Hydration and Fluid Loss Assessment

There are many techniques that are adopted to assess hydration status in field and lab based applications, some commonly used examples are bioelectrical impedance analysis, blood indices, total body water measures (deuterium oxide), visual urine colour assessments (via pee charts), body fluid osmolality measures via urine, tear drop, or saliva, the assessment of body mass fluctuations during training and of course thirst perceptions. At this point it is worth noting that there are conflicting recommendations in research to date when considering thirst perceptions of athletes, and its’ worth as a hydration assessment method. Some studies (4) have suggested that thirst perception alone is suffice, on the other hand numerous studies have clearly documented the inadequacy of ad libitum fluid consumption during training. In one previous study (5) showed that only 27% of the fluid lost during training was replaced via ad libitum fluid consumption. It may be advisable to avoid the use of thirst perceptions in a hydration strategy until further research is carried out.


In a field-based practical setting, typical assessments involve a combination of assessments to create an effective strategy:


  • Body mass fluctuations- Given that 1 g of water = 1 ml; this provides a platform for a pragmatic approach to estimating fluid losses. But be sure to use the results obtained in conjunction with an assessment of the environmental and physiological variables previously detailed. Will an assessment of body mass reductions during intense exercise in the heat give a reading appropriate to devise a strategy for less intense training in cooler environments?


  • Urine Osmolality Analysis- This is a relatively cost effective method that involves the analysis of a small drop of urine using a digital refractometer, whereby a digital reading gives an indication of the athlete’s state of hydration.


  • Urine colour analysis- Again an inexpensive method that is easy to implement by any athlete at any time of day. A pee chart is used to give a general indication to hydration status based on comparisons drawn from a spectrum of urine colours. (Please note this technique can give inaccurate colour readings in certain cases, such as post-effervescent vitamin consumption, or digesting large quantities of beetroot amongst others).


Hydration for Endurance. 
Successful preparations for endurance exercise involve commencing training in a state of euhydration and maintaining a hydrated state throughout, especially when training in the heat. Adequate hydration can be used as part of a strategy to reduce the likelihood of fatigue, particularly during the latter stages of competition. The formulation of an individualized strategy is important, the athlete needs to assess exactly what works for them well before the day of the competition, less than adequate fluid intakes are likely to result in decrements in performance, with excess fluid intakes likely to result in hypernatremia which in certain cases can be fatal. The health of the athlete must be of paramount priority above all aspects of the strategy; with a rise in fatalities in endurance events over recent decades, we must learn from previous occasions in competition. For example in 2002 excessive fluid intakes lead to the death of a female runner during the Boston marathon, highlighting the importance of a safe and personalized approach to hydration.


Realistically speaking it is unlikely that significant performance decrements will occur during an endurance event when subject to small losses in total body fluid. The athlete should aim to limit body mass losses to no more than 2% during exercise, or aim to replace no more than 80% of fluid losses. In the majority of cases this approach will avoid sever bouts of hypohydration and hyperhydration, as well as adequately maintain hydration levels in preparation for performance.
Carbohydrate/electrolyte drinks have become widely used especially over recent years with mass marketing and advances in research examining the positive effects on endurance athletes. As carbohydrate reserves deplete throughout prolonged exercise bouts, particularly during the latter stages, it becomes ever more appealing to replace carbohydrate, salt and fluid losses simultaneously. The concentration of carbohydrate drinks typically arrive at around 4-8% (4-8 g/ 100ml), and aims to supplement blood plasma carbohydrate levels during exercise. Recent studies have recommended intakes of concentrations no higher than 6% to avoid gastrointestinal disturbances (6).


The electrolyte content of carbohydrate drinks can also aid in the replacements of salts lost through sweating, and the maintenance of a hydrated state during training. Particular importance can be placed on sodium and potassium losses. Salt losses can vary greatly between athletes with sweat concentrations between 40-60 mmol. L -1 for sodium, and 4-8 mmol. L -1 for potassium. During a study involving the analysis of salt losses in soccer players during training (7), sodium chloride losses varied between 2-10 g during a 90 min training session, further reinforcing the requirements for individualized salt replacement strategies. During training a practical suggestion for assessing the relative levels of salt losses during training could involve the use of black clothing, resulting of the visual appearance of salt crystals post training. The more salt crystals are present, the greater the importance placed on adjustments in the athlete’s diet to supplement their salt losses. Excessive salty sweaters should aim to avoid consuming high levels of salt in one instance, but rather consume the necessary levels on a longitudinal basis to avoid the likelihood of blood pressure and other associated health and risks.


Other Sporting Applications for Hydration.
In this article endurance based activity has been the focal point; however the maintenance of a hydrated state has many practical applications throughout sport and varying populations. Young athletes need particular attention, as their body surface area is greater per unit of body weight, which increases their capacity for sweat loss especially when exercising in the heat. Educational interventions have been effective in improving fluid intakes and subsequent performance in young populations (8). To support this; I have known from personal experience the benefits of educational strategies to improve hydration status pre and during training, when offering nutrition support for the Junior British Climbing team. Dehydration has been associated with disturbances to cognitive performance, which could have a dramatic effect on sports such as rugby, hockey and soccer amongst many others that require constant decision making. Previous studies have identified that a 2% loss in body mass can negatively affect speed and efficiency regardless of the environmental conditions in young athletes (9).


Differences in gender can prompt the need for more personalized strategies. Given that women on average have lower total body-water content than men, a change in body weight for women will cause a greater percentage change than that of men. Athletes that compete in combat and weight restrictive sports report less than desirable practices on ‘weigh-in day’. The use of diuretics, lengthy periods spent in saunas, the use of sweat suits; and restricted fluid intakes all contribute towards often extreme levels of dehydration in order to make weight. Recent case studies involving a professional boxer (3) and a professional jockey (10) have highlighted the use of weight management strategies that result in the avoidance of such extreme dehydration practices on weigh in day.