If it is a myth, it is a very widespread stubbornly persistent myth. If you have links to scientific sources debunking it as a myth that would much appreciated.

I came across it most recently in Tom Brady's athletic health book [0]. If I remember correctly, he states that you should drink double the amount of coffee consumed to offset the dehydration effects of coffee. This is simply to emphasize the ubiquitous nature of the myth, as he is someone who has presumably been surrounded by medical professionals for most of his athletic career, and who should have known better if what you say is correct. He singles out proper hydration as a major factor in his peak performance, something he claims younger athletes ignore.

[0] https://www.amazon.com/TB12-Method-Lifetime-Sustained-Perfor...

It does not surprise me even one bit that a professional athlete's book on health and nutrition contains health and nutrition myths, especially given other stuff that he has promoted.

Do you have links to scientific sources acknowledging it?

I participated in a time tracking exercise and I found that like-clockwork, without fail, I would urinate 1 hour after a cup of coffee at the office. On the way back from the bathroom, I'd get myself another cup of coffee. I had no idea I was having 5 or more cups of coffee every day.

I later tried to cut down or reduce my coffee intake. Most times I always resumed from a combination of missing the enjoyment/routine, flavour, and ability to focus. I finally at least cut out espresso shots, and mostly have a green tea in the morning and a coffee in the afternoon, and sometimes an (orange pekoe/earl gray) tea or a decaf coffee in the early evening.

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...

I have experimented with caffeine for years (periods of off and on) and noticed dehydration (it is a known diuretic after all). The problem with many of these types of studies that claim there is no effect of x from their inability to detect an effect of x is that they don't rule out sub-populations who may experience the effect. In other words, it's possible for there to be no effect on average, but for the population to be comprised of different groups who do and don't experience the effect. This is particularly relevant to the linked studies which (aside from the first one) have very small sample sizes (n=19, 21, 12) that would not differentiate sub-populations.

After reading the abstracts and skimming some of the studies you linked, I don't personally find the quality of evidence convincing enough to discount what seems to be a commonly reported anecdote about the effects of caffeine.

As an example, the second study makes a glaring statistical error (equating p>0.05 lack of a difference between groups with equivalence of groups) that renders the entire conclusion invalid.

The fourth study (which probably has the best methodology aside from the small sample size) actually did find caffeine has an effect on water retention.

The other studies use various indirect measures of hydration (blood osmolality and urine metabolites) where it is unclear if sub-clincical diuresis would actually impact these measures.

I believe the ABV for beer being hydrating is <3.4% or <3.2%. So it’s a very conservative session beer or one of those ultra low calories beers.