https://www.ncbi.nlm.nih.gov/m/pubmed/27225921/
So that link describes how it works in science jargon, I’ll try and simplify it for you. So your body is made of cells and those cells have channels, proteins, and a bunch of other things that interact with the environment with the sole purpose of keeping your ass alive. One of the most important things the cells regulate is water balance, so the blood can send stuff to other places and the cells don’t shrink up. The way the body does this is with dissolved salts, arguably the most important being Na+ (sodium). It uses this salt to direct water flow. So if there is a lot of Na+ in one area, water will rush to it (due to osmotic pressure).
This is the fundamental way kidneys work. With the power of hormones, water/salt concentration, etc they use salts to move water back into the body while leaving toxins (such as ammonia) which is excreted as urine. Link to a good illustration of this
http://umich.edu/~elements/web_mod/viper/kidney_function.htm
So how does caffeine mess this up? First it competes with receptors in the kidney making the kidney less effective at controlling the amount of water for use in urine (making it more watery). Second, it prevents Na+ from being reabsorbed by the kidney, which lowers the pressure for water to re enter the body (making the pee more watery).
With all that said, your question is a wives tale, it’s based in science but not fully fledged out. Caffeine is a diuretic, but it’s a tolerant diuretic. With several consecutive days of caffeine consumption your body will compensate and not be affected. On top of that you would need to drink ~4 cups of coffee worth of caffeine, all at once to see a significant diuretic effect. Notice I didn’t say 4 cups of coffee, I said 4 cups of coffee worth of caffeine. The water in the coffee counteracts the diuretic effect. The amount of diuretic volume loss of water the coffee has is less than the volume of water in coffee.
Also, an Americano is roughly equivalent to a black coffee.
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