Frequently Asked Questions

Many people still believe that drinks which contain caffeine, such as coffee, tea and soft drinks, can act as a diuretic and promote dehydration. Here are answers to some frequently asked questions.

1. Drinks containing caffeine will cause your body to lose even more water than normal – true or false?

False. Despite what you often read in popular magazines and travel guides, there is no evidence to support this claim. In fact, a moderate intake of coffee and other caffeinated drinks does not lead to dehydration and makes an important contribution to daily fluid intake.

2. Caffeine containing drinks are a main cause of dehydration during the heat of summer – true or false?

False. The main factors affecting water loss from our bodies are surrounding air temperature and humidity. If the temperature increase, and we sweat more and this loss is greater still if we undergo prolonged exercise in a warm environment.

3. How much water do we loose on average per day?

(a) 1150 ml  (b) 1800 ml (c) 2500 ml

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

4. How much do we need to drink?

Fluids are an important part of our diet and to maintain our body's water balance we should be drinking on average 6-8 cups (1.5-2litres) of various fluids throughout the day, and that will include caffeinated drinks such as coffee. This 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.

5. Who might be at risk of dehydration?

People at particular risk of dehydration, and its consequences, are the very young and the elderly. Even physically active healthy children may be at risk during periods of hot weather. They have a relatively greater skin area, so sweat more to keep cool.

The elderly will often reduce fluid intake after lunchtime to avoid getting out of bed during the night. The sensation of thirst is often also reduced in elderly people, so they may not always drink enough to replace normal fluid losses.

There are also some occupations, such as fire fighting and some factory work, where those undertaking hard physical work in hot environments need to top up their fluid levels on a regular basis, as do we all when in hot and humid climates.

6. What are the signs and effects of dehydration?

Frequent headaches, tiredness, constipation and nausea, are all typical symptoms of dehydration. A dry mouth or dry lips are also an indicator that our fluid intake is insufficient. Another useful guide is the colour of urine – it should be straw coloured.

Doctors also agree that ongoing mild dehydration does have a negative effect on general well-being. Severe dehydration however is detrimental to health and is associated with the less efficient functioning of the heart and kidneys and generally makes us feel unwell.

7. So, what makes us thirsty?

We are consciously stimulated to drink when our blood volume falls and there is a rise in the concentration of sodium in the blood. This mechanism is rather insensitive in us and we may already be slightly dehydrated before we are prompted to drink.

8. Where do we get caffeine from in our diet?

Caffeine is present in many products. Main sources of caffeine in the diet come from tea, coffee, cocoa, chocolate and cola and “high energy” drinks (specifically those which contain large amounts of caffeine as well as high levels of sugar).

9. What happens if you drink 3-4 cups of coffee a day – does your urine output stay increased?

No, regular consumers of caffeinated drinks, including coffee, have a higher tolerance to the mild diuretic effect, so the overall long term effect on hydration is very similar to water.

In the same way as we are advised to eat and enjoy a variety of foods, so we should enjoy a variety of drinks. Fluids are an important part of a healthy diet and we should be drinking at least 6-8 cups (1.5-2 litres) of fluid a day to maintain our body's fluid balance. Drinking coffee can make a positive contribution to that intake, as it has also been shown to have other health benefits.

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