Does your water bottle tell you how much to drink? Here’s when to obey your thirst instead

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Do you have a water bottle beside you right now?

Is it etched with words of encouragement, urging you to “keep chugging” because you’re “almost there”?

If so, you’re not alone. The popularity of reusable water bottles has recently exploded.

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It’s not just about sustainability anymore. They have become a way to express yourself through colour and design.

And they might even give you a much-needed sense of control.

“The world is very intimidating. There’s a lot going on. There’s a lot that feels out of our control,” University of Queensland marketing expert Ann Wallin says. 

“This is something we can measure — how much water we drink.”

Dr Wallin says water bottle brands benefit from health advice telling people they should be drinking 2 litres of water a day. 

For people who find meeting this recommendation a slog, it’s tempting to see a shiny new water bottle as a solution. 

But water bottles aside, where do these recommendations even come from, and what happens if you don’t meet them?

Why we’re told to aim for 2 litres a day

The origin of the 2L-a-day advice is hard to pin down, but similar guidance has been around since at least 1945.

And there is some rationale behind such recommendations.

In Australia, they’re based off the highest median water intake according to the 1995 National Nutrition Survey. 

So basically, the recommendation aligns with what many people already drink each day. 

For adults, 2–3L is common. 

These recommendations apply to all beverages, not just water. Milk, tea, coffee and even alcohol can contribute to total daily intake.

Of course some of those are diuretics, meaning they’ll make you urinate more often, so they won’t be as hydrating (or healthy) as plain water. 

Overall, beverages make up about 75 per cent of your daily fluid intake. The remaining 25 per cent comes from the food you eat. 

Glass of water on a pink table

Plain water isn’t the only way to meet daily hydration goals.(Getty Images: mrs)

“We don’t tend to think of water as a nutrient, but it’s the one nutrient that we are likely to become unwell from the fastest,” Monash University sports dietitian Alan McCubbin says. 

Water helps carry other nutrients and oxygen to cells, protects organs and tissues, lubricates joints, flushes out toxins and regulates body temperature. 

So why does it feel like a slog to drink the recommended 2L a day? Or, for others, like 2L a day is not nearly enough?

“The amount of fluid that we actually need is going to vary quite a bit from person to person,” Dr McCubbin says. 

Along with age and gender, how active you are, your body composition, if you’re pregnant or breastfeeding and where you live can make a difference too. 

So if the recommendations don’t apply to everyone, how can you be sure you’re drinking enough? 

Well, our bodies have developed a vital mechanism for that — thirst.

“The body has evolved very strong mechanisms to regulate all of this,” Dr McCubbin says.

“In most cases, we can trust those mechanisms.”

The science of thirst

If you’ve ever gone too long without water on a scorching day, you know how powerful thirst can be. 

You daydream about a cold bottle of water, or an ice-cold glass with condensation gliding down the side.

(If that made you thirsty, it’s time to take a sip from your trusty water bottle.)

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A number of things happen when we’re running low on water.

“As we lose water, we lose salt as well,” Dr McCubbin says. 

“But we lose proportionately less salt compared to the water, which means that our blood gets saltier and more concentrated.”

Receptors in the brain and carotid arteries, which run either side of your neck, pick up this change.

A hormone is released from the pituitary gland to suppress urine production.

Your blood volume drops, and likely your blood pressure too, triggering another cascade of events.

Your kidneys release the enzyme renin into your bloodstream, which triggers a process that generates the hormone angiotensin II.

Among other effects, angiotensin II alerts the hypothalamus to set off the sensation of thirst.

Once you’ve gulped down enough water, the concentration of water in your blood increases and your urge to drink settles down. 

But it is possible to overcorrect and drink more water than your body needs, which is why Dr McCubbin says it’s not necessarily a good idea to down 2L of water in one go. 

“You’re just going to produce more urine to get rid of what the body perceives as excessive water,” he says.

When to go beyond thirst

There are exceptions to the “listen to your thirst” rule.

Young children have an immature thirst mechanism and might not always notice when they’re thirsty, while the sensation of thirst can become diminished in older people.

Some medications can act as diuretics, or affect your perception of thirst. 

And neurodivergent people can have trouble translating signals their bodies are sending, like thirst, hunger and satiety cues. 

Athletes might also need to think beyond thirst, especially when general health isn’t their only priority. 

“If we’re trying to optimise performance, then there may be situations where drinking to thirst is not adequate,” Dr McCubbin says. 

This is particularly important for athletes involved in longer duration sports in hot and humid environments. 

Athlete drinking water

Humans have lots of sweat glands to regulate temperature, but it means we need to consistently restore water. (Getty Images: Commercial Eye)

However, over-hydration in athletes is also a concern.

While uncommon in your everyday office worker, drinking too much water is possible, especially in endurance athletes. 

It can lead to a potentially fatal disorder called hyponatremia where water dilutes essential sodium levels in your blood.

This is why many athletes will supplement with electrolytes to stay hydrated without throwing off their sodium balance. 

Whether you’re an Olympian or a regular person trying to do the best for your health, moderation is key.

“For most people, thirst will get them to to where they need to go, and plain water should do the job in 95 per cent of cases,” Dr McCubbin says. 

“Just make sure there is water available. There might be situations where that does mean carrying around the big water bottle.”

Water bottle collage

Reusable water bottles can be found for anywhere between a few dollars to over $100. (Getty Images: Daniel Grizelj)

At the end of the day, marketing expert Dr Wallin says, there’s nothing wrong with having a companion to help you hydrate, as long as you don’t fall into the trap of believing you can’t trust your thirst response. 

“People receive a lot of varying advice, so it can feel as a consumer as though it’s quite complex to be healthy,” she says. 

“I think health and wellness marketing is often really effective when it simplifies that advice.”

Listen to Dr Norman Swan and Tegan Taylor discuss the latest hydration craze on RN’s What’s That Rash? And subscribe to the podcast for more.

Get the latest health news and information from across the ABC.

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