Is there a link between dehydration and our experience of pain?

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In this report I talk with Dr Toby Mundel, senior lecturer in sport and exercise science at Massey University in New Zealand. Toby and his colleagues have been looking at the link between levels of hydration and how it affects pain perception. And this is important because chronic pain is a huge health issue with around about one in five, one in six people suffering from some form of chronic pain. This has big implications for loss of productivity and the medical costs associated with that. So, have a read or listen to the conversation between myself and Toby about some of the research that he and the team that he's been involved with have been doing, looking at the relationship between levels of hydration and our pain experience.

Toby Mundel: This area of research follows on of my research programs which is the hydration platform and I've got to admit that ever since being a teenager I've been very sensitive to a lot of things and one of those things is, for example, hydration, how well hydrated you are or not and the consequences of being either over-hydrated or well-hydrated or under-hydrated and what effect that has, not only physically, but also psychologically because typically the effects of hydration, dehydration in particular, tend to get seen more from a cognitive psychological perspective and only later do the physical consequences come about. So, really what I wanted to do, as most of my other research has been exercise and physical physiological, I wanted to extend this hydration research into a more psychological, cognitive domain. Because that's typically where most people would see an effect.

SJ: So, what you're looking at here is the effect that hydration levels have upon people's perception and experience of pain. Both acute pain but also longer lasting, chronic pain.

TM: Yeah, that's right. We wanted to look at how the way that most people would become dehydrated, which is by not drinking enough, how that reflected on their pain experience, like you just said.

SJ: This is interesting because we hear a lot about people not drinking enough and recommendations on how much we should drink during a day in order to keep our wellbeing, our attention, our general sense of feeling okay with the world. But this is a very specific application of thinking about the impacts of adequate hydration.

TM: Yeah it is, you're right. I think since ... for decades, there's been a constant media message and so-called experts about how much we should be drinking and I guess the health costs and consequences of not drinking enough. I think at this point it's really important to mention the fact that there isn't some kind of epidemic of chronic dehydration. It's not that people are unhealthy with not drinking enough but because there's been such an amount of research done on hydration, largely because of course, you can sell products, you can sell water, you can sell lots of other drinks ... that we now know the consequences of either being well-hydrated or being hypo-hydrated.

SJ: So, it'd be interesting to see and track the number of papers that have been published correlated to the popularity of bottled water being sold as opposed to free or, at least, very cheap water being drunk out of the tap.

TM: Yeah, that's right and not only that but I think another area which really accelerated the research on hydration or dehydration was sports drinks. And now we see in society, we see lots of people with a standard sports drink or energy drink in their hand and we think, is that what you should be having?

SJ: Yeah, I get into this debate quite a lot with runners. I run quite a lot and often we have quite a discussion around, well, when is it that you should be drinking a sports drink? When would it be useful in terms of performance and recovery? As opposed to just sticking to water. And really, water's pretty okay until you get to a certain level of distance or time that you've been running.

TM: Yeah, I always have to be careful what I say because how the body responds to exercise is my passion and my career but in general I think I like to stick with, in healthy people the body's very clever and it has withstood, over time, a lot of stress and strain and hydration's one of those. The body's made up of 70% water, we have a lot of water in our body, therefore even if we become dehydrated we've got a huge reserve to draw on as long as it's only transient.

SJ: So, we have this interesting intersection here with this idea of hydration and how the body seems to be able to adapt, so long as it's a transient interruption of flow. But generally, as long as our hydration levels are okay, our body seems to be coping okay. But then, we have very specific stressors that we know that we're exposed to in our environment and the two big ones here are musculoskeletal challenges that we have, that we face through our daily lives or perhaps in our occupations, through sitting poorly or our workstation is not being setup correctly or other strains. If you've got a more active job, the way that you carry yourself, the way that you lift and interact in your environment. And then, we've also got more general stresses and strains that have been identified as real challenges for people as they go through their lives. So, we have these two big ... we know that these are big exposures that people come across, in terms of their risk of developing issues with their wellbeing and we have levels of hydration superimposed on top of that.

TM: Yeah and this is really where I hate using the word novel but this is where this area of research is quite new and hasn't covered a large amount because a lot of the hydration literature has looked at, what are the cognitive consequences, what are the physical consequences. So, in other words, what about decision-making, what about mood and what about physical performance? Can you exercise better or ... whereas not a lot has been looked at in terms of something like pain. And the other thing about the hydration issue, of course, is that it's something that everyone does, should do and therefore, in one sense, any kind of research onto hydration could speak to anyone, whether they're healthy, young, old, clinical population because it's something we typically do throughout the day or everyday. So, I was really interested to see that there haven't actually been a lot of research on hydration and it's links to pain but what there was, there was sort of an intermediary whereby hydration, in particular dehydration, affects many of the mediators of things like musculoskeletal pain or pain that we have in chronic conditions.

SJ: And this surprised me when I came across your research, Toby, because chronic pain affects a really large proportion of the population. Could you talk a little bit about the numbers and the proportions we're talking about here and the types of pain that people are experiencing?

TM: Sure, so chronic pain, in terms of definition, is pain that lasts longer than six months and the prevalence is between about one in five and one in six worldwide. So, that's 20% of the population, potentially, that have some sort of persistent pain. The causes of which, of course, are hugely varied and could be things like arthritis, cancer, like you said, musculoskeletal disorders, migraines. We also have pain that's more acute in the sense that many of us injure ourselves, we go through surgery that's ... pre and post surgery, we have some form of pain. So, like you say, the prevalence of pain is quite large and then when you marry that up with what people should be doing in terms of drinking, we're talking to and about a large number of people worldwide.

SJ: Of course, when you were talking there I was thinking about some of the recommendations that we're given when we have to undergo something like surgery and one of those recommendations is to limit our food intake and often our fluids as well. So, I'm curious as to how that might impact things like how we sense pain or recovery from painful issues when we are undergoing or recovering from surgery.

TM: That's right, there's lots of instances where our normal food and fluid patterns are disrupted. One of those is surgery, like you mentioned whereby we're told nil by mouth and, of course, that's for medical reasons, both before and after surgery.

Other instances, of course, are any disruption of our fluids. If, for example, there are many occupations whereby out of convenience you might not want to go to the toilet often. So, I typically bring up occupations like if you're a pilot, anything where large amounts of concentration are or if you're in an environment that's very hot and you have to sweat a lot or very dry, again, like being in an airline cabin. But there are lots of occupations where taking on fluid and food is not something that we necessarily think are difficult because for a lot of us that sit at a desk and can eat and drink roughly when we want, go to the toilet when we want, it's not problem. Yes, it starts to bring up problems and issues in people and populations that you haven't actually thought about because we think about eating and drinking as being something that all of us have access to and shouldn't really be a problem but actually there are quite a number of people who are recommended, told not to drink or for whom drinking might not be something they want to do. I know lots of family members whereby going to the toilet is a big burden to them and they might therefore prevent themselves from drinking so as not to have to go to the toilet a lot. And, of course, that brings about dehydration and the consequences of it.

SJ: Yes, and as you're talking now, often people, if they're a parent, they're managing their children's fluid intake as well. And often they may choose to minimize that fluid intake because it's not convenient for that child to be heading towards the toilet for one reason or another because of all the different things parents have to manage. That's interesting as well. I'm thinking about all those self management things that are really implicit and we may not necessarily be fully conscious of.

TM: Yeah, that's right. This is again, one of the areas ... when people ask me about, what do you do, what's you're area of research? I generally say, well, things that don't really matter too much in life, whereas this, saying to someone, you should or shouldn't drink and what kind of drinking and what are the consequences of doing that, like I said, it speaks to a lot of people. Of course, we have to look at different populations differently. So, like you said for instance, the young, the elderly should be treated slightly different to healthy adults and in particular there are differences ... sex differences, there are also differences between healthy and diseased populations.

SJ: So Toby, let's move on to how you started tackling finding out more about this link between hydration levels and how pain is experienced.

TM: Okay, well, we're part of a group so we have got physiologists on the one hand, looking at the body, we've got psychologists on the other hand, looking at the brain, as it were. What we wanted to do was really have a look at, first and foremost, how does hydration and pain normally come about? Now, hydration and dehydration is quite simple because experimentally dehydration is usually brought about by giving people diuretics so that they urinate too much by getting them to sweat in heat or by exercising them and any combination of the three. And the thing is that this is something that doesn't actually happen in real life to a lot of people much. Therefore, we wanted to say, actually dehydration's often brought about by people simply not drinking enough, they're eating fine, they're performing their normal daily routine. So, that's the first thing, we wanted to actually mimic the way in which people became dehydrated. Unfortunately, pain is slightly more difficult because there are lots of different types of pain, ways of bringing it about and also different populations experience different kinds of pain. I guess one of the things is that pain doesn't have to be associated with some kind of damage and the cold pressor test has been previously shown to be the best surrogate of chronic pain that disease states might bring about. So, the cold pressor test is simply putting your feet or hands into cold water and that brings about a very painful, cold stimulus. So, that was the first thing is that we wanted to make sure that our interventions, pain and hydration or dehydration, were relevant to the vast majority of the people and weren't just some experimental paradigm.

SJ: Sure. So, you've got the cold pressor task which is widely used and you've described it, so you've got a good way of triangulating your results to the wider literature about how pain is experienced and perceived but you've also got this quite ecologically valid way of trying to mimic or reproduce how people become dehydrated in their daily lives and you're trying to put those two together.

TM: Yeah, that's right and what we wanted to also do is, again, previous research ... there's been so much research done on hydration and dehydration but previous research hadn't, for example, controlled people's diet, caffeine intake and sleep. And what we wanted to do, all known things that effect pain and also affect. And so, we wanted to try and as much as possible cross a very well experimentally controlled study with an ecologically valid study. Yeah, that's where we tried to keep anything that would affect pain constant and then we wanted to induce our intervention by the way in which most people become dehydrated, which is just not drinking enough.  Our dehydration intervention was just getting them to have no fluids or high water content food for a total of 24 hours. Now, in practice that was actually quite simple because half of that 24 hours is, of course, when we're sleeping which is when most people don't take on fluid or food. And so, in reality it was a 12 hour abstention of fluid and high water content foods. So, it's like drinking very little throughout the course of a working day.

SJ: So, one you then deprived these people of water and you looked at their reactions to this cold pressor task and you'd controlled for a many of the other variables that are linked to how people experience pain, what did you find in terms of the differences between those people who had had enough water and those people who had not and how they experience pain?

TM: Okay, so from a physical point of view, the physical responses of our participants as soon as they put their feet into ice-cold water, they responded very similarly to all the other studies that have been done when you give someone a painful stimulus. So, we saw their blood pressure and their heart rate skyrocket and they had very obvious signs of painful experience. So, in the physical study that looked at the physiological responses, the only really noteworthy result was the fact that peoples brains blood flow was slightly altered in that blood flow was reduced compared to their normally hydrated trial. So, the only real impact of this was because the cold pressor test is used as a clinical, often, a clinical measure, not so much for pain but for testing peoples cardiovascular responses, that arm of the study just said, well, if you're gonna be using the cold pressor test then we suggest you standardize peoples hydration going into the test because actually their hydration state, whether they're well-hydrated or dehydrated, might influence the results of the cold pressor in terms of cardiovascular responses. So, that was the physical side of things. More from an affect and a psychological point of view, we found that, again unsurprisingly, when you've got your limbs in very cold water, this is between nought and three degrees, peoples pain responses were quite high. But we found that the more dehydrated they became that  predicted their actual pain response. In essence, when people were dehydrated and also the people who became even more dehydrated, their pain sensitivities was much higher.

SJ: Okay, so that's interesting. I think you also measured another thing didn't you? It was this idea of how much people worry about pain.

TM: That's right yeah, in between what we found, there's been one or two studies before looking at catastrophization, catastophizing is really how much you amplify or worry. And we had hypothesized that by increasing your anxiety and worrying about things, that might be how the pain is increased or decreased as it were. And we found that that was a very good proxy in the sense that catestrophization was higher when people came in dehydrated. So, there does appear to be a good link between ... well, previously there'd been a link between anxiety and depression and hydration as are in pain. So, we took that a step further and said, well, not so much looking at general anxiety and depression but what about a specific catastrophization, a specific over-worrying and that seemed to be true.

People, when they were dehydrated, they worried, ruminated more and this also led to an increased pain stimulus or sensitivity.

SJ: That's an interesting finding. The fact that these people who are dehydrated tended to worry about pain more. Do you think that there's a causal link there? Or what's the relationship there between those two variables?

TM: Well, this is where it gets a little bit tricky because there does seem to have been research looking into the structural brain responses of dehydration and pain and I would like to say that it is a causal one, however, these were state conditions and one of the problems is that we had a relatively even split of people who were, I don't want to call them habitually, but who even at baseline before intervention, they seemed to be quite high catastrophizers or worriers. Versus, there were some that were lower.

And so, when we looked at the data more closely and we've got another paper that spoke to this. It was those that already had high worrying catastrophization who seemed to be affected more by the hydration intervention. So, in a way that does speak to it being causal and I don't want to say genetic but it's a pre-existing thing whereby people are probably naturally bigger worriers and therefore if you affect their behavior in any way in their control, that might lead to a detriment.

SJ: So, this catastrophizing, this pain worrying, seems to be more of a trait or a personality characteristic that people bring along with them to this interaction between the dehydration state and going through a stressful, painful experience that seems to amplify their pain experience. Is that one way of seeing this?

TM: Yeah, I'd say that's a very good way of summarizing it. What we hope to do in the next phase is exactly look at that, whether this is a trait thing whereby if you were to chronically change something, so for example, if you were to, over a number of weeks, have similar interventions, would those people who naturally worry more display consistent responses if you changed their hydration, for example, versus people who have naturally lower worrying and catastrophization.

SJ: So, you're bringing me onto the crux here, Toby. Who should care about this? 'Cause I could think of a few implications here around possible interventions but who should care about this and what are the implications of what it is that this line of research that you're engaged in is pointing you towards?

TM: The obvious thing is basically, anyone that has pain, preferably chronic, but whereby there are some kind of interventions that are already being done, cognitive behavioral therapy or analgesic, people taking pain killers. So, that could ... the painkiller speaks to anyone who is trying to treat pain but cognitive behavioral therapy ... anyone with chronic pain pre, post surgery, that has some kind of condition where they're managing the pain, clinically in particular, if there is some kind of practice in treatment to reduce someone's pain, if they are under-hydrated, if they're badly hydrated, that may be confounding some of their pain issues.

So, to answer your question, really anyone that seems to be experiencing persistent pain and wants to improve it or at least wants to have any kind of treatment have a better effect, should potentially look at not being dehydrated but actually in drinking enough so that the pain is reduced, in terms of the results of our study.

SJ: Of course, that brings up the question of, how much is enough? And that's really difficult but do you have  ... are there any recommendations around how people can tell if they are adequately hydrated or not?

TM: So, the first thing ... by answering your question the first thing I want to say is, our studies specifically looked at when people came in normally hydrated versus dehydrated. Now, there was one interesting finding which is that normally people are actually often a little bit dehydrated. Chronically, a large part of the population doesn't drink, I don't want to say, doesn't drink enough, but we spend a large part of our day not drinking enough. We tend to then, compensate.

So, what we haven't looked at here is, what happens when you really hydrate people well. Effectively, if you hyper or over-hydrate them ... now, that brings about a problem in itself in that we've done a bit of a pilot study and people are creatures of habit and if, generally speaking, they normally drink a certain amount, if you try and increase that, I don't want to say, force water down their throat but if you give them too much fluid, they might actually experience that as being just as negative because of some of the consequences.

For instance, having to go to the toilet more or feeling bloated. So, that's a really important thing whereby what we haven't tested is, does being very hydrated, well-hydrated, have a positive or opposite effect of being dehydrated?

To answer your other question, there are very simple ways in which people can measure their hydration. One of the simplest ways is just take your body weight. One of the best was of looking at how your diet, that includes food and fluid, goes is first thing out of bed, hop on a pair of scales. If you do that three of four days a week, what you'll see is a general rolling average. And as long as that doesn't change too much or day on day, week on week it stays the same, then you're probably reasonably hydrated.

Another very simple way which, of course, most people don't necessarily want to do but it's the obvious way is just looking at your pee, looking at your urine. Depending on how concentrated and yellow it is, that tends to suggest being a bit more dehydrated, like with our first thing in the morning, when we have had a night of not drinking versus when people's urine is relatively clear, it also doesn't smell much, that tends to suggest that people are better hydrated. So, there's some very simple ways that you can look and measure your own hydration state.

SJ: Thanks so much Toby. I was wondering if there is anything else that you wanted to add about the possible implications in terms of cost savings, 'cause that's one of the other big implications for chronic pain is that it actually costs quite a lot to manage, not just for the individual but also for the society who's trying to provide the health system to care for that population.

TM: Recent studies have shown that the cost per person with chronic pain is about $11,000 per year. So, when we're talking about something simple like drinking a little bit more, we could be saving society and individual people quite a lot of money just by simply having them drink a little bit more or being well-hydrated.