This week we have Harriet Carroll, an Honorary Research Fellow in the Clinical Research Centre at Lund University, who gained a PhD in hydration research at the University of Bath. So grab some water or (if you are British) a cup of tea… and let’s hear what she has to say!
Key question: Do I really need to drink 8 glasses of water a day?
You might have heard the recommendation that you should drink as many as 8 glasses of water every single day. Although that statement is not necessarily true (we will get into that) it still has its uses, as we are about to find out…
Guidelines to drink 8 glasses of water a day are mostly based on average fluid intakes (and a bit of theory) from large scale population studies. In these studies, researchers found that people who drink a lot and people who did not drink very much fluid had roughly the same plasma osmolality. So what is plasma osmolality? Plasma osmolality is a measure of how concentrated your blood is and you can think of this like diluting squash. If you put lots of water in your squash, you have an unconcentrated drink (akin to well-hydrated blood), but if you don’t put very much water in you have a very concentrated drink (or dehydrated blood). From these findings, researchers concluded that it did not matter how much people drink, as their osmolality stays roughly the same. However, we now know is that plasma osmolality remains stable until you are in very extreme circumstances. So perhaps it is not the best marker to base our recommendations for water intake on... Despite knowing this, the guidelines have not changed much and part of the reason for this is because hydration is hard to measure. For example, blood osmolality can remain stable across a range of fluid intakes, but you can see overnight the difference in your urine colour (a marker of urine concentration) after not drinking for 8 hours when sleeping. Your urine gets darker when you stop drinking simply because there is less “disposable” water, so instead of excreting the extra water, your kidneys reabsorb the water back into your blood. They do this through a hormone called arginine vasopressin (“AVP”). This hormone is really sensitive; even the taste of water can reduce your blood AVP levels! But (as with urine colour) because it is so dynamic, AVP levels do not always reflect hydration status; instead it often reflects how much you have recently drunk.
We could also look at health outcomes, but this again gets tricky... Although health can mean different things to different people, my focus is on metabolic health, which influences your risk of type 2 diabetes and cardiovascular diseases.
In terms of these outcomes, the evidence of hydration is mixed depending on the type of study that is conducted. First we have observational studies. These track people’s behaviour, including how much they drink, and correlate these behaviours with outcomes like type 2 diabetes risk. Most of these studies show that people who drink more water have better health outcomes. These studies do not provide causal evidence though, and so the findings could also be explained by other factors. For example, people who drink more water, often also smoke less and do more sport.
Secondly, we have controlled studies. These are causal studies because everything else is controlled and just fluid intake is manipulated (lots of water compared to not very much water). Overall, these studies show that drinking water does not seem to impact metabolic health, in the short term (typically a few days). A summary of the research can be found here. A recent study which combined all relevant controlled studies also confirms no effect on health markers overall. Together, these findings suggest that in a natural setting, people who drink more water often have healthier lifestyles. So whilst water intake may not causally influence health, it may be a marker of other healthful behaviors.
Take home message: We do not have enough evidence yet to make strong causal claims about the optimal amount of fluid we should be drinking for health. However drinking a bit more each day might be healthful and is unlikely to cause harm. Therefore drinking 8 glasses of water (or any non-alcoholic fluid) a day seems like a reasonable target.