Coffee and Hydration – the Facts

Somewhere in the world today it is a hot sunny day, and amongst all the warnings we are given about the consequences of such heat, there is always one about not becoming dehydrated.

The caffeine in coffee has a mild diuretic effect, increasing the frequency of urination but not the amount of fluid passed. For many years health and exercise experts thought that, as a consequence, coffee and other caffeinated beverages promoted dehydration and didn't count as a source of fluid in the diet. We now know this is not true and there is no scientific evidence to support these views. Caffeine in fact is no more a diuretic than water. (8)

The truth is that a moderate intake of coffee and other caffeinated drinks does not lead to dehydration and does count towards daily fluid intake.

How did this myth start?

The normal average daily loss of water is around 2500ml of which urine accounts for approximately half. A further 800ml is from water loss through the skin and most of the remainder is from the lungs. This amount will of course vary and, in conditions of extreme heat and exercise, daily fluid loss may exceed 10 litres as we sweat more.

Urine output is often used as an indicator of dehydration. Earlier studies (1, 2,) only collected urine samples over a 2-4 hour period after caffeine consumption, not over 24 hours. It is now known that the period over which urine is collected is a significant factor. The small increase in urine output after 3 hours is compensated for by a decrease in output later in the day: i.e. total urine output over the day remains unchanged (3). In one study (4) the effect had already disappeared within 4 hours.

What is the latest evidence?

Professor Lawrence Armstrong, an avid runner and well respected scientist , decided to review the scientific literature to see whether abstaining from coffee and other caffeinated drinks was scientifically justifiable. In his published report (5) he concluded:

· After drinking a caffeinated drink, the body does retain most of the fluid ( up to 84% has been reported in some studies)

· Moderate consumption does have a mild diuretic effect, but the overall effect is very similar to water.

· Regular consumers of caffeinated drinks have a higher tolerance to the diuretic effect

· There is no evidence that consuming caffeinated drinks causes an imbalance of fluids and electrolytes, such as sodium and potassium, in the body, or that it is detrimental to health or exercise performance.

In 2003 (6), a further review of scientific studies published between 1996 and 2002 concluded that: “The most ecologically valid of the published studies offers no support for the suggestion that consumption of caffeine containing beverages, as part of a normal lifestyle, lead to fluid loss in excess of the volume ingested or is associated with poor hydration status”.

These conclusions are further supported in the most recent review (7) where it was noted also that in relation to hydration, there was no evidence to support caffeine restriction.

So how much should we drink?

Fluids are an important part of our diet and to maintain our body's fluid balance we should be drinking on average 6-8 cups (1.5 - 2 litres) of various liquids throughout the day. This will include caffeinated drinks such as coffee. The amount will need to be increased to around 8-12 cups (2-3 litres) in hotter weather, during and after exercise, or when experiencing high body temperatures associated with a fever.

Moderate intake of caffeinated drinks (250 - 300 mg caffeine/day – equivalent to 3-4 cups of coffee) will have no effect on hydration and will count towards this daily fluid intake (3). This lack of any chronic effects on hydration was also confirmed over a longer period in another study (8).

When we drink more fluids, our urine output is greater in response to that increased intake. Fluid balance is maintained as long as our intake matches our losses. However, the mechanism can be rather insensitive and we may already be slightly dehydrated before we are prompted to drink.

Conclusion

Despite being widely believed, the claim that caffeine containing drinks promote dehydration is simply not true.

The latest evidence, published in scientific and medical literature, concludes that moderate intakes of caffeine (250-300mg/day), have no long term effect on hydration and can actually count towards daily fluid intake. According to Armstrong: “Caffeinated fluids contribute to the daily human water requirement in a manner that is similar to pure water”. (9)

Good news for all of us, who enjoy a cup of coffee as part of a healthy lifestyle, to know that drinking coffee does make a positive contribution to that intake as well as contributing other health benefits.