Diet sodas, typically prepared with artificial sweeteners such as sucralose or aspartame, give you a low-calorie alternative to traditional sodas made with sugar. Unless you choose a caffeine-free variety, however, you still risk dehydration if you consume the drink in large amounts. While it may be hard to think of a fluid being dehydrating, diet sodas with caffeine can have diuretic effects.
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Your body relies on a number of key hormones and your kidneys to maintain fluid and mineral balances in your blood. When you need to conserve water, your kidneys signal your brain to release an anti-diuretic hormone that helps you retain water. This makes your urine more concentrated. Caffeine affects the production of anti-diuretic hormone. Because the hormone will not be sent to your kidneys, they will release additional water, which can have a dehydrating effect.
Although they contain dehydrating caffeine, moderate diet soda intake should not cause significant dehydration. For caffeine to have noticeable effects, you have to drink a lot of it -- several cups. Because a 12-ounce diet cola contains about 45 milligrams of caffeine, you would have to drink at least 11 12-ounce diet sodas to drink enough caffeine for it to be dehydrating. However, if you live in a dry and hot climate, you might be more sensitive to caffeine's dehydrating effects, notes the University of Arizona.
A number of research studies have found that the caffeine in diet sodas can be a diuretic, but not a significantly powerful one, according to "The New York Times." A 2005 study published in "The International Journal of Sport Nutrition and Exercise Metabolism" followed 59 active adults over the course of 11 days and measured their urine levels after consuming caffeine and when they did not consume caffeine. The researchers did not detect a significant difference in urine volume over the course of the study.
Caffeine affects some people more than others. If you feel as though you are more thirsty or less hydrated after drinking one or several diet sodas, consider switching to their caffeine-free counterparts or giving up soda altogether in favor of water or other decaffeinated beverages.