It’s a shame this is downvoted on HN of all places. I have seen various research that suggests both caffeine (in coffee & tea) and even alcohol (in lighter beers) do not dehydrate. I’ll stick with caffeine since that’s the topic.

The primary study that spread the idea that caffeine dehydrates is from a 1928 study with a sample size of three men[0]. It may very well be that caffeine does dehydrate, but I have yet to see overwhelmingly conclusive evidence to make such a claim as matter of fact. For now, I believe it’s safer to err on the side of moderate and regular caffeine consumption not causing dehydration to a perceptible level. This could of course be entirely incorrect, but I haven’t come across research that has convinced me otherwise so far. My understanding is the reason a cup of coffee (or tea’ does not dehydrate the consumer is because it is almost entirely water.

I think an important frying to remember is that science is a field of research and discovery, so we should try to avoid arriving at conclusions and refusing to accept differing opinions. Look at how we treated Ignaz Semmelweis for an extreme example of that type of fallacy.

[0] https://jpet.aspetjournals.org/content/33/2/167

More studies:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3886980/

https://pubmed.ncbi.nlm.nih.gov/11022872/

https://pubmed.ncbi.nlm.nih.gov/21450118/

http://www.ncbi.nlm.nih.gov/pubmed/9194998

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjourna...