I am confused about coffee. I remember reading somewhere that coffee is only a mild diuretic and doesn't contribute to dehydration. But after I drink a cup of coffee or an espresso drink, my mouth feels dry and I need to pee more frequently. So I am hoping you can clarify this. Is coffee a diuretic? And how exactly do diuretics work? Is coffee dehydrating, or is it the caffeine?
— Confused Coffee Lover
Don't worry, you are not the only one out there confused about coffee! For years, medical experts had been saying that the caffeine in coffee acts as a potent diuretic, which leads to excessive urination resulting in dehydration. But, like you mentioned in your question, recent research has been published on this topic and the results appear to contradict the prior notion about coffee. Most studies have shown that in moderate amounts, the caffeine in coffee only has mild diuretic effects and does not lead to dehydration. To learn why, keep reading.
A diuretic is any drug that elevates the rate of urination. There are specific diuretic medications that are used for multiple medical conditions including high blood pressure, glaucoma, and heart failure. There are several categories of prescription diuretics, and they differ in the way they act on the kidney's water absorption mechanisms. All diuretics increase the excretion of water from the body, and each category of diuretics does so in a distinct way. There are many adverse effects from taking prescription diuretics, and these include dehydration, potassium abnormalities (high or low levels), low sodium levels, and acid-base disturbances. Diuretic medications should only be taken if a physician prescribes them to you for a medical condition, given their adverse effects.
A recent study on the diuretic effects of caffeine, by a scientist at the University of Connecticut, appeared in The International Journal of Sports Nutrition and Exercise Metabolism. Based on a review of 10 scientific research studies, the scientist concluded that when coffee or a caffeinated beverage is consumed, the body retains some of the fluid and that caffeine consumption causes a mild diuresis that is very similar to the diuretic actions of water. He also reported that there is no evidence that consumption of caffeinated beverages causes fluid or electrolyte abnormalities. Another interesting point he made is that a person who regularly consumes coffee/caffeine has a higher tolerance to the diuretic effect. This means that someone who regularly consumes coffee/caffeine would have to consume more coffee to have a diuretic effect compared to someone who does not drink coffee regularly. Additionally, the diuretic risk for coffee/caffeine is determined by multiple factors including the amount consumed and a person's tolerance to caffeine.
Another study, reported in the same journal in 2005, involved research scientists studying 59 active adults over an 11-day span while controlling their caffeine intake. The participants in the study were given caffeine pills on some days and on other days were given a placebo pill. They found that there was no evidence for dehydration or fluid status changes based on no significant differences in levels of electrolytes or total urine volume.
The take-home point here is that coffee and caffeine consumption in moderation can have a mild diuretic effect, but that its diuretic effects are similar to that of water. Thus, caffeine consumption in moderation does not lead to dehydration. Regardless, caffeinated beverages can make you jittery, sleepless, or anxious. Water is always your best bet for staying hydrated. Not only is water caffeine-free, but it is also calorie-free (which counts as a big positive point in my opinion, compared to the high calorie levels in most sugary caffeinated beverages or fancy espresso drinks).
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