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Fluid balance

Fluid balance vodcast

There are widespread guidelines on the amount and types of fluid we should be drinking in order to remain well hydrated. We’re often told to drink 2 litres of fluids a day and limit our consumption of caffeinated beverages, including coffee
July 24, 2012

Vodcast transcript

(Dr Trisha Macnair) There are widespread guidelines on the amount and types of fluid we should be drinking in order to remain well hydrated. We’re often told to drink 2 litres of fluids a day and limit our consumption of caffeinated beverages, including coffee.

Interestingly, there is very little evidence to support this specific recommendation.

So, what does the evidence show and what should we, as health professionals, be advising our patients and clients?

Joining me today is Dr Andrew Blannin of the School of Sport and Exercise Sciences at the University of Birmingham in the UK.

Andy, we hear a lot about how active individuals should avoid caffeinated beverages in order to stay well hydrated, and yet caffeine as a stimulant can improve performance.

Perhaps you could just talk us through the aspects of how performance is improved by coffee?

(Dr Andrew Blannin) Of course. The beneficial effects of coffee on sports performance are due to the caffeine within the coffee. In 2011, the European Food Safety Authority concluded that caffeine was indeed an ergogenic aid and that’s because it’s been shown to improve endurance performance, endurance capacity and ratings of perceived exertion.

(Trisha) How much caffeine do we need to see an effect?

(Andrew) Performance benefits can be seen with moderate intakes of caffeine, so 3mg/kg body weight, which is equivalent to 2-3 regular cups of coffee.

(Trisha) And what about hydration in these active individuals? Does coffee, or caffeine, have a negative effect on their fluid balance during exercise?

(Andrew) There is no evidence of any significant detrimental effect of moderate caffeine consumption on sports performance, nor is there any evidence that caffeine consumption adversely affects hydration status or our ability to regulate body temperature during exercise.

When we exercise there is an increased demand of the exercising muscle and we redistribute blood away from organs such as the kidneys towards those muscles. The outcome of that is that we produce less urine when we exercise so exercise has an anti-diuretic effect. The evidence suggests that athletes consuming caffeine to improve their performance don’t produce more, or less, urine than they would otherwise, so there is no evidence to support the notion that active individuals should avoid consuming caffeinated beverages such as coffee.

(Trisha) Now we’ve been talking about active individuals. What do we know about coffee and hydration in the general population? Is it true that coffee, like water, can contribute to our daily fluid requirements?

(Andrew) Research on fluid balance mainly focuses on athletes, and that’s because athletes are under significant pressure to be in fluid balance.There is mixed research on fluid balance in healthy individuals in the general population and that’s because many of those studies have significant weaknesses, such as the ridiculously large amounts of caffeine that they require their participants to consume. For example one of those studies required the participants to consume 642mg of caffeine, and that’s equivalent to consuming about 8 cups of coffee.

So whilst there might be a mild, short-term diuretic effect of consuming coffee, that seems to be mainly restricted to the studies where they recruited non-coffee drinkers or people that had been desensitised  to caffeine. When you look at the studies where they recruited regular coffee drinkers, the amount of caffeine that they would need to consume to induce this diuretic effect is extremely large so the scientifically rigorous literature suggests that moderate coffee consumption, about 3-4 cups per day, can indeed contribute to our daily fluid requirements. And that’s not surprising given that black coffee contains more than 95% water.

(Trisha) It sounds like more research is needed on the effects of caffeinated drinks on fluid balance in the general population. What aspects would particularly complement current understanding?

(Andrew) Most of the research focuses on athletes during exercise and what we need are more studies that look at the effect of coffee on fluid balance in the general population doing day to day activities.

(Trisha) I understand this is the scope of your current work, perhaps you can just tell me a bit about what you are looking at and why?

(Andrew) There is a need for a large scale study looking at the effects of moderate consumption of coffee in regular coffee drinkers. We want to see if there is a detrimental effect of consuming coffee on their hydration status; so would these people be better off if they consumed water rather than coffee? To achieve this aim, we are comparing the effects of consuming black coffee with the consumption of equal volumes of water in men who are regular coffee drinkers.

(Trisha) What else is unique about the study?

(Andrew) Another important aspect of our study is that we are using a battery of hydration measures, such as the use of stable isotope tracer techniques so that we can measure total body water. We also want to look beyond just a few hours, so we are making these measures 48 hours later. Our aim is to provide scientific evidence to inform the guidelines around the use of coffee in contributing towards our daily fluid requirements.

(Trisha) It sounds very pertinent to the current debate – when do you expect to have some results?

(Andrew) The results should be published in the next 6 to12 months.

(Trisha) We look forward to hearing more in due course but perhaps we could summarise now what we have heard so far. You have explained how drinking moderate amounts of coffee doesn’t cause dehydration, and in fact can contribute to our daily fluid intake. You’ve also explained how drinking caffeinated coffee can be beneficial to sports performance.

So, as an expert scientist, Andy, what advice would you suggest health professionals pass on to their patients and clients about drinking coffee and staying hydrated?

(Andrew) So, factors such as temperature, humidity and activity levels will affect our fluid requirements.

And although we are working on a large scale study to provide conclusive evidence, the majority of the literature suggests that moderate consumption of coffee, about 3-4 cups per day, can contribute to our daily fluid requirements.

(Trisha) Thank you Andy for this overview of current research into coffee, sports performance and fluid balance.