Hydration for athletes: a controversial topic! – Andy Blow

A lot of controversy still surrounds the optimal hydration for athletes. Andy Blow takes us through the history of this conflated science and provides a more balanced perspective, highlighting the need for individualisation.

We humans are a sweaty species. We are, in fact, the sweatiest animals on earth. Although many other mammals also have the ability to perspire, none do it quite as brilliantly as we do. One of the most convincing theories to explain just why it is we sweat so much is that this ability allowed our African ancestors to move, hunt and gather during the hottest part of the day – a niche that other predators could not exploit primarily due to the inherent risk of overheating. It worked, so the theory goes, because sweating is a far more effective cooling mechanism than panting (the primary method of heat dissipation favoured by most other mammals) and, as a result, we could still function in temperatures that forced predators and other competitors to laze in the shade.

The downside of sweating, of course, is that it results in relatively large amounts of fluid and electrolyte loss from the body, so in places where other mammals can exist for days at a time without drinking significant amounts, humans need to consume fluid and salt regularly in pretty large quantities to survive and thrive, especially in hot climates.

Whilst very few humans still actually hunt and gather in the midday sun, our ability to sweat is an evolutionary legacy that persists and gives us the potential to be very good endurance athletes today. When exercising vigorously, a great deal of excess heat is produced by our working muscles and the evaporation of sweat from the skin is how most of that heat is transferred to the environment, preventing our core body temperature from becoming dangerously high. This fact essentially explains why even a moderately, well-trained human runner can outrun a dog over a long distance in the heat, even though dogs are much faster runners than us over shorter distances or in cold conditions.

Sweat outputs

Sweat output, and therefore hydration for athletes, can vary dramatically from person to person during exercise (which depends a lot on genetics, work rate, environmental conditions, clothing and other factors), and values in the region of ~0.5l/hr to ~2.0l/hr have been reported as relatively normal for adults, with some outliers below and above that range (1).

While there is no real argument about our sweat losses needing to be replaced at some point to allow the body to maintain homeostasis over an extended time period, the question of how much and what athletes should drink – specifically during sporting activities to counteract sweat losses and to maximise performance – has become one of the most hotly debated and polarising topics in sports science and nutrition in the last few decades.

Hydration for athletes – a historical perspective

Prior to the 1970s, it was generally thought that any drinking during activities was counterproductive to sporting performance. Although they occasionally did it, athletes in the early to mid-1900s were largely discouraged from consuming anything, as quotes such as this one from the legendary marathon and ultra-distance runner Arthur Newton exemplify: 

“Even in the warmest English weather, marathon runners should limit themselves to a single drink, two at most.”

The largely ‘nil by mouth’ approach remained essentially in vogue for about three-quarters of the 20th century, and then shifted quite dramatically in the mid-1970s (soon after the invention of Gatorade – the first commercial sports drink) when research, much of it funded by the emerging sports drink industry, started to point to the fact that dehydration could actually be harmful to performance, and that drinking plenty of fluid to prevent it occurring was in fact a very good idea.

What essentially then occurred was one of the classic ‘pendulum swings’ in opinion about hydration for athletes, that are all too often seen in sports science. We went from a situation where the basic advice to athletes flipped from ‘try not to drink’ to ‘drink as much as you can’ in a relatively short space of time, with very little acknowledgment of there being a middle ground between these two extreme viewpoints! We’ve since seen similar things happen with carbohydrate intake, as another classic example.

By the 1990s, even esteemed organisations such as the American College of Sports Medicine were recommending that athletes should:

“….start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.” (excerpt from the 1996 Position Stand on Exercise and Fluid Replacement)

As summarised in his 2012 book Waterlogged (2), renowned sports medic and researcher Professor Tim Noakes was one of the first to pick up that this switch from telling athletes to try not to drink at all to suggesting they need to drink as much as they could tolerate, could be problematic.

Along with some other forward-thinking colleagues, Noakes highlighted and documented a worrying increase in recorded instances of hyponatraemia (sometimes known as water intoxication) that were starting to occur in several sporting and occupational settings. He went on to pull together evidence showing that the over-consumption of fluids had been the primary cause of a number of what should have been very preventable deaths in athletes, service people and other individuals, who either followed advice to drink ahead of thirst instincts, or who were forced to take in fluids in a bid to avoid them becoming dehydrated at all costs.

More recent research on hydration for athletes, conducted at Ironman triathlon events, has continued to show that hyponatraemia is relatively common in endurance athletes, with about 10 per cent of finishers demonstrating a significant level of dilution of blood sodium levels at the end of races (3). This adds more proof to support Noakes’ basic hypothesis that over-drinking and hyponatraemia are a significant issue.

Noakes has not held back when it comes to pointing the finger of blame for the rise of hyponatraemia squarely at the sports drink industry and its marketing messages designed to sell more beverages. His opinion on how athletes can avoid hyponatraemia is both simple and clearly a jab back at ‘the industry’ – he actually recommends ignoring its products and most of its research altogether.

Waterlogged

In Waterlogged, Noakes advocates that simply ‘drinking water to thirst’ (i.e. just listening to your body and drinking water as and when you feel like you should) is all that is required to avoid hyponatraemia and maximise performance. In other words, he does not suggest restricting fluid intake, as was the case in the pre-Gatorade era, nor does he advise drinking large amounts over and above what your body ‘tells you’ it needs (be that water, sports drinks or a combination of the two), as was the predominant advice in the 1980s, 1990s and early 2000s.

His idea is appealingly simple and has merit in several common situations when supporting the hydration for athletes. It makes physiological sense at times when sweat losses are moderate and/or exercise durations are relatively short. This is because as you sweat, both water and electrolytes are lost from the body, but because sweat is hypotonic (i.e. it contains less electrolytes than your body’s own extracellular fluid pool from which it is drawn), you always lose proportionally more water than sodium. This loss of proportionally more water causes sodium concentrations to effectively rise in the blood (even though both blood volume and net sodium levels are dropping), and by topping up with just enough water to keep sodium levels diluted to within acceptable levels, hyponatraemia will theoretically not be able to occur. You can then replenish yourself with sodium and additional fluids to return everything to normal after the exercise bout ends and all becomes right once more within your body. This is probably sound advice for anyone engaging in reasonably light activity, where taking in lots of fluids or sports drinks is basically unnecessary.

Where the idea of just drinking water to thirst potentially breaks down, though, is when sweat and sodium losses are very high, as can frequently occur during very long, hot endurance events and/or specifically in individuals who lose unusually large amounts of sodium in their sweat, or have very high sweat rates.

In the face of very large fluid and sodium losses, simply drinking water to thirst can lead to either a gradual drop in blood volume (hypovolemia), which at some point inevitably reduces exercise performance and the ability to dissipate heat, or hyponatraemia (if sodium losses are very high), or in some cases both simultaneously.

We recently published two case studies (4) that demonstrated the occurrence of hyponatraemia in someone with Cystic Fibrosis (CF sufferers have very high sweat sodium losses due to a genetic mutation that affects their sweat glands) and an Ironman athlete (without CF) who also happened to have high levels of sodium loss in her sweat. Neither of these individuals drank to excess, yet both ended up becoming hyponatraemic through a combination of sodium lost in sweat and by consuming low sodium fluids during periods of heavy sweat loss.

It is therefore highly likely (and there is a large body of both anecdotal and peer-reviewed evidence to support the idea) that a certain amount of sodium consumption during long, hot bouts of exercise, along with fluid intake, can be a good idea to help preserve extracellular fluid volume and help athletes maintain performance, as well as minimise the risk of hyponatraemia.

In one relatively recent study (5), a group of Spanish researchers saw a marked improvement in the hydration status, blood sodium levels and performance of triathletes given salt supplements as opposed to placebo pills in an actual half Ironman race. Interestingly, the athletes were all allowed to drink water ‘ad libitum’ (i.e. to thirst as Noakes suggests to be optimal) and they seemed to unconsciously choose to drink more when sodium was also ingested compared to the placebo pills. This outcome makes a lot of sense as sodium concentration in the blood is a large driver of thirst, so it could well have been that the ingestion of sodium helped to preserve their thirst instinct when it would have diminished due to sweat sodium losses going otherwise uncorrected.

There are also many examples in fields outside of sport where sodium is routinely added to beverages to aid fluid absorption and retention when large amounts of extracellular fluid volume are lost. These include the widespread and hugely effective use of Oral Rehydration Salts (ORS) in those suffering sickness and diarrhoea, and historically when salt was added to the drinking water of men engaged in manual labour in hot conditions, such as foundries, mines and other forges, to prevent “… fatigue, cramps and collapse” (6).

Hydration for athletes – closing remarks

More research in this area is definitely called for when it comes to hydration for athletes – especially using groups of athletes with differing individual rates of sodium and fluid loss. Given what we already know about the large range of sweat and sodium losses that we see in different groups of athletes (7), it seems highly likely that inter-individual variations in fluid and sodium requirements will be very significant, but this is not an area that has been widely studied as yet.

What is most important though, in the field of hydration as a whole, is that we try to move away from looking for or promoting extreme and overly simplistic notions that hydration advice can be summarised into a single sound bite.

Scientists have tried telling athletes to simply ‘avoid drinking’ and found that it does not really work well in all situations. They then moved onto the ‘drink as much as you can tolerate’ era and found that not only did it not work brilliantly, but it actually had the potential to kill!

Now it could be argued that we’re moving towards a sort of middle ground with Noakes’ idea of ‘drink water to thirst’. However, it is pretty clear that this is not the one and only answer either.

Optimal hydration practice for athletes is a complex topic. A level of nuance and a high degree of individualisation needs to be applied to get the very best results. Focusing on accepting that there is no ‘one-size-fits-all’ answer would be a great starting point for us all to move more productively towards learning more useful insights about this fascinating area of human performance and nutrition.