A water is a large part of the human body composition, about 60 to 70%.
There is no doubt that drinking water is essential for maintaining good health – there is a consensus. However, the daily amount that should be consumed is still the subject of disagreement among 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 simple terms, to maintain the amount of water in our body, we need to replace the losses,” says Professor Eduardo Barbosa Coelho, from the Ribeirão Preto School of Medicine at the University of São Paulo (USP).
A study by the University of Aberdeen in the United Kingdom, published in November 2022, added fuel to the fire of the debate about the ideal amount of water per day.
The research shows that the recommended water intake of eight glasses, or about two liters, per day rarely corresponds to actual needs and can often be too much. According to the research, the amount needed to be drunk varies between 1.3 and 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 sex, 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 amount of water to drink per day. Generally, for an adult on a standard normal-protein diet, around one to one and a half liters of water will be enough to maintain water balance. However, this amount will depend on the individual’s 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 your health. As described above, there are physiological mechanisms for maintaining water balance and if there is a lack of water, thirst will appear”, says Coelho.
Importance of water intake
For the body to continue functioning, biochemical reactions require 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 to maintain 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 arise, and if you lose more than 15%, it can be fatal,” the researcher continues.
How does water loss occur in the body?
In the process of cellular respiration, human cells convert nutrients and oxygen that enter the bloodstream into carbon dioxide (CO2) and water. Thus, an adult weighing around 70 kg 'manufactures' approximately 700 ml of water per day.
As water losses occur in three ways: one part corresponds to production of secretionssuch 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 Nephrology discipline of the Department of Clinical Medicine at the USP School of Medicine, warns that certain population groups should be more attentive to the amount of water they drink.
“Some groups are encouraged to maintain a certain level of consumption, generally something close to the famous two liters/day, to increase urinary flow. Examples include: individuals with urinary tract infections or with a tendency to develop them: although there is no conclusive evidence, the very reasonable concept has been established that a high urinary flow makes it difficult for bacteria to attach themselves to the urinary tract,” he explains.
“Another group that may benefit from higher fluid intake is people who are prone to urinary calculi—known as ‘kidney stones.’ In the absence of such conditions, there is no basis for the notion that drinking large amounts of water will ‘cleanse the body’ or provide any other benefit,” Zatz adds.
Feeling thirsty
The thirst mechanism is the body's main alert to dehydration. Experts explain that the habit of drinking water regularly 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 water—when we eat or drink other types of liquids—and if for any reason we stop doing so, the thirst mechanism will force us to correct any imbalances. Thirst is a kind of safety net that ensures that water losses never exceed gains, thus preventing the individual from becoming dehydrated,” says Zatz.
The kidneys are capable of modifying 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 abundant liquids and nutrients.
In addition to these organs, the antidiuretic hormone (ADH), also known as vasopressin, produced in the pituitary gland, also acts to regulate urine. When the concentration of water in the body decreases, the concentration of electrolytes, which are minerals, becomes concentrated in the blood.
“Sensitive cells, osmoreceptors, signal the pituitary gland, which produces ADH. This hormone acts on the kidneys, increasing water reabsorption and concentrating urine,” explains the specialist.
In this process, the brain identifies the 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 liquids 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 consuming more than 15 liters per day or from ingesting 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, consequently, 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 makes it difficult for the kidneys to excrete water, 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 kidneys slowly lose their function, 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
Age is one of the factors studied in the article. The body changes in relation to water as it ages. Elderly people, for example, have greater difficulty in saving water when necessary, says Zatz.
“If the elderly person remains alert and active, there are usually no major problems. However, the sensation of thirst is often dulled in these people, especially if they already have some 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 recommendations according to the expert. “It is clear that individuals who practice sports need to consume more, which can be active or motivated by thirst. The same goes for elderly people who remain physically active,” he says.
The issue of water, when analyzed from a biological point of view, depends on several individual factors that together modify the amount needed to be ingested. However, even though there is no predetermined amount, its consumption is important.
“Water is the essence of life and, for this reason, our body is prepared to conserve it. Maintaining health means learning to listen to what your body is telling you. Drink the water you need and avoid excess. A simple way to know if you are drinking too little water is to look at your urine. If it is very orange and in small quantities, it is likely that it is concentrated and a break for hydration is recommended,” concludes Coelho.
(With information from Alessandra Ueno, from Jornal da USP)
