Somewhere in the world today it is a hot sunny day, and amongst all the warnings we are given about the consequences of such heat, there is always one about not becoming dehydrated.
The caffeine in coffee has a mild diuretic effect, increasing the frequency of urination but not the amount of fluid passed. For many years health and exercise experts thought that, as a consequence, coffee and other caffeinated beverages promoted dehydration and didn't count as a source of fluid in the diet. We now know this is not true and there is no scientific evidence to support these views. Caffeine in fact is no more a diuretic than water.