How much water to drink per day?

A summary of the amounts of water to drink to maintain a good state of health.

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 at all.

On average, humans lose about 2.3 liters of water per day, the value of which is divided into the following values: urine disposes of 1-1.5 liters, sweat 0.3 to 0.9 (depending on physical activity), respiration about 0.4 liters and feces 0.1-0.2 liters. Even exhaled air is essential for maintaining the body's water balance, since we lose about 350 milliliters of this fluid with exhalation, 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 this 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, based on science and recent research.

The body and water

The 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 fluid, but in dilute solutions (such as fluid media within the human body), both terms can be used interchangeably with comfort.

Persons 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 any given time.

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 encompasses 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 glucose is considered correct when the value is less than 5.6 mmol/L: beyond this, prediabetes/diabetes is considered. On the other hand, urea, the end product of protein metabolism, is present 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 must remember that 80% of blood is water. The more water available in the body, the more diluted a sample can be, within the limits of normality.

How much water to drink daily?

On the basis of this extensive but necessary introduction, it should be noted that what is really important is the osmotic balance in our body, beyond arbitrary values in liters of water, when talking about water intake.. A person can lose up to 1% of their body weight in sweat during demanding exercise, so naturally, their water intake should be much higher than someone who has not left the couch all day.

However, there are a number of average 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 average values are as follows:

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

As you can see, the values vary greatly depending on the age group analyzed and also on the government 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 slightly less, 2 to 2.5 liters. This distinction is purely arbitrary, simply based on the fact that men tend to have a slightly higher muscle mass.

The brain and its role in body hydration.

Studies such as Water Intake, Water Balance, and the Elusive Daily Water Requirement (Nutrients Magazine) have attempted to accurately quantify the importance of these figures at the neuroendocrine level. The brain is constantly acting to maintain the internal homeostasis of the organism, so it is to be expected that the plasma concentration of hormones and neurotransmitters varies according to the physiological needs at any given time..

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, is produced in the hypothalamus and is stored and released through the neurohypophysis. It causes vasoconstriction, renal reabsorption of water and acts as an antipyretic/analgesic, among other things.

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

In addition, 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 the neuroendocrine defense methods against dehydration do not appear.

How do you know if you are drinking too little water?

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

Dehydration (mild to severe) is manifested by symptoms such as dry mouth, fatigue, weakness, irritability, dizziness, nausea, headaches, constipation, dry skin and many other events. It can be caused by lack of water intake, but some infectious pathologies (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.. Diabetics who feel signs of dehydration or patients with acute intestinal infections, for example, should see an emergency physician immediately. Sometimes, intravenous fluid therapy is the only possible approach to avoid complications.

Summary

As you have seen, the question "how much water to drink per day?" can be answered in many ways, always taking into account the individual condition. Firstly, intake depends on the person's age, sex and physiological state, since, for example, pregnant women should drink almost half a liter more per day than non-pregnant women.

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

Referencias bibliográficas:

• 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. E. L., Prestwich, A., Sykes‐Muskett, B. J., Hooson, J., & Cade, J. E. (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.