Stop gulping away – you're not short of water

Water is essential for life, but we don't need large quantities to keep us going; on the contrary, over-hydrating can be dangerous. Water and liquids should be drunk and given in moderation, researchers believe.

Text: Helena Mayer, first published in Swedish in the magazine Medicinsk Vetenskap, no 2, 2015.

Do you sip water all the time? You are not alone. Since the 1980s, the picture of people drinking from their water bottles has become more and more common, and in the media we often hear advice that we should drink more water so that we don't go short.

An American kidney researcher studied the reasons for regularly drinking water and published an article in a scientific journal in 2002 where he came to the conclusion that the "water drinking epidemic", which started around 30 years ago, has no scientific basis.

"There is still no evidence that healthy adults need to worry about drinking too little – we are rarely dehydrated under normal circumstances," says Mats Rundgren, former* senior lecturer and researcher at the Department of Physiology and Pharmacology at Karolinska Institutet.

Woman drinking water, genre image.
There is no scientific evidence that healthy adults need to worry about drinking too little water. Genre image: iStockphoto.

An adult human being is made up of approximately 60% water, and for newly born infants the figure is 80%. Water is a good solvent and can be used by the body to transport salts, carbohydrates, and proteins to the right place. Via urine we excrete waste products and regulate the salt balance in the body. Evaporation of the water is also an important means of regulating body temperature - when we sweat, excess heat from the body is used for evaporation so that we are cooled down. New liquid must then be added. Many people want to know a quantity they should drink, to know they have drunk enough.

"Normally we need about one and a half or two litres per day, but on some days you may need up to four litres if you have been working in a warm environment or for other reasons. The best advice to check on liquid balance is to note the quantity and colour of your urine. If you urinate a few times a day and the urine is light yellow, then yes, you have drunk enough,"says Mats Rundgren.

Regulate by urinating more

Many people now work on the principle that more is better, though, according to Mats Rundgren. As long as it is not huge amounts, we can regulate the surplus by urinating more. There is a limit for the kidneys of around 20-25 litres per day.

"In general, you might think that thirst is the best measure of how much you should drink, but this is not really true. Thirst is a powerful driving force when you are really suffering from a water shortage, but studies have shown that we are rarely thirsty for purely physiological reasons," says Mats Rundgren.

Mats Rundgren
Mats Rundgren, credit: Ulf Sirborn.

Instead, explains Mats Rundgren, we drink things like coffee and tea in advance and out of habit, or we drink at meal times and get much of the liquid we need. It happens quite easily, though, that we ignore the sensation of thirst, particularly during sports when we focus on performing instead. The thirst then comes when the shortage of liquid is so large that it has already had an impact on performance.

"This happens at a relatively moderate level of dehydration. Many people try to drink in advance of a long-distance race. There is little point, though, because the body cannot store excess water. Instead we should replace losses with a reasonable quantity of liquid while we are running," explains Mats Rundgren.

There are several cases of people who have been taken to hospital for over-hydrating during marathons.

"These days every doctor has several cases a year of water-poisoning, which used to be very unusual," says Mats Rundgren.

Children and the elderly tend to feel thirst less. Thirst can also be affected by drugs, high temperature and certain diseases such as bipolar disease, which can lead to an extremes of over-drinking.

Healthcare services have also jumped on the bandwagon and given advice to patients that they should drink a lot before an operation or that women should drink more prior to childbirth. Even though water poisoning is rare, there are several cases where women have had symptoms of over-hydrating during labour where both mother and child needed intensive care.

"As well as sickness and pregnancy, various forms of trauma or operation and anaesthesia reduce the ability to excrete water, and those with weak hearts and kidneys are very sensitive to a small increase. So don't be too keen to fill up with liquids," says Mats Rundgren.

Salt content in the blood

In hospital, we usually measure the salt content in the blood to determine whether a patient is over-hydrated, which a low value may indicate. One of the most serious risks associated with this is that the brain can become swollen, which can occur with a surplus of five to ten litres of liquid. Most tissues and cells manage to increase in volume without problems, but the brain has limited space, causing the pressure to rise and, in the worst case, affecting vital functions such as circulation and respiration.

"In general, adults have a fairly large margin of error — we may have too much or too little water without it being medically risky," says Mats Rundgren.

Christer Svensén, credit: Stefan Zimmerman.

Until the end of the 1990s people were given large quantities of liquid prior to operations, anaesthesia and intensive care, because it was thought to increased survival rates.

"Giving liquids has been very common in our work. But we are more restrictive now, because it has been seen that many patients can't manage too much. Instead we carefully monitor patients' heartbeat volume during major operations. By monitoring heartbeat volume increase up to an optimal value for the patient, we can provide the correct volume at the right time," says Christer Svensén, professor and senior consultant in anaesthesiology at the Department of Clinical Research and Education, Södersjukhuset (Stockholm South General Hospital).

He tells us that studies have also shown that the healing process in the area of the operation, such as during colon surgery, is not so good if the patient is over-hydrated because it results in body tissues having worse micro-circulation when they swell. This can lead to poorer recovery or even a new operation.

In normal cases, approximately two litres of liquid per day are given to a patient who cannot drink and more is only given for haemorrhages or other liquid losses. These days patients are weighed before and after surgery, as well as some days later, to check their liquid balance.

With inflammatory conditions such as blood poisoning, however, larger quantities of liquid may need to be added if blood leaks from vessels into the surrounding tissue. The kind of liquid given is also important.

"It is not an easy situation, though, because there is no "one size fits all" and liquid replenishment must be individualized," says Christer Svensén.

Works as a liquid buffer

Two-thirds of the water in the body is inside the cells and one third outside. The water outside the cells is both inside and outside the blood stream and in different tissues and organs. Tissue outside the bloodstream works as a liquid buffer and most water is stored in your muscles.

Dehydration is, despite all these mechanisms, a problem that can affect us, but the symptoms of drinking too little are subtle and rather vague. A headache is one known symptom. Why lack of liquid causes a headache is unclear, but one theory is that it results in an imbalance in the meninges. Concentration problems are also common. Studies have shown that a person with slight dehydration is worse at solving problems under time pressure.

Serious dehydration, however, gives clearer symptoms such as reduced urine quantity, dry mucous membranes, loose, sunken skin and fatigue.

"This can be a problem if you drink too little and sweat a lot, such as during a marathon, staying in a hot climate or in cases of disease. Evaporation of sweat from the skin is the most effective way of losing heat, but body salts are also lost through sweat and need to be replaced. In the end, the blood volume is affected so much that the body can neither maintain blood supply to the brain and kidneys or keep a moderate temperature, which leads to heat collapse," says Mats Rundgren.

Shortage of liquid due to a large haemorrhage can usually be replaced relatively quickly with blood and plasma in the hospital, provided that the bleeding can be stopped.

"These days, though, it is very common that we have older patients who have not been able to eat or drink for several days due to a blockage in their intestines. The whole intestine system usually absorbs five to eight litres of liquid a day, so if the intestine is partially or completely blocked due to an obstacle, such as a tumour, the patient may have a large shortage of liquid," says Christer Svensén.

Such patients often need be operated within twenty-four hours. Because we know that the body is lacking many litres of liquid, it is necessary to restore that balance to a certain extent before the tumour is surgically removed.

"If we anaesthetise the patient and start to operate before we have restored at least some of the liquid deficit, the patient may go into a state of shock. The bloodstream can't compensate for the drop in blood pressure caused by the anaesthetics," says Christer Svensén.

He adds that there is a fine balance, because older patients often have a weak heart or poor kidneys, which further complicates the situation and which can lead to pulmonary oedema if liquid is given too quickly.

"Many people want to have measurements, numbers and schedules to stick to when it comes to liquid intake, and it is the same thing in healthcare. But it is just not possible to prescribe a number of litres of liquid over a certain number of hours. You have to give some liquid, measure the effect, and then increase or decrease the rate of supply based on the individual in front of you," says Mats Rundgren.

* June 2019: Mats Rundgren is no longer affiliated to Karolinska Institutet. 

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