’re not thirsty. Thirst is the body’s general response to dehydration and should act as your cue to drink.
Drinking Too Little ─ Dehydration
It takes only a 2% fall in body water to start feeling thirsty (by which time you are already dehydrated), but even a 1% loss could affect cognitive ability and mood (Waterlogic). The body's signals for dehydration can become weaker over time, and may even be mistaken for hunger.
Anyone may become dehydrated, but young children and older adults are especially vulnerable. In children, the most common cause is severe diarrhoea and/or vomiting. Older adults naturally have a lower volume of water in their bodies and may have conditions or take medications that increase the risk of dehydration. Even minor illnesses, such as respiratory or urinary tract infections, can lead to dehydration in older adults. Mild dehydration can usually be reversed by drinking more water but severe dehydration can require medical treatment.
The symptoms and consequences of dehydration include:
In adults:
- Fatigue
- Dizziness
- Confusion
- Concentration difficulties
- Dry skin
- Muscle cramps
- Bad breath
- Mood swings
- Body aches
- Headaches
- Dark urine
- Infrequent urination
In infants and young children:
- Dry mouth and tongue
- No tears when crying
- No wet nappies for three hours
- Sunken eyes, cheeks
- Sunken soft spot on top of the skull
- Listlessness or irritability
When to see a doctor:
- Diarrhoea for 24 hours or more
- Irritable or disoriented
- Much sleepier or less active than usual
- Cannot keep down fluids
- Bloody or black stool
Dehydration can lead to serious complications, including:
- Heat Injury. When you’re perspiring heavily, e.g., when exercising at high intensity, you may get heat injury, ranging in severity from mild heat cramps, heat exhaustion, or life-threatening heatstroke.
- Urinary and kidney problems. Prolonged or repeated dehydration can lead to urinary tract infections, kidney stones, and kidney failure.
- Seizures. If electrolytes like sodium and potassium get out of balance, electric signals in your body can become dysregulated. This can lead to involuntary muscle contractions, seizures, and even loss of consciousness.
- Hypovolaemic Shock (low blood volume). This is one of the most serious, and sometimes life-threatening, complications of dehydration. Low blood volume due to lack of water causes a drop in blood pressure and thus in the amount of oxygen in your body.
Tips for drinking more water
It's a good idea to drink a glass of water with each meal, before, during, and after strenuous exercise, and whenever you’re thirsty. Try to tie drinking water into a routine. If you’re having trouble quitting fizzy drinks and fruit juices, try alternating water with your favourite soda.
Many foods such as watermelon also have very high water content and will contribute to your water intake, and it can help to use a free water-tracking app like Hydro Coach or Aqualert, or even a smart bottle that can give you regular reminders to drink.
Lastly, try to set an achievable daily water goal based on your bodily requirements, and try to sip 5water throughout the day, rather than trying to drink large amounts all at once.
Summary
While there is no simple, universal answer to the question “How much water is enough?”, the widely recognized “8 cups per day” rule is a good starting point. The actual requirement for water varies depending on age, gender, climate, and level of physical activity. There are many ways to take in enough water each day, but it must be borne in mind that overhydration is as harmful as dehydration.
