Polski
At the end of November last year, news circulated in the media that counting the glasses of water drunk each day had lost its meaning. Such assumptions were to be based on newly published research results. However, is this claim supported by the content of the cited article? Anyone can check for themselves in the source text. However, since access to it requires a subscription, I will try to briefly present the theses to those who do not have one or do not want to wade through several pages of scientific language and data.
Water – does everyone need to drink 2 liters a day?
Indeed, in the article’s conclusion, the authors write that the recommendation that everyone drink 2 liters (or 8 ounces) of water daily, regardless of a number of accompanying conditions, is not supported by the results of their analysis. Admittedly, such guidelines can be found in many guidebooks (sometimes even as “at least 2 liters”), but it’s probably easy to guess that this is an averaged amount, and calculating the actual requirement is a much more complicated process that depends on many factors.
And it is the analysis of these factors that the article is devoted to. The list of authors includes more than 90 names, which is not unprecedented in studies that use a lot of data from different parts of the world. Information from the International Atomic Energy Agency’s International Doubly Labeled Water database was looked at. There are statistics in it of people who were given water containing several percent heavy water (with deuterium, a heavier isotope of hydrogen) or doubly heavy water (in addition with the oxygen isotope 18O), and then their circulation was analyzed. Data such as age, gender, height, weight, body fat mass, physical activity, etc. are included. Derived indicators, such as BMI and the proportion of water in body weight, were also analyzed. Living conditions (temperature, humidity, wind, altitude, etc.) were noted. For reliable results, data were collected on 5604 people (3729 women and 1875 men) from 23 countries with different levels of economic development. There were no participants from Poland (the closest country to us is Germany). However, the proportion was not maintained, as 69% of those surveyed are from the United States. Only 40 people classified as gatherer-hunter and a similar number from the mixed gatherer-hunter-agricultural group were observed. The age of the subjects ranges from 8 days to 96 years.
One of the main parameters calculated was the percentage of water in body weight. In the first months of life, it’s approx. 60% in both sexes, while in 60-year-olds approx. 50% in men and 42% in women. This discrepancy is due to the difference in the averaged proportion of body fat, which contains relatively little water. Muscle tissue, on the contrary, especially its intercellular space.
Water exchange ratio, or how much water to whom?
The main parameter studied was the exchange rate (water turnover), which is the amount of water that passes through the body during the day. Due to the homeostatic maintenance of constant water content in the body, the amount consumed equals the amount excreted.
This ratio also correlates with non-fat mass, so it is generally higher in men, exceeding 4 liters per day (l/d) on average between the ages of 20 and 40. In women, the highest values are an average of 3 l/d and apply to the 25-65 age group. The differences disappear in old age. In children, statistical disparities between boys and girls also occur, although they are not as great. For newborns, the ratio is the lowest, but reaches 28.3% of total body water weight, while in adults it is only 9.9%.
When children and people over 60 were excluded from part of the analysis, the correlations of the water exchange rate with the variables came out stronger. Rather unsurprisingly, this parameter is higher in people who are more physically active or live in warmer environments. Both promote sweating, and the loss needs to be replenished. When analyzing the effect of air temperature on the water exchange coefficient, a proportional increase in values was noticeable if it was warmer than10°C. In negative temperatures, the ratio is slightly higher than around 0°C. Latitude was also among the factors analyzed. The effect of temperature corresponds to the geographical distribution, as while it decreases as you move away from the equator, it begins to increase again in the polar regions. A correlation was found between the two variables, which is essentially trivial. There is also a weaker correlation with relative humidity and altitude, but no correlation was found with wind conditions. The exchange rate increases its value in summer.
A factor that raises the value of the ratio is pregnancy and lactation. The largest increase occurs in the third trimester. Among the record holders, whose exchanges exceeded the value of 7 liters per day, were two pregnant women. In addition, such values were recorded by several sportswomen, but also by women who do not stand out in any way, but who took part in the survey during hot weather. Nevertheless, in general it is men who have a higher water exchange rate than women. The record holders exceeded the value of 10 liters. They are athletes or Indians living in a traditional way in the Ecuadorian part of the Amazon. In men, records are not associated with a large BMI, while in women they often are.
The formula for water exchange, or how to count how much to drink.
Combinations of various lifestyle factors mean that the water exchange rate is generally lower in wealthier, more heavily urbanized societies, as measured by the Human Development Index (HDI). People who do sedentary work in air-conditioned rooms drink less water. In richer societies in general, the variability of this coefficient and its dependence on external conditions is weaker.
Taking the results into account, the researchers proposed a mathematical formula for the water exchange ratio (WT):
WT (ml/d) = [1076 × ustandaryzowany poziom aktywności fizycznej] + [14,34 × masa ciała (kg)] + [374,9 × płeć] + [5,823 × wilgotność (%)] + [1070 × wskaźnik usportowienia] +[104,6 × HDI] + [0.4726 × altitude (m)] – [0.4726 × altitude (m)]. [0,3529 × wiek (l)2] + [24,78 × wiek (l)] + [1,865 × temperatura (°C)2] – [19,66 × temperatura (°C)] – 713,1
To count it, you need to know the relevant coefficients (for example, for gender, it takes two values: women, as drinking less water, receive a value of 0, and men 1). It is expected to explain roughly half of the variability in performance. WT shows that an average twenty-year-old citizen of a highly developed country, living at sea level, with an average air temperature of 10°C, humidity of 50%, weighing 60 kg and not engaged in competitive sports has a rate of 2.7 l/d, while for a twenty-year-old, seventy-kilogram athlete, living in a highly developed country, at an altitude of 2000 meters above sea level, with an average air temperature of approx. 30°C and humidity up to 90% WT is 7.3 l/d.
Of course, these calculations can hardly be taken as strict recommendations. However, they show that simple and universal advice like “2 liters a day” is too simplistic and too general to be put into practice. The entire survey is of very high descriptive value, but not exactly a source of guidance. It examined how much different people drink and how much they excrete. However, whether these amounts were appropriate for their health needs was not investigated. Even assuming that healthy people were studied, it is not clear whether their habits are worth following. It also assumes that the people taking part in the study had unlimited access to water and drank as much as they needed. However, this is not obvious in the real world. Or do athletes drink too much because their type of activity makes them pay more attention to such issues? Or maybe people in rich countries drink less not just because of climate stability, but for whatever other cultural reasons. These questions are not answered by the authors of the article, as they themselves report.
