Staying hydrated is more than a buzz-term — but how much water should you drink daily? 

He said he drank 150 ounces (4.4 litres) of water “on a given day” and close to twice that amount — about 2.3 gallons (8.7 litres) — when he exercised.

However, there is evidence that too much water consumption can lead to water intoxication.

 A paper in the Journal of Clinical Pathology (2003) outlined how initial symptoms are similar to psychosis, with inappropriate behaviour, delusions, hallucinations, confusion and disorientation. 

If untreated the symptoms may progress from mild confusion to acute delirium, seizures, coma and death.

Wellness influencers often encourage us to drink more water, preferably from an elegant stainless steel thermos. You would think the evidence for the benefits is overwhelming, but it is pretty limited.

In a 2004 report, the US National Academy of Medicine suggested that men drink 13 8oz cups daily (about 3 litres) and women drink nine 8oz cups daily (slightly more than 2 litres). However, the evidence for this advice is minimal.

We know water is essential, but how much evidence is there that drinking more water is better?

A paper published in JAMA Network Open this month attempts to break down the evidence.

Researchers from the University of California, San Francisco, scoured the medical literature for randomised control trials of water intake. These trials take a group of people and randomly assign some to the intervention — in this case, drinking more water — and others to continue doing what they normally do.

Of the 1,500 papers reviewed, only 18 met the rigorous criteria required to be included in the analysis.

Of the 18 trials in the review, none included the often promoted energy, wellness, and skin tone, though one study examined overall quality of life.

So, what did the studies find?

One of four weight loss studies showed that increasing water intake did not affect weight loss. Two studies showed an effect, but drinking water was combined with a low-calorie diet, making its impact difficult to measure. 

In one study, participants were asked to drink half a litre of water before meals, and that group did lose more weight than the control group – about a kilogram over 12 weeks. Overall, the evidence linking water intake to weight loss was mixed.

Of the studies that examined the link between water intake and fasting blood glucose, one trial suggested that higher pre-meal water intake lowered blood glucose levels; the other (which looked just at increasing water intake overall) didn’t. Again, the results were mixed.

Regarding headache relief, one study showed that increasing water intake by 1.5 litres per day improved migraine-related quality of life but didn’t change the number of headaches per month. In contrast, the other study showed no improvement.

For preventing urinary tract infections, the results were mixed with one positive trial and one negative one.

The kidney stone trials showed the clearest benefits. Increasing water intake to achieve more than two litres of urine daily was associated with significantly reducing kidney stone recurrence. Kidney doctors consistently advise people with a history of kidney stones to drink more water.

What about quality of life? In one study, participants were asked to drink 1.5 litres extra water per day, while made no changes. Six months later, there was no difference in scores on a quality of life survey.

The researchers pointed out that all the studies in the review were relatively small, with most enrolling less than 100 people. The effect of extra water would need to be potent to detect it with such small sample sizes.

Prof F. Perry Wilson, a nephrologist at Yale New Haven Hospital, Connecticut, explains in ‘How Medicine Works and When it Doesn’t’ (2023) that our bodies are exquisitely tuned to manage how much water we carry.

“As we lose water throughout the day from sweat and exhalation, our blood becomes a tiny bit more concentrated — the sodium level goes up. Our brains detect that and create a sensation called thirst. Thirst is one of the most powerful drives we have.

“Animals, including humans, when thirsty, will choose water over food, over drugs and over sex... We drink when we are thirsty. And that may be enough.”

Wellness influencers often claim that drinking more water flushes toxins out of your system. Perry explains that’s not how it works. “The clearance of the blood happens way upstream of where your urine is diluted or concentrated.”

For the wellness, skin tone, detoxing — the evidence points in one direction: Drink when you’re thirsty. Drink a bit more if you’d like, but there is no need to go overboard.

• Dr Catherine Conlon is a public health doctor