The common belief that consumption of alcoholic beverages, such as wine, results in dehydration is fully supported by scientific research. When we consume an alcohol-containing drink, the alcohol level in our bloodstream can rise very rapidly. When our blood alcohol level rises, a metabolic regulatory mechanism kicks in and our pituitary gland stops releasing one of the hormones that it stores up—a hormone called antidiuretic hormone, or ADH. When release of ADH is blocked, our kidneys know to start increasing the release of water from our body in the form of increased urination. Unless we compensate for this increased loss of water, we will become partly dehydrated. The metabolism of alcohol can also interfere with our water balance in other ways—all of them pointing in the direction of increased dehydration. Dehydration is one common contributing factor to hangovers.
Of course, there are enormous differences in the amount of alcohol that different individuals can consume. People vary in their ability to metabolize alcohol, and the amount of dehydration caused by a specific amount of alcohol will vary from person to person. In addition, it is important to take into account the strength of the alcoholic beverage. A glass of wine containing 14% alcohol is going to challenge a person's metabolism (and hydration level) more than a similar glass with 10%. However, for any given person, larger amounts of alcohol always cause greater dehydration than smaller amounts (just as you might expect).
I haven't seen any research whatsoever to determine exactly how much additional water drinking would be recommended for different levels of alcohol consumption. However, I'd be surprised if a person needed to make changes in his or her total daily water intake to help offset the amount of alcohol contained in the six-ounce glass of wine typically enjoyed at dinnertime. The routine consumption of wine or other alcoholic beverages, however, would definitely make it more important for a person to pay attention to daily water intake and to increase his or her water intake above the eight glasses of water routinely recommended.
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