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How Much Fluid Do You Really Need After Exercise?

fluid

Individuals are often advised to consume 1.5 times the amount of fluid lost during exercise and the recovery period.

By Ben Desbrow

The science behind rehydration following strenuous exercise is often lost in market hype.

You find yourself exercising in a very warm environment for 1–2 hours. You’re hot, your sweat has soaked your clothes and your mouth is as dry as a chip, but you have no access to a drink.

You're told you can stop. You can drink but you can’t eat! You are given one drink in a volume you can only just tolerate – 800 mL every 15 minutes. You’ve been told roughly that you’re drinking “a new type of sports drink” but you’re not sure exactly what it contains. Regardless, you guzzle it down because you know that for the next 4 hours you will again be deprived of food and fluid.

At the end of the 4 hour rest period you’re again required to exercise. This time it’s a maximal effort for 45 minutes. After what seems like an eternity, the pain finally ends. You’re exhausted, hungry and it’s 2 pm.

The first of your four trials has ended, you are able to eat solid food again and you are welcomed to the unique world of rehydration science by one of the people who’s been looking after you for the past 7 hours. “See you back here same time next week!” says the researcher.

Does this sound like torture? Does this seem a little impractical? When does this occur in real life?

This example describes the most commonly used study design to assess the rehydration potential of beverages. While not all studies of fluid recovery after exercise employ involuntary drinking protocols and the avoidance of food and beverage mixtures to assess fluid retention, most of the fundamental knowledge in this area has been derived from studies like the one described above. Why? Primarily because researchers focused on how much fluid is required to rehydrate the body following exercise and the direct comparison of beverage constituents over factors influencing ad libitum drinking behaviour.

So what have we learnt from research studies to guide fluid recovery advice following exercise?

During intense exercise individuals are unlikely to drink sufficient fluids to match sweat losses. This often means that the individual finishes exercising with a fluid deficit.

When the time between successive exercise or workouts is short – such as physically demanding work or training in the morning and again that afternoon – a fluid deficit leading into the second task can lead to a subsequent impairment in physical performance. However, the evidence that fluid loss impairs subsequent mental performance, such as reaction times and decision-making, is less certain – particularly when no heat-related stress is evident for the second task.

For complete rehydration, the volume you drink must be sufficient to replace losses through sweat as well as additional water losses during recovery from the exercise. Individuals are often advised to consume 1.5 times the amount of fluid lost during exercise and the recovery period. Note that weighing yourself before and after the exercise session gives an estimate of fluid loss, with a 1 kg weight loss equalling a 1 litre fluid loss.

How accurate this advice is for an individual is highly variable, depending on the duration over which the post-exercise drinking occurs. Faster drinking equates to greater additional fluid losses. For example, our laboratory has never observed complete fluid recovery in exercisers who have consumed 1.5 times their fluid losses in a 1-hour period, whereas it’s more likely that the same volume of the same drink consumed over 2 hours will allow complete fluid recovery.

Once a sufficient volume of drink is ingested, it is the drink composition that dictates rehydration success. It is well-known that the addition of sodium to rehydration drinks helps with restoring fluid balance after exercise. The sodium content of commercial beverages is usually limited due to issues with palatability: it is much easier to consume sodium within food.

Milk is a very effective post-exercise rehydration solution that enhances the restoration of fluid balance after exercise-induced dehydration by up to 50% compared with the same volume of traditional sports drinks. The effectiveness of milk as a rehydration solution is related to a number of its constituents. Cow’s milk is naturally high in sodium. However, just adding sodium to milk provides no further benefit to its fluid restoration capacity. It appears that the protein within milk is important for fluid retention, and compared with an energy-matched amount of carbohydrate has been shown to be more effective for post-exercise rehydration.

However, when individual constituents of milk beverages have been tested in isolation, such as whey protein, the effectiveness on rehydration appears less convincing. This suggests that greater emphasis should be given to the natural rehydration potential of milk rather than adding a single ingredient within milk to other non-milk-based drinks to try and improve their rehydration potential.

When dehydrated, drinking beverages that contain small amounts of alcohol (≤2% by volume) or caffeine causes only subtle losses in fluid compared with the same volume of beverage without these ingredients. Thus it is better to consume fluids with these ingredients rather than avoid fluids altogether.

What is equally important is that there are many factors such as thirst, personal taste preferences, gut tolerance, cost, social acceptance and beverage availability that all play a role in influencing the volume of a beverage you choose to consume following exercise or exertion. Factors influencing ad libitum drinking are as important to consider as a particular drink’s nutrients given that ultimately a beverage has to be consumed before its ingredients can influence the restoration of body fluids. For example, would gastrointestinal intolerance reduce the volume of a milk beverage consumed compared with a commercial sports drink? If so, does the smaller volume of milk consumed negate any benefit conveyed by its improved nutrient profile?

The lack of investigation into factors influencing voluntary beverage consumption is one reason our research group decided to look at the problem from a different perspective. We started experiments on a beverage with a high degree of social acceptance that is voluntarily consumed in large volumes following exercise – beer. Our recently published study demonstrated that a low alcohol beer with added sodium offers a potential compromise between a beverage with high social acceptance and one that avoids the fluid losses observed when consuming full strength beer.

Interestingly, when presenting the results from this study to scientific audiences the most commonly asked question is always: “Is the beer with sodium better than a sports drink?”. While this trial deliberately did not make a non-alcoholic comparison to avoid any “switch to beer” message, it seems even scientists seek the simple and marketable answer when the real answer is that “it all depends on the individual”.

What this study does highlight, however, is that by combining what we know about beverage ingredients in controlled environments, the potential exists for evidence-based innovations within consumer beverage markets. Happy drinking!

Associate Professor Ben Desbrow is Discipline Lead of Nutrition and Dietetics at Griffith University, Gold Coast.