Dehydration Myths: a Thirst for Knowledge

In the latest SwimCoachStu post, I have shared the work of a fellow coaching colleague, Coach J. Macpherson-Stewart (SwimCoachJM-S) – from Free Style Swimming Club, who debunks the prominent lore surrounding hydration in swimming and, more widely, sport. 

Over the weekend, at a local District Championship meet, I became very aware of the large volume of water bottles – filled with a variety of fluids and, of various sizes – that lay across the poolside. This is just one example of the many dogmatic practices which continue to exist within the swimming sphere. It is a belief-based habit which has the potential of having severely detrimental effects on an athlete’s performance and, health. SwimCoachJM-S continues…

I was once at a swimming presentation where the then Scottish Director of Coaching and Development implored his audience of coaches to “throw kids off the poolside” if they didn’t bring a water bottle with them. Continuing that afternoon on the poolside itself I watched as a member of that same audience stumbled over one of those very water bottles referred to earlier in the day and nearly came a cropper; I saw swimmers interrupting the focus on the set they were doing by the distraction of finding and then drinking from their water bottles; and then I watched the subsequent trail of the same swimmers leaving the poolside to go to the loo. Mmm…

The reason for my cynical “mmm…” is that all this flies in the face of science. With few exceptions, the prevailing requirement to bring a water bottle on to the poolside at each session and the encouragement swimmers then get to make sure they drink regularly from them is an unnecessary pre-occupation particularly knowing the kind of predictable consequences I witnessed that day.

The demand for every swimmer to bring a water bottle with them into the training pool has its roots in practices used in the running world which have in turn been driven by the commercial interests of the sports drinks industry. Tragically, the cost of this to the sport of running has been high indeed with over 1,600 reported cases and 12 deaths in the last 15 years (including two in separate London Marathons) from exercise-associated hyponatremia (EAH) – a condition brought about by drinking too much.

Despite these horrifying statistics, the mythology that supports the practice is still being widely repeated revolving as it does around three underlying beliefs: that dehydration will inevitably occur in all athletes who exercise for anything other than a short of amount time; that dehydration is the single most important factor explaining reduced performance levels during prolonged exercise; and that dehydration plays the central role in any instance of collapse and heatstroke there may be in endurance athletes. Since the development of the drink Gatorade in the late 1960’s, this faux science, for that is what it is, has been cynically, and very profitably, peddled by the sports drinks industry even though the wider published evidence paints a quite different picture. In his definitive text, Waterlogged*, Professor Tim Noakes has finally put the record straight to reveal the extent of this misinformation and to demonstrate quite unequivocally that the problem is a problem that would never have existed had there not been millions of dollars to be made. So what are the facts and how should they be interpreted specifically with respect to swimming training?

First of all, our unique evolution as a species has resulted in the human body being amazing well adapted to deal with periods of transient dehydration for up to as much as 8 hours. This evolutionary adaptation arose from the need of Homo sapiens to run long-distances in the heat to pursue and kill energy-rich animals for food. The result was a superior capacity to regulate body temperature when exercising in hot conditions which allowed our ancestors to even run antelope to ground in the heat of the middle of the day.

But let us return and consider each of those erroneous underlying beliefs in turn. Evidence shows that we carry a substantial fluid reserve that simply does not need acute replacement during exercise. This reserve takes the form firstly of metabolic water that is released in two ways: as a by-product of the cellular oxidation of carbohydrate, and as water molecules that are released from their chemical bond with stored glycogen as it is freed and used; and secondly as additional free fluid contained in the intestine. The water reserve referred to here is considerable and even conservative estimates suggest that this available reservoir is likely to easily be in excess of 2L in a mature adult. What this means is that during prolonged exercise (a 2 hour swimming training session, for example) losses of up to 2kg in body weight should not be immediately associated with dehydration and should not be expected to have any deleterious effects on performance. This reserve is easily replaced by drinking normally after the end of the session, often simply with the next meal.

With respect to the second underlying belief, evidence shows that dehydration has little effect on body temperature response during marathon running, for example, and drinking more does not necessarily contribute to better performance. There is no direct evidence that exercise performance is impaired in those who lose weight during exercise provided they drink to the dictates of thirst (I shall come back to this later).

Finally, there is no evidence that dehydration plays any role in the causation of heatstroke. Rather it appears to be increasingly the case that a complex combination of factors contribute to its occurrence the discussion of which goes beyond the scope of this article. Suffice it to say that it is associated with moderate to high intensity exercise of relatively short duration often undertaken in unkind environmental conditions.

What emerges from this is that the natural behaviour modification mechanism that has evolved, that of thirst, is a very sophisticated and perfectly adequate means through which we can maintain our levels of hydration within safe limits. When you reach a point of about 2% dehydration, you will begin to feel thirsty and start looking out for something to drink. Even if there’s nothing available, it’s not the end of the world – in a normal training or racing context there is plenty of time to rehydrate once you’ve finished. At this level of dehydration there are no untoward effects or dangers, and should your levels of hydration continue to decrease, so your desire for water will increase – in other words you will know when you absolutely need to stop to get a drink. The best advice is quite simply, drink according to your thirst – you need do no more than this.

If now, however, we turn our attention to how this applies to swimming training in particular, there are several additional factors that clearly distinguish it from most other popular sports and these need first, to be identified and then to be considered in the light of the above. Perhaps the most significant difference lies in the fact that swimmers are water-cooled. Typical pool temperatures of 28-30°C and the relatively high specific heat of water mean that convective/conductive heat transfer from a moving swimmer to the water is substantial, the faster the swimmer’s speed through the water the greater the heat loss. Body heat is actually lost some 25-30 times faster during swimming than during cycling or running at equivalent ambient temperatures, the greater the temperature gradient between the skin and environment, the greater the rate of that heat loss.

Secondly, the normal major avenue of human heat dissipation during exercise, sweating, is compromised in water since without evaporation no heat can be lost from the body via this mechanism though limited evaporation from wet body parts above the water will still take place. Sweating does certainly occur in the water during training but is unlikely to account for amounts anywhere near the 2% thirst threshold level mentioned above. Thirdly, the total muscle mass involved in swimming is less than in many other non-aquatic sports and consequently the metabolic heat produced is likely to be proportionately less. Fourthly the age and maturity of the swimmers involved needs to be taken into account: because of their larger surface area-to-body mass ratio, children lose heat more readily than adults in the same situation. The thickness of the subcutaneous fat layer also plays a significant role with the result that swimmers who are small and lean are likely to lose body heat more rapidly.

Finally, we need to add one more factor to the mix: immersion diuresis. Explained simply, a combination of cold and the external water pressure brings about a rapid increase in plasma volume which the body then attempts to correct by increasing urine production. The natural consequence of this is an increase in the frequency of trips to the loo though the effect has been shown to be significantly less in trained swimmers when compared with sedentary controls.

Well, dear reader, it may well be that it is your head that is now swimming! We are faced with a series of seemingly conflicting influences which ultimate combine to determine our individual levels of hydration during training. The key word here is individual. There is no fluid replacement protocol that is going to suit even one single member of any particular swimming squad. There is however, a single individual solution….

No-one at Free Style is expected to bring a water bottle with them to training and they certainly will not be thrown off the poolside if they don’t! Not that water bottles are banned, but once swimmers have a better understanding of the whole area, they find out for themselves what best suits their needs. An odd water bottle sometimes turns up (probably more as a result of parental concern) and we have no problem with that providing the swimmer only drinks from it if they become thirsty – sip-aholics are given short shrift! Should anyone during a session become thirsty then they are quite at liberty to return to the changing room at an appropriate point to get a drink. It has been a few years now since anyone has even done this. In fact the real problem for us, and I suspect for many other Scottish swimmers, the younger ones in particular, is actually quite the opposite from the non-existent problem that water bottles presume to address and that is, how to stay warm enough in the water to train effectively. The water cools us so effectively on occasions that body heat loss is greater than its heat production. Some of our leaner, mainly younger, swimmers have had to be encouraged to wear close-fitting thermal tops in training and on occasions we have been known to take warm showers between sets rather than end up uncomfortably cold in the water and consequently losing focus.

So where does that leave us then? During land training the correct advice is quite clear: listen to your body and only drink when you are thirsty from an available source of water which may or may not be your own water bottle, and at the same time understand as well that any other advice is the result of targeted manipulations by industries whose principal focus is their own commercial well-being and not necessarily yours. In swimming sessions leave your water bottle at home if you find that generally you do not get thirsty and in the unlikely event that you do, wait until you have finished the set you are doing and then take a few gulps from the nearest water fountain, or simply wait until the session is over – you’ll have plenty of time and opportunity to rehydrate then and you will come to no harm nor will your performance be affected in the interim. That’s it – the mythology has been exposed!



*Noakes, Tim (2012) Waterlogged: The Serious Problem of Overhydration in Endurance Sports. Champaign, Il: Human Kinetics.

Hang up those Question Marks

If Charles Darwin had chosen not to bother questioning the origins of life on Earth, the assumption might have remained that a supernatural being, ‘God’, shaped each living organism on our planet. Of course, this belief has been since ‘blown out of the water’ due to the debate which ensued from Darwin’s Theory (or rather, fact) of Evolution through Natural Selection.

Questioning one’s beliefs and, debating the opinions of others, has transformed civilisation over the past 500-years; it remains our societies most important tool in progressing peace, social justice, and, science – although, this is not an exhaustive list.

Debate can reveal to an ‘opponent’ the gaps which permeate their knowledge and, may, lead them to fill-in said gaps of ignorance with the appropriate information. Criticism of an idea can lead the originator to question the accuracy of their belief and, can assist in improving it – this may entail abandoning it all together. At the very least, engaging in an open discussion about a concept can improve the advocate’s articulation of it to others.

An important distinction to clarify is the difference between the critique of an idea and, an ad hominem attack on a person. The former involves scrutiny of a thought created in one’s mind – it has no feelings, it does not care how much you criticise or ridicule it. The latter involves fallaciously rebutting an opposing point by attacking the person, rather than debating the argument itself; even if the criticism of the individual is accurate, it has no relevance on whether the claim made is valid.

Ad hominem non-sequitur:
“You’re an ugly person. Therefore, you’re wrong” – the perceived beauty or, suggested lack of, has no bearing on whether the opponent’s argument is sound or not.

Scrutinise what the person is saying, not who is saying it.

That said, there are many unfortunate individuals and, organisations, in society who cannot bear to hear an opposing opinion – particularly, one which confronts a long-held view. They wish to remain in the safety of those who agree with them and, run away from, or verbally attack, anyone who dares trespass into their blissfully ignorant world.

What is the worst that could happen? You’re proven wrong, through the use of rational argument – based on evidence. As I see it, you are left with two options: 1. You can continue to deny, in the face of logic and the evidence, that the critic’s view is not accurate and continue to shamefully remain within your ‘safe zone’ or, 2. Explore the person’s claims through your own research, and, if the evidence appears to be valid, accept it, utilise it and, voilà, you have improved your view/opinion/model/strategy/, etc.

No one’s opinion is infallible; rejecting to hear an argument against your view suggests you believe it to be so.

This skill can also be applied to your own ideas. Indeed, it is often essential to dispute one’s own beliefs before challenging those of others. How do you know you’re right? How do I know what I know, except that I’ve always been taught it is so, and, I’ve never been told otherwise?

As the philosopher, Bertrand Russell once said, “In all affairs, it’s a healthy thing now and then to hang a question mark on the things you have long taken for granted.” When is the last time you hung up those question marks on your long-held beliefs?

Depressingly, in the world of swimming, the art of critical thinking and scientific scrutiny has not permeated many levels nor is it a skill employed by all swimming coaches – I would boldly claim that the majority do not. That said, this is not entirely the fault of coaches. Unfortunately, the pool of scientific studies into competitive swimming is relatively limited and, can be rather difficult to find – not to mention that most coaches aren’t taught how to review or analyse, research papers. This a major shortfall on the part of swimming organisations and sporting bodies – a topic I wish to write about in the near future. Instead, coaches often resort to belief-based practices: copying other club programmes and reading swimming ‘manuals’ which are themselves based on dogma. Only now, as the evidence sources (and reliability) increases, we see the debunking of many traditional training practices – training which YOU likely use in your club programmes.

There has been no better time to debate the training prescriptions of other coaches, research the science available (and learn how to do it properly!) and, start questioning your own beliefs. No matter how long you have coached for, no matter what your track record is, no matter how strongly you believe you are right, hang up those question marks!

Yours in Swimming,


Belief-based Vs Evidence-based Coaching

Hello to all my loyal and new readers,

Firstly, I would like to apologise for my recent absence, I have been busy experiencing a coaching epiphany of somewhat, of which I shall explain below; as well as being buried in my coursework. I would also like to mention that in this post, I will not be focusing on my series regarding the ‘Body in Swimming’, which I hope to return to soon. Instead I am going to try and convince some of you to rethink the way you coach – something I am finding to be a rather bold and daunting task.

Belief-based coaching

How much of what you coach or train swimmers by, do you know is based on scientific evidence? How did you come about concluding that what you are preaching is the right way of doing things?

Belief-based coaching includes the use of personal experiences, in that, it worked for some swimmers so it must, therefore, work for the rest (especially if those swimmers are your ‘top’ performing swimmers). This reasoning is concluded from trusting the knowledge you have acquired is reliable and, from using your interpretations of what you have read or heard from other coaches – in particular, ‘senior’ coaches. If a coach’s methods are shown to have not worked, then the blame will be placed on the athlete, i.e. the swimmer did something wrong to have produced the unexpected performance – rather than it being the coach’s training method to be at fault.

Unfortunately, belief-based coaching plagues many swimming organisations. The training is not based on accurate sports science or scientific evidence. It is, as Brent Rushall describes it, “subjective, biased, unstructured and mostly lacking in accountability.”

I am as guilty as most coaches for applying training methods based on my own interpretations e.g. Instruction from other coaches and following ‘what everyone else does’. This is not the way we should be coaching swimmers; instead, we should be applying methods of training based on evidence.

Evidence-based coaching

Evidence-based coaching relies on scientific studies and research as the basis for training. Rushall, a champion of evidence-based coaching, describes how coaching principles should be decided from: “several independent published scientific studies that report similar findings about human behaviour and therefore, deemed to be of substantive and reliable merit.”

Evidence-based coaching principles also allow for training effects to be reliably predicted, tested and verified – this is something which is uncommon in belief-based coaching. The latter camp often argues that it is tough to predict/measure certain effects/results and often hide behind this – blaming errors on uncontrollable factors rather than their practices.

A Vicious Cycle

The problem with many swimming organisations is that many refuse to receive evidence that may change their beliefs and structure. They then continue to feed incorrect knowledge to their various members (clubs/districts/regions) who then apply these belief-based methods.

These organisations often defend their beliefs by pointing out the successes of their athletes. However, as described before this is the problem with belief-based coaching – they wrongly support their methods using the observations of a few; again allowing for the spread of error throughout the organisation.

An important phrase to note is “the few.” Although belief-based methods may work for some swimmers, many athletes are left behind. They often conclude that the reason why some swimmers don’t perform well is simply that “they haven’t got it”; this is a ridiculous notion.


Many myths exist in swimming that are very long-standing and, are mainly due to individuals and organisations not scrutinising their knowledge. Up to 20% of what we learn today will be inaccurate in one years time; this percentage only increases with relation to years.

Ignorance should not be an excuse. It should be seen by all coaches and organisations as their responsibility to ensure that they evaluate their practices, seek to analyse their knowledge and use evidence as the basis of their sessions.

It is easy to appreciate why many individuals follow the beliefs of their organisations rather than attempt to dispute, analyse or evaluate it through the use of research. Extracting information from scientific research, studies and, papers, is a skill that many coaches do not currently possess. Those responsible for educating coaches should ensure that they 1) encourage the use of evidence-based coaching 2) provide coaches with the skills to apply it.

It’s Down to You

I hope a few of you that read this article will realise the dangers of belief-based coaching and will look to employ evidence-based.

It can be tough to take the jump from belief-based to evidence-based coaching; however, It is every coaches responsibility to ensure that they use evidence and science for choosing, developing, and justifying a training strategy in order to give swimmers the best possible chance of achieving their potential.

Yours in Swimming,


Please note SwimCoachStu posts are all of my  own opinion and are not necessarily endorsed by other clubs or organisations which I may be affiliated to

A brilliant resource for the evidence-based coaching approach can be found Swimming Science Journal

I would like to thank a fellow evidence-based champion for inspiring me to write this.


Rushall. B. S (October 2003) Coaching Development and the Second Law of Thermodynamics (or Belief-Based Versus Evidence-Based Coaching Development [Online] San Diego University