So what is the net gain or loss? Surely we can quantify it, based on caffeine or alcohol content.
Oh, caffeinated drinks are still a net gain - in fact some studies like these ones claim that the net gain is almost the same as water - but it’s worth bearing in mind that caffeine does have a mild diuretic effect so it’s not quite as good as drinking water. This study actually claims its results show there was almost no difference in hydration between caffeinated and non-caffeinated drinks, nor any difference between calorific and non-calorific drinks, so you’re definitely correct that fruit juice is just as good as water. Even coffee is probably not too far off. From personal experience I find I feel noticeably less hydrated a few hours after drinking caffeinated drinks than I am after water though.
Alcohol has a more significant effect, particularly stronger alcohol like wine or spirits, as found in these studies and this study. Very weak beer (like 3%) is still a good net gain; I suspect spirits are a net loss. In between, I’m not sure.
From the link: “Body weight, urine and blood assays were measured before and after each treatment.” That measures the amount of water going into and out of the body as a whole, but doesn’t say anything about where it goes in the meantime. Body weight measurements can’t tell the difference between cellular and intercellular water, for instance. I also have to wonder what the effect of possible additional intake on the off days of the study might have been. It would be more useful if they’d been able to keep the subjects on a regimen for the whole time rather than only a couple of days per week.
I’d like to see something that measures the amount of water getting into the cells themselves before going back into the blood and to the kidneys, if that’s possible. I’m sure there must be some way to measure cellular and intercellular water levels. Because there might be just as much water in the bloodstream, and hence the urine, but as you and I and others have noticed, the tissues feel dehydrated after caffeinated drinks, and especially after alcohol. Even sugary non-caffeinated drinks can have that effect in my experience, though fruit juice is less likely to do so than, say, Kool-Aid. It’s not just diuretics as such that can cause that, but other chemicals as well.
I always thought this was touted as a weight loss method, not something about actually needing that much water. The idea is that water can help mediate between meal hunger. I admit that drinking a couple glasses of water when I’m hungry makes me not hungry. I just don’t do it because I also feel sick.
This is all you need to tell her - if she doesn’t immediately realise she’s talking twaddle about “only pure water counting” then she’s beyond reasoning with on this matter.
I drink maybe two glasses of water a day. The modern obsession with “hydration” is ridiculous, all it does is make you piss 20 times a day.
Although I tend to agree with you, there is some evidence that high water intake lessens the risk of bladder cancer (as well as kidney stones).