Fluid loss during exercise presents a challenge for the cardiovascular system which needs to transport oxygen in the blood to the working muscles so that exercise can continue at a high intensity. However, body temperature increases during prolonged exercise, particularly when it is performed in hot conditions. Therefore, blood flow needs to be directed to the skin for evaporative cooling so that cooler blood can then flow around the rest of the body and help to reduce body temperature. In addition, fluid loss reduces blood volume which makes it more difficult for the cardiovascular system to shunt blood to different parts of the body. A reduction in the volume of blood coming back to the heart means that the heart can’t pump as much blood out in each beat. In order to keep total cardiac output high during exercise, there is a gradual increase in heart rate at a given exercise intensity to compensate for the reduced stroke volume. This gradual increase in heart rate during exercise is called cardiovascular drift. As a result of these challenges to cardiovascular function, the cardiovascular system is likely to reach maximum capacity earlier, or at a lower exercise intensity when fluid loss is significant during exercise. This can potentially have a negative impact on exercise performance.

Fluid loss during exercise can potentially cause reductions in the amount of fluid inside cells and also in the interstitial fluid. These fluid shifts and associated changes in electrolyte concentrations can disrupt cell and organ function. This is particularly problematic in the brain and dehydration can cause impairments in cognitive function which can have a negative influence on exercise and sport performance.

Hydration can also have a negative effect on exercise performance when too much fluid is consumed during exercise. This can be as simple as carrying too much weight during exercise. If fluid consumption exceeds requirements during exercise, athletes often don’t have an opportunity to excrete this excess fluid and it adds to their body weight during exercise. This is particularly relevant for weight bearing activities such as running. Excess fluid that sits in the gastrointestinal tract or bladder during exercise can also cause discomfort and potentially reduce exercise performance.

A more severe consequence of excess fluid consumption is hyponatraemia, which is a reduction in the sodium concentration of the blood. This condition can result in nausea, collapse, loss of consciousness and even death. Sodium is lost in sweat during exercise and these losses can be quite significant during prolonged exercise. This reduces the amount of sodium in the blood and the concentration will reduce. The concentration of sodium may decrease even further if large volumes of low sodium fluids are used to replace fluid losses during exercise. Interestingly, the risk of hyponatraemia in endurance events appears to be higher in competitors of lower ability because of the longer time taken to complete the race. This can result in greater sodium loss through sweat and also greater opportunity to consume a larger fluid volume.

Fluid loss from various fluid compartments in the body is likely to impair function in tasks related to exercise and sport performance, however, there is still much debate about the exact amount of dehydration that impairs performance in different tasks. Historical guidelines have suggested that a level of dehydration equivalent to a fluid loss of 2% of body weight is likely to impair performance in a wide range of exercise and cognitive tasks. However, meta-analyses have found that this level of fluid loss only has a negative effect on endurance capacity during exercise tasks performed at a fixed intensity. Conversely, adverse effects on endurance time trial performance, which is more like exercise tasks in most sports, may not occur until 4% of body weight is lost. Another recent meta-analysis has suggested that any impairment in cognitive task performance as a result of dehydration is not likely to occur until fluid losses are greater than 2% of body weight, if at all.

The level of dehydration required to impair exercise performance is quite variable and different studies often produce conflicting results. This could be due to the differences in the way dehydration is induced in most studies. Dehydration can be induced by performing exercise without drinking and/or exposing participants to heat stress. These methods of inducing dehydration mean it is difficult to know if any performance impairment observed in the study is due to dehydration or simply due to previous exercise or heat stress. It is also very difficult to conduct blinded studies because people can usually tell if they are consuming fluid or not. The methodological challenges and the conflicting results of different studies make it difficult to draw definitive conclusions. In addition, significant variation in sweat rates and sweat electrolyte composition are observed when individuals perform an exercise task. Given this variation, it is possible that there is also variation in how different levels of dehydration influence performance in different individuals.

Clearly the variation in study findings makes it quite difficult to make specific recommendations about how much dehydration is likely to impair performance and also recommendations about the amount of fluid to consume during exercise. Fluid consumption during exercise needs to finely balance the need to reduce any negative effects of dehydration while also preventing any negative effects of excess fluid consumption. In addition, athletes often have limited opportunities to drink during exercise, particularly in competition. Therefore, a variety of hydration strategies may be effective for different athletes during exercise. Some studies show that simply drinking according to thirst may be just as effective as a planned fluid intake strategy designed to match fluid losses or restrict fluid losses to no more than 2% of body weight. Others have suggested that drinking to thirst may be appropriate when exercise intensity is moderate and exercise is performed in mild ambient conditions. However, a planned drinking strategy may be necessary when higher intensity exercise is performed in the heat.

Drinking according to thirst has several advantages for athletes. Drinking to thirst allows athletes to consume fluids at a rate that best suits their own comfort and subjective feelings at any given time during an event. It is also likely to be more effect at reducing the risk of excess fluid consumption. However, recommendations for athletes to drink to thirst during training and competition are problematic because there is not always an opportunity to consume fluid immediately when feeling thirsty in many sports.

Recommendations to have a planned fluid intake strategy to restrict fluid loss to no more than 2% of body weight can also be problematic because body mass losses aren’t easy to measure in many sports. Athletes must rely on previous measures of fluid loss under similar conditions in order to implement this strategy. There is also limited evidence to suggest that a specific level of dehydration is likely to impair performance across all exercise tasks. Therefore, there is no fluid intake strategy that will suit all athletes in all conditions. Burke (2019) has developed a framework (see below) to guide drinking strategies during exercise. The framework outlines specific factors that should be considered when deciding on a fluid intake strategy that either promotes fluid intake or moderates fluid intake in order to prevent excess consumption.

Athletes wanting to avoid any negative impact of dehydration should use monitoring tools to assess hydration status as well as performance to develop an understanding of how fluid balance affects their own individual performances during a range of training and competition scenarios. Specifically tailored hydration strategies can then be developed using this information.