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The complexity of the obvious.

Updated: Nov 21, 2022

One of the most common questions i’m often asked is “How much water should I drink per day?” It might sound contre-intuitive, but in my opinion, proper estimation of daily hydration needs is probably as complex as individual macro and micro-nutrients requirements.

I decided to dedicate this particular blog post to hydration, after reading a really cool study just published in the journal Current Biology. For those interested in human body evolutionary changes, I'll summarise it in a couple of paragraphs.

The main finding of the study is that humans use 30-50% LESS water each day than other primate relatives. For reasons not fully understood, we have evolved as water conservers.

Perhaps it’s our body’s ability to conserve water that may have enabled our hunter-gatherer ancestors to venture farther from open water sources and pursue a physically demanding foraging strategy for search of food.

Researchers found that the average person processes around 3 litres, or 12 cups, of water each day. But a chimpanzee or gorilla goes through twice that much. It is really surprising because humans have 10 times as many sweat glands as chimpanzees do. So we have a greater sweating capacity, but our water turnover is much lower. And it seems that our reduced needs in water amount are due to diet, climate and/or behaviour changes over the course of human evolution.

Interestingly, among humans, physically active hunter gatherers subsisting on wild-food diets have a higher water turnover (3.8 L/day) compared to modern sedentary people (2.9 L/day). And that’s where it gets a little bit more tricky given the complexity and huge variability among people in total daily water intake. But i’ll get back to this point later.

For now, it seems logical that we have evolved to use reduced amounts of water compared to apes.

One hypothesis to explain the physiological change suggests that our body’s thirst response was re-tuned so that, overall, we crave less water per calorie compared with our ape relatives. Even as babies, long before our first solid food, the water-to-calories ratio of human breast milk is 25% less than the milks of other great apes (we have more concentrated/less dilute milk, but we have no idea about the differences between humans and apes in total volume of produced milk).

Another curious hypothesis is related to the structure of our nose! 👃

Fossil evidence suggests that, about 1.6 million years ago, humans started developing a more prominent nose, different from much flatter noses in apes.

Our nasal passages help conserve water by cooling and condensing the water vapour from exhaled air, turning it back into liquid on the inside of our nose where it can be reabsorbed. Having a nose that sticks out more may have helped early humans retain more moisture with each breath.

It’s all really fascinating and fun except that the average total water turnover is not that simple to estimate. And actually to what extent does the average estimate apply to various populations, and in particular to you?

In this specific study, researchers controlled for many variables, such as body weight, sex and calories burned per day. It's a great and smartly designed study!

But I'm always a little bit sceptical about extrapolating any study data to the general population, especially for body water regulation. The variability in water intake among people is really huge, i guarantee you would be surprised. And it’s not only influenced by sex, age and geographical location. You can see that the water balance homeostasis is subject to many variables.

Our common sense suggests that a young active man living in the South of Spain will most likely consume (and require) a different amount of liquid than an elderly woman living in Siberia.

I’m also not convinced that our current recommendations for adequate total water intake are really adequate. They are based largely on median water intakes from national population surveys and without solid physiological evidence linking total water intake to hydration biomarkers in urine, saliva, or blood. Needless to say, it remains very difficult to accurately establish individual water needs, which are influenced by factors including body size, activity level, dietary habits, metabolic rate, climate, and urine concentrating capacity.

Little is known about the hydration biomarkers in ‘average’ living conditions, when water losses are moderate and intake is the major determinant of water balance.

For example, in healthy young women, total water intake variability is large, ranging between <1.0 to >4.5 L for 24h.

Total sweat loss during sedentary work activity (e.g., 8h of computer programming in an air-conditioned environment) may amount to <0.2 L/24h, whereas the total sweat volume during a 164-km ultra distance cycling event often exceeds 9 L during a 9-h ride.

Well, it’s an extreme example, and sport hydration is a totally separate complex subject. But just imagine that in sport you also have to account for dry/cold/hot environment, intensity, duration, clothing, altitude, athletes acclimatisation status, sex and body weight, and potentially the hormonal status for women. And even for the same sport specific activity, there will be a significant variability among athletes. My point here is that “the average” doesn't really mean much.

Some parameters influencing our daily hydration needs are a bit more simple to estimate than others. For example, the body mass is an important one. Large individuals require a greater daily water intake than small individuals.

On the other hand, other parameters such as beverage osmolarity and the content of sodium chloride, protein, and/or energy are much more difficult to assess, but they have a big influence on the percentage of water retained.

But to fully appreciate the complexity of the water regulation process, let me show a list of factors that influence the release and blood concentration of our primary water-regulating hormone, called Arginine vasopressin (AVP).

We are bombarded by health magasins and dietary advice that we need to drink 1.5-2 L/ day. And I'll be the one encouraging you to be more mindful to your hydration. People are carrying bottles of water everywhere, and taking frequent sips from them and it’s fine, as long as you don't force it. But actually, no empirical research provides definitive answers and no universal consensus exists. As you have seen, the dynamic complexity of the water regulatory network, and inter-individual differences, are the primary reasons why widespread consensus regarding the daily water requirements has not been reached to this date.

It’s true that insufficient daily total water intake could contribute to a number of negative health outcomes. But how much water is optimum for you, who is now reading this post and asking this question, is really difficult to answer. To do so it’s important to have at least some details of your food composition, your sex, weight, age, physical activity and medical history. I don't have a very restrictive view on our daily liquid intake, especially for sedentary healthy adults living in a mild climate. And I would be surprised that evolutionary development left us with a chronic water deficit that has to be compensated by forcing fluid intake. I also believe that caffeinated beverages (don't forget teas) should count to daily fluid intake. And, to a lesser extent, the same may be true for mild alcoholic beverages, such as beer consumed in moderation.

From an evolutionary perspective, we are clearly moving toward a more chronic sedentary lifestyle, and we also have an increasing proportion of dry industrialised food in our diet. Would it mean that we will continue to evolve to be more physiologically adapted to conserve water?

Time will tell.

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