Why is it that people are shaming coffee and coffee drinkers! Some people believe that for every one cup of coffee that you drink, you require 2 cups of water to offset the dehydrating effect of that one cup of coffee. Others would state the obvious; "do you realize how I made this pot of coffee? With a pot of WATER!" Being that coffee is my favorite beverage, I wanted to set a few things straight about whether or not coffee dehydrates you. Although this is not the only critique against the beverage, more "facts" can be found in my blog post titled "Caffeine and Endurance".
The available literature suggests that acute ingestion of caffeine in large doses (at LEAST 250–300 mg, equivalent to the amount found in 1–2 cups of coffee or 4–6 cups of tea) results in a short-term stimulation of urine output in individuals who have been deprived of caffeine for a period of days or weeks. A profound tolerance to the diuretic and other effects of caffeine develops, however, and the actions are much diminished in individuals who regularly consume tea or coffee. Doses of caffeine equivalent to the amount normally found in standard servings of tea, coffee and carbonated soft drinks appear to have no diuretic action, in this case.
The most ecologically valid of the published studies offers no support for the suggestion that consumption of caffeine-containing beverages as part of a normal lifestyle leads to fluid loss in excess of the volume ingested or is associated with poor hydration status. Therefore, there would appear to be no clear basis for refraining from caffeine containing drinks in situations where fluid balance might be compromised.
Well that clears some things up. Why have I heard this before then? Carla, are you telling me that all the other studies were just "wrong"?
Well the other studies that previously showed that caffeine dehydrates you, were flawed in the following ways: 1. Habitual drinkers were given a washout period, so when they measured the diuretic effects of caffeine a week or more post-washout period, they did find more urination and decreased body weight in these people. However, as stated above, this effect does not occur if not given a washout period. In layman's terms, if you drink coffee daily, you won't experience dehydrating effects of it. If you never drink coffee or caffeine-containing beverages, you will probably see a dehydrating-effect for the first few days while you adapt.
2. Body weight in some studies was not accounted for, so subjects were given an amount of coffee containing caffeine in milligrams rather than in mg/kg body weight. It is much more valid to control for weight when administering caffeine, because giving one person 250mg isn't the same as giving someone two times their size 250mg.
3. Many previous studies had their control group drinking decaffeinated coffee which contains other active ingredients that may have played a role in the results of those studies. More recent studies do not use any form of coffee for their control groups
All in all, having a cup of coffee or two per day shouldn't cause life-long dehydration, and despite popular myths, you do NOT need to drink 2 cups of water for every 1 cup of coffee that you drink.
Bell et al (1988). Effects of caffeine, ephedrine, and their combination on time to exhaustion during high-intensity exercise. European Journal of Applied Physiology and Occupational Physiology.
Burke, L. M. (2008). Caffeine and sports performance. Journal of applied physiology, nutrition and metabolism.
Ganio et al (2009). Effect of Caffeine on Sport-Specific Endurance Performance: A systematic review. Journal of Strength and Conditioning Research.
Harland, B. F. (2000). Caffeine and nutrition. Journal of nutrition.
Shearer, J. (2014). Methodological and metabolic considerations in the study of caffeine-containing energy drinks. Journal of Nutrition Reviews.