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Coffee has no effect on dehydration

Good news for coffee lovers. Drinking moderate amounts of coffee doesn’t cause dehydration, and contributes to the overall amount of fluid we consume in a day.

BMJ Group News

What do we know already?

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There’s no clear guidance about the ‘right’ amount of fluid we need to drink a day to avoid becoming dehydrated. There haven’t been any good-quality studies that have shown that drinking a certain amount of water - the often recommended two litres a day, for example - is good for health, or that drinking less than this amount can cause dehydration.

There are other unanswered questions about fluid and dehydration that mean that people can get the wrong messages. Does the fluid we drink need to be water? Do other popular drinks, such as tea or coffee, contribute to our daily fluid intake?

To find out if drinking coffee can cause dehydration, researchers looked at the effect of coffee drinking on hydration levels in 50 healthy adult men who normally drank between three and six cups of regular-strength coffee a day. The participants were tested in two phases. In the first phase they drank either 200 millilitres of black coffee or water every day for three days. In the second phase, 10 days later, the participants who had initially drunk coffee switched to water and vice versa.

During the two three-day test phases the participants were told not to do any physical activity apart from walking, and were given a controlled amount of food and beverages to consume at set times. They were also weighed, given blood and urine tests, and tested to measure the total amount of water in their body, to look for signs of dehydration.

In the 10 days between the two phases, the participants were able to consume their usual diet and as much coffee as they normally would. The researchers then compared people who drank coffee with people who drank water to see which group were more dehydrated.

What does the new study say?

There was no significant difference in the change in total body water or body weight between men who drank coffee and those who drank water.

There was no difference in the amount of urine produced by men who drank coffee compared with men who drank water, and no difference in the amount of fluid in their urine or in their blood.

How reliable is the research?

The men in this study were put through a battery of tests designed to measure fluids in the body. These tests have been used in previous studies and are a generally accepted way of measuring signs of dehydration. This is more reliable than asking people if they feel dehydrated, which could be affected by the knowledge they are drinking coffee or water.

The researchers say the tests would have been more reliable if the men were kept in a hospital, to be more certain of what they were consuming and how much physical activity they took part in. But this would not have reflected people’s real life experiences.

The researchers excluded women from this study, as the researchers said there was a possibility that the changes in women’s blood hormones could have affected the results. So we don’t know if the results apply to women.

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