How much water should you drink a day?

Water is essential for life and conditions practically all the biological functions of the human being. Our body is made up of 60% water, but more specifically this percentage increases to 70% in the brain, 80% in the blood and 90% in the lungs. Without this fluid, blood pumping, thinking and breathing would be impossible, so life itself could not be achieved in any way.

On average, human beings lose about 2.3 liters of water per day, the value of which is divided into the following values: urine is responsible for discard 1-1.5 liters, sweat 0.3-0.9 (depending on physical activity), respiration about 0.4 liters and faeces 0.1-0.2 liters. Even the exhaled air is essential to maintain the body's water balance, since we lose about 350 milliliters of this fluid with expiration, in the form of vapor.

As you can see, we are an open system that loses water, heat and organic matter at all times. For these series of reasons, it is necessary to maintain a constant intake of nutrients and water, but do you know how much water to drink per day? Here we give you an answer, taking into account recent science and research.

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Related article: "Excretory system: characteristics, parts and operation"

Body and water

Osmolarity is a measure used to express the total concentration of substances in solutions, with the parameter osmoles / liter. Osmolality, on the other hand, uses osmoles / kilogram of liquid values, but in dilute solutions (such as are the fluid media within the human body), both terms can be used interchangeably with comfort.

People with a plasma osmolarity between 285-295 mOsm / kg are considered hydrated, regardless of the level of urine produced and water intake. The brain directly modulates both vital functions based on the concentration of body fluids, so knowing the osmolarity of the blood plasma is "enough" to be able to estimate the state of a patient at a time dice.

The most common formula for establishing plasma osmolarity (osmolality) is as follows:

OSMp = 2 [Na +] + [glucose] + [urea]. Normal = 290 ± 10 mOsm / kg H2O

This formula includes the 3 solutes most present in blood plasma, which are sodium, glucose and urea. The normal range for blood sodium levels is 135-145 milliequivalents per liter, while the glucose is considered correct when the value is less than 5.6 mmol / L: beyond this, the prediabetes / diabetes. On the other hand, urea, the end product of protein metabolism, occurs in plasma ranges of 12-54 mg / dl.

All these values depend on the percentage of solutes present in the plasma, but also on the water available to the body, since we remember that 80% of blood is water. The more water we have available at the body level, the more diluted a sample can be shown, within the limits of normality.

How much water should you drink daily?

Based on this lengthy but necessary introduction, keep in mind that what is really important is the osmotic balance in our body, beyond arbitrary values in liters of water, when talking about intake. A person can lose up to 1% of body weight in sweat when doing strenuous exercise, so Naturally, your water consumption will have to be much higher than that of another who has not moved from the sofa in the whole day.

In any case, there are a series of mean values that can be established in the general population. The European Food Safety Authority made a list in 2010, with the intake of milliliters of water per day, in different age groups. The mean values are as follows:

Newborns from 0 to 6 months of age: 680 milliliters daily, in the form of breast milk.
Children 6 to 12 months of age: 800 milliliters to 1 liter a day.
Children from 4 to 8 years old: 1600 milliliters a day (1.5 liters).
Boys from 14 to 18 years old: 2.5 liters a day.
Girls from 14 to 18 years old: 2 liters a day.
Adults: 2 to 2.5 liters per day.

As you can see, the values vary greatly according to the age group analyzed and, also, based on the governmental sources consulted. We can conclude that most men should ingest 2.5 to 3 liters (depending on their demand and physical activity), while women require a little less, 2 to 2.5 liters. This distinction is merely arbitrary, simply based on the fact that males tend to have slightly higher muscle mass.

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The brain and its role in hydration of the body

Studies like Water Intake, Water Balance, and the Elusive Daily Water Requirement (Nutrients Magazine) have tried to accurately quantify the importance of these figures at the neuroendocrine level. The brain is constantly acting to maintain the internal homeostasis of the body, so it is to be expected that the plasma concentration of hormones and neurotransmitters varies according to the physiological needs at each moment.

With this idea in mind, circulating vasopressin levels have been quantified in patients at different times of the day. This hormone is the main regulator of body fluids, it is produced in the hypothalamus and is stored and released through the neurohypophysis. It causes vasoconstriction, water reabsorption at the kidney level and acts as an antipyretic / analgesic, among other things.

The intensity of the neuroendocrine response (using circulating vasopressin levels as a parameter) allowed us to contrast that the Typical brain mechanisms of dehydration begin to activate, on average, when the person consumes less than 1.8 liters of water per day. day.

What's more, up to 71% of adults consume less than this daily value in some countries, showing mild signs of dehydration throughout the day. These values show that the majority of the population does not respect the minimum Daily Total Water Intake (TWI L / 24H) so that neuroendocrine defense methods do not appear in the event of dehydration.

How do you know if you drink little water?

The normal oscillation of water in the body during a day is less than 2% of the patient's body weight. Thus, hypohydration is defined clinically as a state in which the person loses more than 2% of weight per day due to this state. Dehydration (the next step of the clinical event) is considered when the loss of body water is above 3%.

Dehydration (mild to severe) manifests with symptoms such as dry mouth, fatigue, weakness, irritability, dizziness, nausea, headaches, constipation, dry skin and many others events. It can be caused by lack of water intake, but some infectious diseases (such as bacterial infections that cause vomiting and copious diarrhea), Hormonal imbalances (effects of chronic diabetes) and even organic problems (in the liver, kidneys, etc.) can lead to dehydration in the patient.

Sometimes avoiding dehydration is a matter of responsibility, while in other cases, it does not depend at all on the patient's disposition. Diabetic people who feel signs of dehydration or patients with acute intestinal infections, for example, should see an emergency doctor immediately. Sometimes intravenous fluid therapy is the only possible approach to avoid complications.

Resume

As you have seen, when asked "how much water should you drink a day?" You can answer multiple things, always taking into account the individual condition. In the first place, the intake depends on the age of the person, the sex and physiological state of it, because for For example, pregnant women should drink almost half a liter more daily than non-women. pregnant.

In general, it is estimated that about 2 liters of daily intake in women and 2.5 in men are enough, but the This figure can increase considerably more if we are talking about athletes or people with a very high body mass index. high. In any case, below 1.8 liters of intake, neuroendocrine responses begin to be shown. associated with dehydration, so this is a clear “cut-off point” of reference for all adults functional.

Bibliographic references:

Armstrong, L. E., & Johnson, E. C. (2018). Water intake, water balance, and the elusive daily water requirement. Nutrients, 10 (12), 1928.
Liska, D., Mah, E., Brisbois, T., Barrios, P. L., Baker, L. B., & Spriet, L. L. (2019). Narrative review of hydration and selected health outcomes in the general population. Nutrients, 11 (1), 70.
Andersson, B. (1978). Regulation of water intake. Physiological reviews, 58 (3), 582-582.
Hooton, T. M., Vecchio, M., Iroz, A., Tack, I., Dornic, Q., Seksek, I., & Lotan, Y. (2018). Effect of increased daily water intake in premenopausal women with recurrent urinary tract infections: a randomized clinical trial. JAMA internal medicine, 178 (11), 1509-1515.
Vargas-Garcia, E. J., Evans, C. AND. L., Prestwich, A., Sykes-Muskett, B. J., Hooson, J., & Cade, J. AND. (2017). Interventions to reduce consumption of sugar-sweetened beverages or increase water intake: evidence from a systematic review and meta-analysis. Obesity Reviews, 18 (11), 1350-1363.