They're as much a part of the Gen Z uniform as baby tees and flares but how necessary are these gallon-sized bottles of water and, fundamentally, how much water do we need to consume in a day to stay healthy?

On Today with Claire Byrne, Sarah Byrne, dietitian from, responded to a recent Irish Times article separating fact from fiction.

In the article, Dr Joel Topf, a nephrologist and assistant clinical professor of medicine at Oakland University, Michigan, states that for most young, healthy people the best way to stay hydrated is simply to drink when you're thirsty. Does Sarah agree?

"Absolutely. It’s really, really good advice. You can actually, if you want to, drink a little bit more and there’s no problem with it. And we do know that there’s huge long-term benefits to getting those one and a half to two litres of water that we talk about. We know that it reduces risk of getting kidney stones, we know it reduces risk of bladder cancer. But this thing of having to drink 3, 4, 5, 6 litres of water a day – there's really no evidence to support that."

And are there other conditions that adequate water consumption can stave off?

"If you look at something like kidney stones, which can just come from having concentrations of certain compounds in your urine [...] If you’ve fluid going through you're less likely to develop them and if you’ve ever had a kidney stone, you know you don't want another one and drinking lots of water definitely stops you from getting another one. [...] We look at conditions like gout, which is a little bit to do with collections of uric acid in the body and again, plenty of water helps treat it."

So, what is the H2O sweet spot?

"For overall health, really the research is telling us that for women around 1.6 litres of water a day, for men around 2 litres".

Sarah clarified that this doesn't mean sitting down and chugging it straight:

"It doesn't have to be water, so your small glass of fruit juice at breakfast time is going to count. If you put milk on your breakfast cereal that’s going to count. You’re looking at drinking maybe 1.5 to 2 litres of liquid a day and you will then also get water from your food. You look at different fruit and vegetables and things like that. I'd a thing up on my Instagram page there a while ago about bananas being about 75% water and most people couldn't believe it."

And what of the suggestion that drinking water can distract you from snacking?

"I think if you're genuinely hungry drinking water doesn't do a whole lot, but we do know that sometimes, particularly mid-afternoon, the signals around thirst and hunger can be a little bit mixed up so I often see people who think they're craving sugar but if they actually drink water, it seems to just go away."

And the... er... Status Yellow warning?

"On its own the urine colour is a guide [...] Most people will notice that first thing in the morning their urine will be a little darker. And that’s because you would have sweated really quite a lot of water into your bed overnight [...] it's about 500mls."

And dietitian Sarah explained that she does consciously try to drink a couple of glasses of water, but that as a devoted tea drinker, she’s covered in the hydration stakes:

"Alcohol is the only one really to watch because we do know that if you’re drinking a few drinks you're going to get dehydrated [...] Some people genetically, tea and coffee can make them a little more dehydrated but it’s unusual, so for most people tea, coffee will hydrate you as well."

And then to tackle the other hydration buzz word – electrolytes:

"For most people who are going out for a walk and doing a bit at the gym I wouldn’t really be worrying about them. Where we would look at electrolytes would be people [...] exercising for long periods. So, someone who’s running a marathon, people doing quite long distance running.

"What can happen there is they’ll be sweating as they go, and you don't just lose water in your sweat, you're going to lose salt, you're going to lose these electrolytes as well and these electrolytes help to balance water in the correct places in the body.

"You have a different level of water in the brain compared to your blood – different levels of water in different places in the body and they’ve to be carefully managed, and we do that with things like sodium and potassium. So, if you've sweated out your sodium and potassium and then you drink just water you’ve really lowered those levels in your body and actually it can cause difficulties."

And what about too much water?

"If you drink an awful lot of water, you dilute these electrolytes in your body and [...] these help to hold water in the correct places in the body, so if you wash out (particularly) a lot of sodium by drinking, now, we’re talking [a] huge amount of water–like 8 litres in an hour–you can actually even cause a bit of brain swelling which, in rare cases [..], it can actually kill somebody."

You can listen back to Claire’s full chat with Sarah Keogh here.