(Disclaimer: The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of the club)
Dehydration myths: a thirst for knowledge
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.