Water versus Fluid: What Counts, What Doesn’t
In terms of hydration, the words water and fluid are often used interchangeably. However, other liquids, like coffee and fruit juice, are also classified as fluids. Is there scientific evidence to support that these beverages can be included towards meeting fluid recommendations?
The words water and fluid are often used interchangeably when discussing the intake of liquids by mouth to maintain good hydration. Water is indeed a fluid, but it is just one type. All liquids, including broth, fruit juice, coffee and soft drinks, are classified as fluids.
In meeting daily fluid recommendations, advice is often given to avoid beverages that contain caffeine (e.g., tea, coffee, cola drinks) or alcohol because they are “dehydrating.” However, is there scientific evidence to support this recommendation?
Caffeine and fluid balance
Studies have shown that among people who have not been exposed to caffeine for a period of days or weeks, drinking a large amount of caffeine-containing beverages in a short period of time (e.g., 2-3 cups of coffee or 5-8 cups of tea) results in an increased urine output. However, this does not happen to people who routinely drink caffeinated beverages. They develop a tolerance for caffeine and have a consistent urine output whether or not the fluids they are taking in contain caffeine or not. Even among those who do not routinely drink caffeinated beverages, there is no diuretic effect when caffeine-containing beverages are consumed in normal serving sizes. Therefore, the available evidence indicates that caffeine-containing beverages are no less hydrating than other fluids and can count toward fluid recommendations.1
Alcohol and fluid balance
The body’s fluid balance is largely regulated by a hormone called vasopressin. Also known as the anti-diuretic hormone (ADH), the pituitary gland releases it when the body needs to conserve fluid. In essence, ADH signals to the kidney that it should concentrate the urine and keep the body’s blood volume up. Alcohol blocks the release of ADH by the pituitary gland in a progressive fashion. In other words, the more concentrated the alcohol is in a beverage (and/or the more beverages consumed), the more urine the body releases and the gap shifts from balanced, to less hydrated, and, when alcohol is taken in excess, to dehydration. It is not possible to develop a tolerance to this effect2
. Because alcohol-containing beverages have the potential to adversely impact fluid balance, they should not be included toward meeting fluid recommendations.
The Weight Watchers Good Health Guidelines recommend drinking six glasses of fluid a day. All fluids, except alcohol-containing beverages, count toward meeting this guideline.
This content is reviewed regularly. Last updated December 17, 2011.
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Maughan RJ, Griffin J. Caffeine ingestion and fluid balance: a review
. Journal of Human Nutrition and Dietetics. 2003;16 (6): 411 – 420.
Eggleton, MG. The diuretic action of alcohol in man
. J. Physiol. 1942 Aug 18;101(2): 172-191.