1. Introduction

1A body water deficit will have a negative impact on exercise performance, but the when this occurs will be influenced by the environmental conditions, the exercise type and duration and the individual physiology of the athlete. Athletes are usually encouraged to begin exercise euhydrated and to consume sufficient amounts of appropriate fluids during exercise to limit water and salt deficits (Sawka et al. 2007).

2. Methods of assessing hydration status

2An individual’s body water content is most appropriately determined using tracer methodology with the use of deuterium oxide. The determination of hydration status has received increasing attention over the past 10 to 20 years. Blood borne parameters and urinary markers have been widely studied areas, with a research into the use of bioelectrical impedance analysis, and latterly saliva, also being undertaken. The choice of hydration status marker will ultimately be determined by the sensitivity and accuracy with which hydration status needs to be established, the technical and time requirements, and the expense of the method. However, consideration must also be given to other conditions or complicating factors that may influence the parameter of measurement. At present, measures of urine concentration are typically the favoured methods used in athletic settings but they may be unreliable in situations when body water content is changing rapidly. Where practical, a combination of two or more different measures may increase the chances of correctly classifying an individual.

3. Methods of assessing water loss

3It is common practice to use body mass changes as an index of body water content changes and thus of changes in hydration status. This is based on the assumption that 1 kg of mass loss is equal to 1 litre of sweat loss. Any fluid intake and urine or faecal loss need to be accounted for and appropriate corrections made and the dehydration level can then be calculated as the reduction below the baseline body weight. The sweating rate is calculated as the change in body weight (with appropriate corrections) during the exercise period.

4Potential sources of error in using body mass change to quantify sweat loss include loss of water and therefore mass from the respiratory tract and mass loss due to substrate oxidation. Although individually these losses may be small, their overall effects are not insignificant in many exercise situations. The decision whether or not to correct for substrate oxidation and respiratory water loss is usually determined by the relative magnitude of the different avenues of mass loss, the precision required in the expression of the data and the purpose for which it is being determined (Maughan et al. 2007a). Indeed, respiratory water loss and metabolic water produced are generally of similar magnitude and approximately cancel each other minimizing the need for correction.

4. Methods of assessing electrolyte loss

5A wide range of values for all of the major sweat electrolytes has been reported in the literature, reflecting variations between individuals, differences due to the experimental conditions and differences due to the collection methods. Sweat composition has been investigated using a variety of collection methods. The two main methods used involve collection of sweat from a specific body region using some form of enclosing bag, capsule or absorptive patch, or a variation on the whole body-washdown technique.

6There are regional variations in sweat composition so when regional collection is used collection from both trunk and limb regions are recommended. Also, regional sweat collection procedures typically provide higher sweat electrolyte concentrations than from the whole body-washdown technique.

7In most of the whole body sweat collection methods, total sweat loss is calculated from the change in body mass and the body and any clothes worn washed after exercise with distilled water and the electrolyte content of that water measured. Alternatively, subjects have worn a close-fitting plastic bag or have exercised within a large plastic bag which makes a small room around. Clearly, some of these procedures are more suited to laboratory research with others being able to be used easily in the field.

8The majority of published data from athletes has been obtained using regional collections from absorbent patches that collect sweat. Whilst this is not a particularly difficult technique, clubs and players wanting to investigate their sweat electrolyte losses are likely to need specialized help to do so (Burke, 2005). Besides providing approximate sweat electrolyte losses, this approach, at a minimum, identifies those athletes with electrolyte rich (“salty”) sweat and who need to pay particular attention to electrolyte replacement. When it is not possible to determine electrolyte losses in this way, it may be possible to subjectively identify players with very high salt losses. That is, they may complain of the very salty taste of sweat in their mouth or that they have eye irritation when salt gets in their eyes or salt stains may be visible on clothing worn during training or matches.

5. Water and electrolyte losses: typical findings

9In recent years there have been several published papers reporting the results of “in-the-field” hydration monitoring, sweat collection and subsequent estimation of sweat water and electrolyte losses in athletes (e.g. Maughan et al. 2005, 2007b; Shirreffs et al. 2005, 2006; Palmer & Spriet, 2008; Horswill et al. 2009; Yeargin et al. 2010; Shirreffs et al. 2006; Kilding et al. 2009). Some of this data is from training sessions and some from competition.

10When pre-training or pre-game/match hydration status has been assessed it has typically been done by assessment of urine specific gravity or osmolality. Most research has suggested that the majority of players appeared to start training in a euhydrated state, with a urine osmolality less than about 700 mosmol kg^-1 or a specific gravity less than 1.020 (Sawka et al. 2007). However, the data does suggest that some players appear to start training and/or matches hypohydrated and only in some research have these players gone on to consume more fluid during the training and/or match than their euhydrated teammates.

11When mean values for sweat volume losses are considered, they detract from the considerable variation in both sweating response and drinking behaviour between players, as illustrated in Figure 1, which shows data for three different football teams taking part in their club’s 90-minutes training sessions. This variation, however, does not appear to be due to any difference in body size between players. Therefore, other factors like activity rate (metabolic rate), heat acclimatization status and genetic differences probably contribute to this large variability.

Figure 1: Estimated sweat body mass loss (% dehydration) from three football teams training for 90-minutes. Each circular symbol represents a different player and the red circular symbol represents the mean within each club.

12The sweat sodium concentrations measured in team sport players generally fall within the normal range reported in the literature of around 10 to 80 mmol/l. However, as for the sweat volume losses, the mean values detract from the considerable variation in sweat sodium concentration and sodium losses between players. As shown in Figure 2, there are examples of players losing about five times as much sodium as others.

Figure 2: Estimated sweat salt loss from ten football teams training in a variety of environmental conditions for approximately 90-minutes. Each symbol represents a different player.

13In conclusion, therefore, both the volume of sweat lost and the amount of sodium lost with it are highly variable in athletes doing the exact same exercise at the exact same time.

6. Practical recommendations

14From knowledge of the effects of water and electrolyte loss on athletes it seems prudent to recommend that athletes consider the benefits of limiting their body mass loss due to water loss during exercise to less than about 2%, unless the aim is to exercise when hypohydrated. However, the activity pattern in many sports may make this recommendation difficult for some athletes. In this case, careful consideration must be given to providing drinks that are palatable and encourage drinking.

15During exercise, some players will lose considerable quantities of electrolytes – particularly sodium – and may need to replace these during the exercise session. If sodium-containing beverages do not suffice, athletes may want to consume small amounts of salted snacks between periods of exercise to replace salt losses and stimulate drinking.

16Some specific recommendations are summarized in Table 1.