Understand the issues behind the recommended amount of water per day

THE Water it is a large part of the composition of the human body, about 60 to 70%.

There is no doubt that drinking water is essential for maintaining health – this is a consensus. However, the daily amount to be ingested is still the subject of disagreements between the medical and scientific communities.

“Because it is so critical to life, there are physiological processes that strictly control the balance of salt and water in the body. In a simplified way, to maintain the amount of water in our body, it is necessary to replace the losses”, says Professor Eduardo Barbosa Coelho, from the Faculty of Medicine of Ribeirão Preto, University of São Paulo (USP).

A study by the University of Aberdeen, UK, published in November 2022, added more fuel to the fire of the discussion about the ideal amount of water per day.

Research points out that the recommended water intake of eight glasses, about two liters, per day rarely corresponds to real needs and can often be high. According to research, the amount needed to be drunk varies between 1.3 to 1.8 liters per day, depending on age, climate and where the person lives.

One study published in the journal Science indicates that the volume for daily consumption is determined by several factors such as gender, age, physical aspects, air humidity, temperature and even the Human Development Index (HDI).

In this sense, there may not be an ideal amount of water ー like the famous two liters ー, since the influence of multiple points modifies the needs of each individual.

“There is no ‘normal’ or recommended value for daily intake. Generally, for an adult on a standard normoprotein diet, about one to one and a half liters of water will be sufficient to maintain fluid balance. However, this value will depend on individual metabolism, age, body fat distribution, environmental conditions, physical activity and other factors that affect water loss. There is a widespread idea that drinking water is good for health. As described above, there are physiological mechanisms for maintaining a water balance and if there is a lack of water, thirst will appear”, says Coelho.

Importance of water intake

For the body to keep functioning, biochemical reactions need water and are fundamental: from the exchange of CO2 for O2 in breathing to digestion.

“It transports nutrients and oxygen through the bloodstream, maintains the correct concentration for maintaining electrolyte and acid-base balance, regulates body temperature, maintains cellular structure, including the cell membrane and the structure of proteins and nucleic acids (DNA ) and is vital for the excretion of toxic substances through urine and feces,” he explains.

“Water is so critical to life that if you lose more than 4% of your total body water, symptoms of dehydration will set in, and if you lose more than 15% it can be fatal.”

How does water loss occur in the body?

In the process of cellular respiration, human cells convert nutrients and oxygen that arrive through the bloodstream into carbon dioxide (CO2) and water. Thus, an adult weighing about 70 kg ‘manufactures’ approximately 700 ml of water per day.

At Water losses occur in three ways : one part corresponds to secretion production such as saliva, gastric juice and sweat, for example, the other part is eliminated in the breathing and a last part is present in the urine and feces .

Although there is no ideal volume of water to be ingested, Professor Roberto Zatz, from the discipline of Nephrology at the Department of Internal Medicine at the Faculty of Medicine at USP, warns that certain population groups should be more attentive to the amount of water.

“Some groups are usually encouraged to maintain a certain level of consumption, generally something close to the famous two liters/day, to increase the urinary flow. Examples are: individuals with a urinary tract infection or with a tendency to develop it: although there is no conclusive evidence, the very reasonable concept has been established that a high urinary flow makes it difficult for bacteria to settle in the urinary tract”, he explains.

“Another group that may benefit from a higher fluid intake are people who are prone to forming urinary stones ー ‘kidney stones’. In the absence of such conditions, there is no basis for the concept that the habit of consuming high amounts of water serves to ‘clean the organism’ or brings any other benefit”, adds Zatz.

thirsty feeling

The thirst mechanism is the body’s main alert for thirst. dehydration . Experts explain that the habit of drinking water recurrently is only necessary, from a biological point of view, when there is an indication that the body is starting to become dehydrated, that is, when thirst appears.

“We usually already ingest more than enough ー when we eat or drink other types of liquids ー, and if for whatever reason we fail to do so, the thirst mechanism will force us to correct any imbalances. Thirst is a kind of safety net that guarantees that water losses never exceed gains, thus preventing the individual from becoming dehydrated”, says Zatz.

The kidneys are able to modify the concentration of urine by altering the amount of free water eliminated. Coelho explains that we can eliminate from 150 ml, in conditions of extreme water loss or lack of intake, to 20 liters of urine in situations of abundance of fluids and nutrients.

In addition to these organs, the antidiuretic hormone (ADH), also known as vasopressin, produced in the pituitary gland, also acts in the regulation of urine. When the concentration of water in the body decreases, the concentration of electrolytes, which are minerals, is concentrated in the blood.

“Sensitive cells, osmoreceptors, signal the pituitary, which produces ADH. This hormone acts on the kidneys, increasing water reabsorption and concentrating urine”, explains the specialist.

In this process, the brain identifies blood concentration and thus controls the rest of the body’s water elimination mechanisms. The researcher explains that it is very difficult for excess water consumption to be dangerous, however, there are some specific circumstances that can lead to this.

“The kidneys have a great capacity to eliminate excess water, which allows a maximum intake of more than 15 liters per day. This means that, in general, drinking more fluids than necessary does not cause major problems. However, there are some situations of water intoxication, a kind of ‘water overdose’. This condition can result from drinking more than 15 liters a day or drinking so quickly, for example five liters in half an hour, that there is no time for the kidneys to eliminate the excess”.

An excessive accumulation of water in the body causes dilution of solutes and, as a result, brain swelling and a serious neurological condition that can have a fatal outcome, says Zatz.

The researcher adds that there are also risk groups, such as some cancers, which abnormally produce a hormone that hinders the excretion of water by the kidneys, with water intoxication being one of the probable first manifestations of the disease.

Chronic kidney disease, also known as chronic renal failure, is a condition in which the slow loss of kidneys occurs, either due to hypertension, diabetes or inflammatory processes in most cases. In advanced stages, the ability to eliminate excess water is compromised.

the age factor

The age issue is one of the factors studied by the article. There are changes in the body with respect to water as age varies. The elderly, for example, find it more difficult to save water when needed, says Zatz.

“If the elderly remain lucid and active, there are usually no major problems. However, the sense of thirst is often dulled in these people, especially if they already have a serious neurological condition, such as previous strokes or dementia. For this reason, it is often necessary to offer them water several times a day, a procedure observed by good caregivers and nursing homes ”, he explains.

As for young people, there is no need for any special recommendation according to the expert. “It is evident that individuals who practice sports need to consume more, which can be active or motivated by thirst. The same goes for seniors who stay physically active.” it says.

The question of water, when analyzed from a biological point of view, depends on several individual factors that together modify the necessary amount to be ingested. But even if there is no predetermined value, its consumption is important.

“Water is the essence of life and, for this reason, our organism is prepared to conserve it. Maintaining health is learning to listen to what your body says. Consume the water you need and avoid excesses. A simple way to tell if you’re drinking too little water is to look at your urine. If it is very orange and in small quantities, it is likely to be concentrated and a break for hydration is recommended”, concludes Coelho.

(With information from Alessandra Ueno, from Jornal da USP)