Hydration: Hyper, Hypo, Eu and De?

Over the years it has been drummed into endurance athletes to drink, drink, hydrate, drink. But what do we really know about hydration? How do we do it and is it really that important?

Background: water isn’t water isn’t water

We have three main components for water storage in the body: intracellular fluid (within cells), extracellular fluid (between cells) and water in our blood (blood volume). These three compart-ments work in concert to shift fluid to where it is needed depending on osmolality, hormone levels and more complex functions. Body water is used for heat transfer, cooling via sweating, as a medium for metabolism, to maintain cell volume and blood volume for bodily functions (Cotter, Thornton, Lee & Laursen, 2014). When we are severely hypohydrated (decreased body water) we risk reduction in mental capacity, exercise capacity and may increase the risk of kidney stones. When no water is ingested – eventually we die, true story.

Conversely, hyperhydration (excess body water) is generally safe in a non-exercise setting, with side effects including frequent urination, discomfort and disrupted sleeping patterns (Cotter et al., 2014). However, exercise-induced hyponatraemia (low blood sodium), is potentially life-threatening. Factors that increase the occurrence of hyponatreamia are when large volumes of low sodium fluids are ingested, when race times are long (>4hours) and sweat rates are low (Cotter et al., 2014). Hyponatreamia is also exacerbated when beginning exercise with low blood sodium, in smaller (particularly female) and older athletes and when taking serotonin reuptake inhibitors (SSRI) or non-steroidal anti-inflammatory
drugs (NSAIDs) (Cotter et al., 2014) – the latter being one reason why race doctors tell participants not to use Ibuprofen or Voltaren during ultra-endurance exercise. So yes, if you drink too much water, while exercising in an ultra-endurance event, and you are hardly sweating, you can also die.

Damned if you do, damned if you don’t…what is the answer?

You may have heard of the suggestion that we should limit hypohydration to two per cent body weight for performance (both mental and physical) reasons. The American College of Sports Medicine (ACSM) developed these recommendations based on available evidence. While this may be true in some lab environments, the applicability to real-world situations is less clear – some real-world research suggests that athletes can withstand much greater reductions in body weight with no detriment to performance or thermoregulation.

A meta-analysis of cyclists showed no detriment to performance with a four per cent reduction in body weight via fluid losses (Goulet, 2011). Furthermore, a small study of world-class marathon runners showed the fastest runners were able to sustain a massive 8.8% reduction in body weight (note that this percentage is total losses, whereas runners were adding fluids ~0.55L/hour on average) (Beis, Wright-Whyte, Fudge, Noakes & Pitsiladis, 2012). Now, I am not for a minute suggesting you go out and achieve 6.6kg weight loss (assuming a 75kg triathlete) over 2 hours 6 minutes, as it is important to note the climatic conditions – 15.3 degrees Celcius and 59% humidity average for these marathons. It is possible that during hotter, more humid weather, these losses may have compromised performance and/or cooling.

Talking of cooling ability, another interesting study looked at the influence of wind speed on cycling thermoregulation (Saunders, Dugas, Tucker, Lambert & Noakes, 2005). While replacing 60% of sweat losses during a two-hour cycling study in hot conditions, cyclists were exposed to differing wind speeds from 0km/hour to 50km/hr. They found wind speed was inversely related to core body temperature rise (Saunders et al., 2005). In other words – the higher the wind speed, the lower the rise in core body temperature. They then repeated this study, replacing 80% of fluid losses for the 33km/hour wind speed, finding no difference in core body temperature to the 60% protocol. I can hear your brains ticking over: “If I can convince my partner to buy an industrial fan for my wind trainer or treadmill sessions, I can ride/run longer, harder and arguably safer.” Some food for thought.

Putting this into practice – strategies pre triathlon

Going into a race start euhydrated (normal body water) is recommended. If you can tolerate being a little hyperhydrated, that is okay too – it comes down to personal preference. If you are getting up more than twice during the night you are overdoing the hydration or you are just getting older (fact of life, sorry!). If your urine is clear in the morning (unless you had beetroot or vitamins the night before) and you have no thirst, this is further evidence that your hydration has been adequate (Patterson, 2015). The best way to hydrate the day before is to ensure you are drinking water with each main meal and snack.

Strategies during triathlon

There are two schools of thought currently – drink to a plan or drink to thirst. Personally, I find drinking to a plan provides more structure, particularly if carbohydrates need to be taken as part of a fuelling strategy. The other reason fluid planning works is that our sense of thirst may be disrupted by hot and humid environments, and as we age we may not be as sensitive (Cotter et al., 2014); the only caveat being that the plan needs to be adaptable should climatic conditions change markedly from what you expected, for example, if racing in Melbourne – who knows what the weather will be doing.

How much to drink?

First and foremost is the fact that we can only process a certain volume of fluid per hour. The upper limit is suggested to be ~800mL/hour, depending on body size. Secondly, we need to better understand how much fluid we are losing to guide intake.

Understanding your sweat rate:

This is an easy way to determine how much sweat you are losing in a session. This will change from season to season and depend on fitness, location, environment, etc. Completing a sweat rate trial on a few occasions will allow you to get a more reliable measure of sweat losses (Patterson, 2015).

For example, if Jo Ironman completed a two-hour session, started the session at 75kg, finished it at 73.5kg (losing 1.5kg), drank 500mL and didn’t go to the bathroom, we can say Jo had a sweat loss of 2L or 1L/hour or a 2% reduction. Now as we know, we don’t expect to put back what we lose, but as Jo is a heavy sweater and training for Ironman, increasing fluid intake in this example is warranted. How much? Well, that depends on Jo, though a good initial aim would be upping intake to 600mL per hour (matching 60% losses) then repeating the trial to test tolerance.

Conversely, if Jo went out for a three-hour session, drank 500mL and weighed the same pre and post, either Jo stopped at the bakery and didn’t record it or Jo is one of those athletes who simply doesn’t sweat much and may be at risk of hyponatremia if not careful. Note: we would rarely expect to replace everything we have lost in a training session and this is never the aim of a hydration strategy. Using this sweat rate calculation can be really helpful in determining a race day hydration strategy.

What to drink?

The number one consideration is do you need carbohydrate from your drink or not? If not then you have electrolyte or water options. For in an Olympic distance event, consuming water throughout the race is perfectly fine. For longer events, looking at an electrolyte option may assist hydration and may also reduce the changes in extracellular osmolality caused by consuming vast quantities of plain water. How much electrolyte to put back in? This is the million-dollar question. Sodium concentrations in your typical sports drink are arguably on the low side (10-25mmol/L), whereas recommen-dations from ACSM suggest an aim of 21-30mmol/L (483-690mg/L) sodium. However, for those whose sweat-sodium rates are higher, there may be a benefit to increasing sodium replacement. Note that sweat sodium losses cannot be estimated from the appearance of salt on a black tri suit (there is no good correlation between white stains and sweat sodium losses). Sweat sodium is collected via patch testing, and combined with a calculation of fluid losses. Sweat-sodium patch testing can be performed by Sports Dietitians or Exercise Scientists accredited to specific labs, such as the ‘Sweat Lab’ in Melbourne. Note that like fluid losses, we never expect to replace all the sodium we lose during exercise

If we want to focus just on rehydration, then taking a leaf out of the World Health Organisation (WHO) recommendations for oral rehydration solution sheds further light. WHO suggests a minimum 60mmol/L sodium = 1380mg sodium/L for rehydration (generally following hypohydration due to diarrhoea etc.). Please note that high sodium will affect drink palatability and may become a gastric irritant. Furthermore, if you have any underlying kidney or heart disease risk factors, talking to your specialist or a Sports Dietitian is advisable before supplementing sodium. So, we really must take this suggestion with, dare I say it, a grain of salt. Nonetheless, there are some manufacturers now including greater amounts of sodium in their products, more in line with WHO recommendations, e.g. SOS. While brands such as Infinite Nutrition allow the athlete to select their own sodium intake in their drink mix.

Take Home Messages:

  • Euhydration is important for maintaining bodily functions.
  • Focusing on euhydration before an event is important.
  • Excessive hypohydration should
    be avoided.
  • If you don’t sweat much during ultra-endurance exercise, don’t drink large volumes of water.
  • In an ideal world, limit fluid losses to 2% body weight.
  • Real world – you may tolerate 4% body weight losses.
  • Understand/test your sweat losses at climatic conditions experienced in racing/training.
  • Develop an individual hydration strategy based on your sweat losses and what level of dehydration (within reason) that you can tolerate safely, without diminished performance.
  • Aim to replace 60% losses during exercise as a ballpark, though remember you can only tolerate up to ~800mL fluid/hour.
  • Replacing sodium in events over Olympic distance is warranted: 21-30mmol/L is a starting point; though increase if dictated by sweat sodium losses (assuming no contraindications).
  • If all of this sounds too daunting go and see a Sports Dietitian – we are happy to help you.


Beis, L. Y., Wright-Whyte, M., Fudge, B., Noakes, T., & Pitsiladis, Y. P. (2012). Drinking behaviours of elite male runners during marathon competition. Clin J Sport Med, 22(3), 254-261. doi:10.1097/JSM.0b013e31824a55d7

Cotter, J. D., Thornton, S. N., Lee, J. K., & Laursen, P. B. (2014). Are we being drowned in hydration advice? Thirsty for more? Extreme Physiology & Medicine, 3(1), 18. doi:10.1186/2046-7648-3-18

Goulet, E. D. (2011). Effect of exercise-induced dehydration on time-trial exercise performance:
a meta-analysis
. Br J Sports Med, 45.

Patterson, A. (2015). Hydration and Thermoregulation. Paper presented at the Sports Dietitians Australia Sports Nutrition Course, AIS, Canberra.

Saunders, A. G., Dugas, J. P., Tucker, R., Lambert, M. I.,
& Noakes, T. D. (2005). The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment.
Acta Physiol Scand, 183(3), 241-255. doi:10.1111/j.1365-201X.2004.01400.x


Peter Herzig

Centred Nutrition was founded by Peter Herzig (APD). Peter is a qualified Dietitian and Accredited Sports Dietitian who also has a degree in Exercise Science. Peter set up Centred Nutrition in Brisbane and the Sunshine Coast to focus on a client centred approach; as there is no one solution in nutrition that will work for everyone.

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