Hydration: stop drowning in advice

What are the best-practice recommendations for optimal hydration? SPB examines the research and makes some practical recommendations for athletes who may be confused by the conflicting advice!

When the mercury rises, how much fluid should you drink, when should you drink it and how often? It might surprise you to discover that this is a topic of great controversy and the answer depends on which scientists you talk to! Back in the early 2000s, the conventional advice for athletes was to drink ample fluid during exercise – enough to ensure that any losses in body mass incurred through sweating and respiration were minimal, and certainly less than 2% of body mass (eg 1.5 litres in a 75kg athlete). This was reflected in the subsequent hydration guidelines issued by the highly respected American College of Sports Medicine (ACSM)⁽¹⁾. The ACSM’s ‘Hydration Consensus Statement’ published in 2005 states the following: “A body water deficit of greater than 2% of body weight marks the level of dehydration that can adversely affect performance.”

However, this advice has come under increasing attack from a number of sports nutritionists and exercise physiologists who believe that the ‘hydrate at all costs’ approach is at best misguided and at worst, downright dangerous. Indeed, as early as 2006, the renowned exercise physiologist Professor Tim Noakes claimed in a hard-hitting leading article in the British Journal of Sports Medicine that case against ‘over-drinking’ was proven 20 years previously, and that official advice has been influenced by the marketing needs of the sports drink industry⁽²⁾.

Around the same time, a study on Ironman triathletes (who are arguably extremely prone to the performance-sapping effects of dehydration) was published by Australian scientists. They found that fluid losses of 3% of bodyweight (considerably above the ACSM’s guidelines) didn’t result in significant rises in core temperature or any drop in performance⁽³⁾. As a result, the scientists called on the American College of Sports Medicine (ACSM) and other official bodies to revise their fluid replacement guidelines.

Underdrinking vs. overdrinking

In the last decade or so, a number of further studies on hydration in a wide variety of athletes have been published. But before we go on and look at what the science says about how much fluid is needed to keep optimally hydrated during exercise, let’s just set out why both underdrinking and overdrinking are undesirable. Basically, you can think of optimum hydration as a balance between too much and too little fluid intake (see figure 1). Drink too little fluid during exercise and you risk becoming hypohydrated; drink too much and you can become hyperhydrated. Both of these outcomes are undesirable as they can impair performance and (in severe cases) can become life threatening. Table 1 lists some of the potential consequences of hypo and hyperhydration.

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Figure 1: The hydration balancing act

During training and competition (especially in warmer conditions), there’s a tendency for athletes to lose significant amounts of fluid – ie move towards hypohydration (shown at the red end of the seesaw). Consuming fluid during exercise helps offset this tendency, but consume too much and athletes can begin to become hyperhydrated (blue end), which is equally undesirable.

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Table 1: Potential consequences of hyper and hypohydration

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As can be seen in table 1, both under and over-drinking can dent performance and in very severe cases, be life threatening. It’s perhaps understandable then that there’s so much advice on this topic. However given the conflicting messages, what’s less clear to many athletes is how to stay optimally hydrated to minimize physiological stress and ensure that performance is maintained!

Previous conclusions on drinking and hydration

In an early article on this topic, we reviewed some of the key research on hydration. Some of the findings we reported were as follows:

• A US study that looked in to the effects of 5% (ie quite severe) dehydration on running economy (biochemical efficiency of muscles during sub-maximal running) during a 10-minute run⁽⁴⁾. It found no significant differences in running economy between fully hydrated and 5% dehydrated states at any workload. Likewise, there were no differences in perceived rates of exertion or in post-exercise lactate concentration;
• A study on basketball players performing 2-hour exercise sessions in hot conditions⁽⁵⁾. Compared with full hydration, a dehydration level of just 2% adversely affected the players’ shooting skills and performance in other manoeuvres requiring high levels of motor skills;
• A study on uphill cycling that compared the performance of well hydrated and 3.5% dehydrated cyclists during an uphill ride to exhaustion trial⁽⁶⁾. Although the dehydrated cyclists had better power-to-weight ratios, their core temperatures and heart rates at a given workload were higher, and their times to exhaustion were 13.8% poorer;
• A study on runners that looked at the effects of 5% dehydration vs. full hydration on the hormonal responses to training – specifically the levels of testosterone, cortisol and the testosterone/cortisol ratio⁽⁷⁾. This showed that while a 5% dehydrated state didn’t seem to measurably increase physiological stress during exercise or affect testosterone concentrations before, during, or after exercise, pre and post-exercise cortisol concentrations were increased in the dehydrated runners, indicating an increased likelihood of muscle tissue breakdown (a bad thing!).

In summary, athletes may get away with up to 2% fluid loss, higher dehydration levels could result in reduced performance, even allowing for a potential gain in power to weight ratio. By contrast, athletes whose sports involve a large skill component should aim for minimal dehydration – even a 2% fluid loss may be too much to maintain optimum performance.

Latest thinking

That was then, but how has the data and thinking evolved since, and should they alter our thinking on hydration for performance? In 2011, a review study looked at a large number of previous studies on hydration during time-trial cycling and pooled the data to try and assess the relationship between varying degrees of dehydration and cycling performance⁽⁸⁾. In all, five studies met the strict criteria for inclusion and averaged across all of these trials, the cyclists rode for around an hour at 68% of VO2max (moderately hard) in temperatures of 26C. The key finding that emerged was that when the cyclists drank freely according to their thirst, they tended to perform significantly better than when they didn’t drink enough to meet their thirst needs OR when they drank in excess of their thirst needs – eg when following a prescribed drinking program. Two other findings were that compared with full hydration, losing up to 4% of bodyweight in fluid loss did not alter cycling performances during out-of-door exercise conditions; also, exercise intensity and duration had a much greater impact on cycling performance than did exercise induced dehydration.

Of course, averaging results from previous studies is not the most rigorous method of looking at the effects of hydration on performance. However, other quite recent studies on single groups of athletes provide solid evidence that following a prescriptive drinking formula might not be best for performance after all. For example, researchers studied half-marathon performance in runners who followed either a thirst-driven drinking program, or a prescriptive drinking program designed to ensure that the runners lost no more than 2% of their bodyweight⁽⁹⁾. Ten trained distance runners performed two treadmill half marathons on two separate occasions in temperatures of 30C (hot), while facing a wind speed matching running speed to simulate running outdoors through still air but with different drinking strategies:

• Drinking water according to their thirst (averaging 0.38 litres per hour);
• Following a pre-prescribed drinking program to ensure that they lost no more than 2% of bodyweight in fluid (averaging 1.4 litres per hour).

The results showed that there were no differences in half-marathon times, running pace, average heart rates or core temperatures in the two trials.

Another study on road cyclist compared a ‘free drinking’ strategy to one where the cyclists were instructed to drink strictly to thirst⁽¹⁰⁾. In the free drinking strategy, cyclists drank whenever and however much they wanted. In the ‘drink to thirst’ strategy, the cyclists had to consume fluid as soon as they felt the slightest bit thirsty – ie to eliminate thirst. The conditions were tough – 160km in temperatures of around 37C (that’s nearly 100F). When the researchers compared the drinking strategies for performance outcomes, they found that there were absolutely no differences for average time to complete the event, actual fluid intake, ratings of thirst, perceived exertion or urine concentration. In fact, the only real difference was that the drink to thirst cyclists seemed to need less fluid in the 24 hours following the event. The researchers concluded that specific instructions to drink to thirst’ are unnecessary and that trying to follow such a strategy may only serve to interfere with an athlete’s focus on training and competition!

Further evidence comes from a Dutch study on dehydration in cyclists performing two 40km time trials in hot conditions (35C), and which found not only could a structured and pre-prescribed drinking approach be unnecessary, in shorter events at least, drinking anything at all might not be necessary⁽¹¹⁾. In this study, one trial was performed in which the cyclists were allowed to drink freely while in the other, they consumed no fluid. However, unlike most other hydration studies, the cyclists began both trials in an already dehydrated state – ie where you might assume that drinking during exercise would be advantageous. Perhaps unsurprisingly, when the dehydrated cyclists drank during the time trial, they experienced less thirst sensations and body mass loss. What was less expected however was that even though the cyclists began in a dehydrated state, drinking during the time trial produced no benefits in terms of performance, perceived exertion, core temperature, and overall comfort.

Does dehydration really matter?

The findings from some studies that significant dehydration during shorter events - ie that drinking is unnecessary and doesn’t really have any negative impact on performance - conflicts with the conventional advice,. However, there are other studies that have produced similar results. In an elegant and landmark study, researchers asked 10 cyclists to undergo three separate trials⁽¹²⁾. This study was particularly noteworthy because it used simulated wind conditions (as athletes competing in real outdoor conditions would experience), and it blinded the participants to their hydration status, removing any psychological effects.

In each trial, 10 cyclists were dehydrated by 3% of body mass by performing 2 hours of sub-maximal exercise (walking and cycling) in the heat. Then, the cyclists were infused with a saline solution to replace 100%, 33% or 0% of fluid losses. This left them fully hydrated, 2% dehydrated or 3% dehydrated, respectively. Why use this procedure you might ask? The reason is that the cyclists weren’t aware how much fluid was infused, which meant they had no idea of how well/badly hydrated they were – ie any placebo effect was effectively removed!

The cyclists then completed a 25km time trial in the heat (33°C) but with an ambient wind speed of 32kmh to simulate real, outdoors conditions. During this 25km time trial, their starting hydration status was maintained by infusing saline at a rate equal to their sweat rate. The results showed that although core temperatures in the later stages of the 3% dehydration trial were higher, there was absolutely no difference in performances across all three trials. The researchers concluded that their findings did not support the assumptions underpinning traditional hydration guidelines.

However, as if to highlight the fact that the path of scientific discovery is rarely smooth, not all of the more recent research suggests immediately throwing away your water bottles! For example, a study published in 2015 looked at the hydration status of 119 cyclists before, during and immediately after the (long) 100-mile ‘Hotter’N Hell’ cycling event⁽¹³⁾. In particular, they wanted to find out whether the cyclists’ hydration status was linked to perceived exertion, sensations of thirst and pain, and overall mood (using a standardized questionnaire called the ‘Brunel Profile of Mood States’). As part of this assessment, and to determine physiological dehydration, the cyclists’ urine concentration was measured. Compared to the well-hydrated cyclists, the results showed that regardless of performance, the dehydrated cyclists experienced greater levels of effort and discomfort than did the well-hydrated cyclists. Given the importance of mental strength and stamina during a long race, these results suggest that staying reasonably well hydrated could offer significant benefits by aiding focus and mental strength.

Likewise, another study investigated the effects of just 1% dehydration (that’s a fluid loss of just 700mls for a typical athlete weighing around 70kg) on subsequent performance in a 5km cycling hill climb⁽¹⁴⁾. Ten well-trained cyclists completed two trials. In each trial, they performed 1 hour of steady-state cycling before tackling a 5km climb outdoors in warm conditions. In one trial, they drank no fluid during the 1-hour steady-state ride in order to ensure they were 1% dehydrated before the uphill cycling task. In the other trial, they drank water during the steady-state ride and so commenced the uphill task fully hydrated. Results showed (figure 2) that when the cyclists were fully hydrated, they completed the 5km uphill ride 5.8% faster than when they were slightly (1%) dehydrated (16.6 minutes vs. 17.6 minutes).

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Figure 2: Full hydration vs. 1% dehydration during cycling hill climbing⁽¹⁴⁾

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At a later date, the same research group used a similar protocol to compare a free-drinking strategy during three bouts of 5km uphill cycling climbing (interspersed with flat riding) in the heat (31.6C/89F) to one where the cyclists followed a prescribed drinking plan⁽¹⁵⁾. The results showed that the average cycling speed for the third bout of the 5km hill cycling was greater (by 1.4kmh) when the cyclists had used a prescribed drinking strategy rather than a free drinking one – seemingly at odds with other research on this topic. One explanation however is that both of these studies were lab-based (ie performed indoors), where the effects of dehydration appear to be amplified.

Conclusions and recommendations

The debate on hydration during exercise is still ongoing. In recent years, researchers have carried out a number of field studies suggesting that in real-world conditions (ie outdoors with a degree of wind cooling – not in artificial laboratory conditions), more than 2% dehydration is NOT a problem for endurance athletes – contrary to a number of existing guidelines. Indeed, some studies have suggested that up to 4% dehydration does not hinder performance. Likewise, new research suggests that the previous advice not to rely on thirst and instead to follow a regular drinking plan seems questionable. For athletes who might be feeling somewhat confused, have no fear – the panel below provides the key evidence-based, best-practice guidelines, for staying optimally hydrated this summer with the minimum of fuss!

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Practical recommendations

The more recent data on hydration and performance suggests that the ‘drink fluid at all costs’ approach advocated in the early 2000s may well be unnecessary and that perhaps thirst sensation is better at determining when and how much fluid to drink. Here then are some practical guidelines

Shorter events (under 2 hours)

• Rather than following a rigid program of drinking, choosing when and how much to drink is likely to be as equally effective for performance – and less hassle too!
• Contrary to popular opinion, using your sensation of thirst is a useful way of ensuring you take in sufficient fluid.
• Although the evidence suggests that athletes can be more flexible about drinking on the move, you should always aim to start your training/competition well hydrated.

Longer events

• With more opportunity to become significantly dehydrated (over 3%) a more considered approach may be required in warm/hot conditions. Using training sessions as a guideline where you drink as you please, compare your body mass before and after 2-3 hours of training. If you’ve lost no more than 2% of body mass, your ‘freely chosen’ drinking strategy is likely to be suitable for longer events. If you’re already 3% or more dehydrated, you might want to consider a more structured fluid intake.
• For very long events where simply staying positive and finishing is the goal, drinking plenty to minimize thirst may help lift your mood by reducing perceived exertion and thermal stress.
• Always start your training/competition well hydrated.

Events indoors or where motor skills/concentration/decision making is required

• Limit any dehydration to less than 2% (using a more structured drinking approach if needed) as the evidence still suggests that motor skills and psychological performance is adversely affected by relatively low levels of dehydration;
• Endurance activities indoors may also require a more structured hydration approach because of the lack of (cooling) winds that are present outdoors.

References

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