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 t

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.