The truth about the ‘eight glasses of water a day’ rule

Influencers have suggested drinking up to a gallon of water per day in order to banish weight, wrinkles and brain fog - getty
Influencers have suggested drinking up to a gallon of water per day in order to banish weight, wrinkles and brain fog - getty

A new survey suggests that 45 per cent of British people sip – ominous drum roll please - just one glass of water a day.

When editor of the Telegraph Magazine, Lisa Markwell, heard this news, she  said: “I can go days without drinking water, a week isn’t unusual,” says Markwell. “Physically I feel... fine, I think. But I do occasionally feel guilty. Could I be slimmer, have supermodel dewy skin and superhuman powers of concentration if I just drank water more diligently?”

NHS guidelines suggest we drink six to eight glasses a day. Influencers have evangelised the downing of up to a gallon in order to banish weight, wrinkles and brain fog. We are drenched in hydration hype, but just how scientific is it?

The eight glass rule

“Water makes up over half our body weight, so staying hydrated is essential for health,” says Dr Emily Leeming, Senior Nutrition Scientist at ZOE, a research programme based at King’s College London. “Even a small reduction in our hydration levels can cause headaches, dry mouth and dizziness. In the long term, it can be linked to urinary tract infections and kidney stones.”

That said: “The ‘eight glasses of water a day’ rule isn’t based on any scientific evidence. Generally, our bodies are really good at telling us if we need to drink more.”

Our hydration needs are highly personal, says Leeming. Plus, they change day to day, depending on factors like temperature and exercise. Thirst sensation also lowers in older age, increasing your risk of dehydration and making it more prudent to drink regularly.

But what of the specific superpowers claimed for water?

Brain power

“Even mild dehydration can temporarily affect your brain, making it harder to concentrate and problem-solve,” says Leeming. No need to think (or drink) too deeply, though. “You can tell if you need to drink more by tuning into your thirst and looking at the colour of your pee,” says Leeming. “When hydrated your pee should look like pale lemonade. If it looks like concentrated apple juice, then drink more.”

Water vs other sources

Nonetheless, hydration is not a “water or nothing” game: “Water, tea, coffee and squash do all contribute towards your overall fluid intake,” says Leeming. “Although the caffeine in tea and coffee is a mild diuretic, it’s in too small amounts to counter the hydrating properties. Food also gives us about 20-30 per cent of our water needs.”

This might explain Lisa Markwell’s miraculous ability to survive and indeed, thrive, without drinking the stuff straight. “I don’t eat a huge amount of fruit, but I do wolf down lots of salad,” she says. “And I also drink industrial quantities of tea - both ‘builder’s’ and fennel.” So let’s drink to that.

Although the caffeine in tea and coffee is a mild diuretic, it’s in too small amounts to counter the hydrating properties - Alamy
Although the caffeine in tea and coffee is a mild diuretic, it’s in too small amounts to counter the hydrating properties - Alamy

Silky skin

“A diet that is good for your general health is good for your skin, so drinking an adequate amount will have benefits for  your skin,” says Dr Anjali Mahto, Consultant Dermatologist. However: “That doesn’t mean to say that you need to drink excessive amounts.”

Sun damage accounts for 90 per cent of premature skin damage, says Dr Mahto. The next culprit is genetics. “Drinking more water is highly unlikely to have any significant impact on preventing premature ageing,” she says. Ditto dry skin. If you want to improve your complexion, put down the four-litre water bottle and pick up a broad-spectrum sunscreen.

Exemplary digestion

“Dehydration is one of the most common causes of constipation,” says Leeming. “The water in your lower intestine gets drawn back in to hydrate the body, making the stools hard and difficult to pass.”

So make sure you drink enough. Drinking more water than your body needs, however, will not confer superpowers on your small intestine: “you usually just pee the excess out.”

Drop a dress size

Common myths about water’s weight-loss powers include the following: one, that we confuse thirst with hunger (so drinking lots prevents us from overeating); and two, that downing a glass of water before a meal will stop us eating so much.

Both are misleading, suggests Dr Saira Hameed, consultant in endocrinology and diabetes at Imperial College Healthcare NHS Trust: “The hunger and the thirst pathways in the brain are completely different. So there isn’t a biologically plausible explanation for mistaking thirst for hunger.”

Meanwhile: “Drinking water, thus leaving less room for food, might work for a short period of time but will not satisfy the body’s need to re-fuel and so it’s likely you will feel hungry again quite soon.”

That said, staying adequately – not excessively - hydrated is “probably somewhat” important for weight management, she says. “Lack of hydration can make us feel tired and sluggish, leading to sugar cravings, and water is also needed for biochemical reactions associated with weight loss, like lipolysis - or fat breakdown.”

Water retention can also result from dehydration.  “This can temporarily stall weight loss,” says Dr Hameed. However as this doesn’t represent an increase in fat mass, ie is not true weight gain unless the water retention is a symptom of serious underlying disease such as heart failure, then the retention should be temporary.  “It’s these fluxes in body water that can dent morale when trying to lose weight and it’s for this reason that I advise my patients not to weigh themselves more frequently than once a week”.

As well as checking the colour of your urine, she advises to “check that your mouth and lips are moist and you are not feeling thirsty.

Organ health

“Inside your cells you have proteins, made up of amino acids,” explains Jodi Stookey, a nutritional epidemiologist who specialises in hydration. “When you’re dehydrated, the cell will break down the proteins into little amino acid pieces, like Lego blocks, so the concentration inside the cell rises, creating an osmotic gradient that pulls water back in and helps the cell to hold water. If you’re hydrated, your cells don’t have to do that. They can use those blocks to make the proteins, protecting the organs and blood volume. That’s just one example of why you want to give yourself water.”

While she too debunks the “eight glass” rule, Stookey’s research suggests there is a measurable threshold for good health, and it’s somewhere between one and two litres of plain water.

“Does it make a difference if it’s plain water or something else? Yes, it does,” she says. That’s because most of the drinks you might snatch off the cornershop shelf contain a higher concentration of chemical particles than your blood. Healthy blood concentration ranges somewhere between 285 and 295 milliosmoles per kilogram. Apple juice is around 736, coke 493, red wine 2573.  So when you drink them, your body has to work harder to get hydration. Your body water has to go to your gut, to dilute the drink so you can absorb it,” says Stookey.


Are you evangelical when it comes to hydration? Let us know in the comments