One of the most popular rules in popular health culture is undoubtedly the amount of water you should drink per day. An amount that is located in eight glasses a day or what is the same: the immovable figure of two liters. We see it in fitness applications, in influencers’ advice and we hear it repeated like a mantra, but the reality is that there is quite a myth behind this.
We are different people. A very common phrase within medicine is precisely “there are no equal people”, and not only because of the external physique, but because of everything that is inside. This forces medicine to focus towards a more individualized idea in its medical advice that must be given, including nutrition or water consumption.
This forces us to have to personalize the amount of water that each person should consume, because a person who is 2 meters tall and weighs 100 kg with a large amount of muscle is not the same as an elderly person who has a much slower metabolism. Logically, the two liters of water mantra cannot be established here.
The origin of the error. To understand why we drink (or think we should drink) so much, you have to travel back to 1945. According to the key review by Dr. Heinz Valtin in the American Journal of Physiology 2002, the myth of the “8×8” rule, that is, 8 8-ounce glasses to have almost 2 liters of water, probably comes from a misinterpretation of a guide from the Food and Nutrition Board.
A guide that indicated that it was always recommended to have an adequate intake of 2.5 liters of fluids per day. But most people ignored the accompanying sentence that said, “most of this amount is in prepared foods.”
What the institutions say. So the question is quite clear: how much should we drink per day? In this case there are different official figures, but they have fine print. One of the examples is the 2010 European Food Safety Authority Panel that established adequate water intake at 2 liters per day for women and 2.5 liters per day for men.
But here’s the key: the EFSA specifies that this refers to total water, that is, the sum of drinks plus food. And there are many dishes that have a large amount of water, such as soup, although fruits also have a lot of water inside.
Even in the United States. If we turn to the recommendations made by the US Institute of Medicine (IOM) in 2005, it suggests that the total water intake should be 2.7 liters per day for women and 3.7 liters per day for men. But again, it includes all the dietary intake that is a day and not just glasses of tap water.
The latest science. If we come more to the present, we also have scientific studies that have sought to dismantle a universal fixed figure set at two liters per day. One of the most important is the one published in Science in 2022 that used isotopes to measure water exchange in 5,604 people, and that showed that real needs vary enormously between people.
One of the conclusions they addressed was that for most people in temperate climates and with sedentary lives, the real water intake needs are between 1.5 and 1.8 liters per day, far from the demands of wellness marketing.
And it is reinforced. It is not a study that is isolated, but also in 2022 the magazine Scientific Reportspublished research that reinforced this idea: they predict necessary beverage intakes of about 1.6 L for women and 2.0 L for men, always depending on factors such as age, sex and body composition.
Is more water better? One of the most repeated arguments by proponents of hyperhydration is that we should drink “before we are thirsty.” Modern physiology, supported by scientific reviews and analysis of urinary osmolarity, refutes this fear we may have.
Specifically, the human body has an extremely sensitive osmoregulation system. When the concentration of solutes in the blood increases by just 2%, well below clinical dehydration, the brain already activates the sensation of being thirsty and releases the hormone necessary to begin conserving water so that it does not ‘leave’ in the urine.
There are exceptions. Unless you are an elderly person (whose thirst sensation is attenuated) or a high-performance athlete in the midst of intense effort, drinking when thirsty is the most accurate and scientifically validated strategy for maintaining water balance.
When you should drink more water. That the “mandatory two liters” are a myth does not mean that water is not logically vital. The most recent systematic reviews and other clinical means confirm that increasing water intake has clear therapeutic benefits in very specific cases that are not universal. These can be the following:
- Having a kidney stone: here the “more, the better” applies since increasing urinary flow is key to preventing the recurrence of this disease.
- Urinary infections: a problem that mainly affects women, and that requires ‘overhydration’ to reduce the risk of new episodes.
- Weight loss: Although the evidence is mixed, drinking water may help with satiety and, marginally, energy expenditure. Although it is not a magic solution against obesity.
More common sense. The obsession with two liters is a perfect example of how an old and misinterpreted scientific recommendation becomes a cultural dogma. The reality, supported by decades of studies from Valtin to the latest isotopic analyses, is that we are not machines that need a fixed tank filling every 24 hours.
In this way, our body’s water needs are dynamic. Water needs are dynamic. If you eat a lot of fruits and vegetables, work in an air-conditioned office, and don’t run marathons every day, forcing yourself to drink 2 liters of extra water will probably only do one thing: interrupting your work to go to the bathroom more times.
The situation. In this way we can understand that a situation such as doing a large amount of physical exercise where a lot of water is lost through sweating logically requires greater fluid replacement. But if we are talking about a sedentary person who does not do much on a daily basis, logically it will not always be necessary to have a large intake of water.
Images | Janosch Lino
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