“How Much Water Should I Drink?”

My personal training clients often ask me how much water they need to drink, and the answer is “it depends”.  Then they say “I heard it’s 8 glasses (around 2 litres) per day”, which is broadly true (4-6 glasses pure water is more accurate), subject to what I’m about to explain below, and depending on the size of the person and the situation. There’s no ‘one size fits all’ answer.


I saw an episode of Food Hospital last week (series 2, episode 5) which made some bold claims about how much water you should drink, and contradicted what I’d previously thought was true, so I researched it in a bit more depth. Problem is, different nutrition experts have different views, which is always confusing for the public wanting guidance on what to eat and drink.

The boldest claim was that the 8 glasses of water a day advice is a myth. I agree that this amount is not set in stone, but calling it a myth implies that it’s totally false, which might lead a lot of people to think that they don’t need to drink any pure water at all. Put it this way, I’d be much happier if my personal training clients drank 8 glasses of water a day than none, as long as they drank it in small quantities throughout the day, around 1/4 litre (250ml glass) at a time.

What are the downsides of following the ‘myth’ and drinking 8 glasses of pure water a day? None! Correction, it’s only a downside if circumstances dictate you should drink more, but we’ll come to that later. And if 8 glasses of pure water is more than your body needs, you’ll pee it out, so regular trips to the loo could be classed as a downside.

The Food Hospital experts claimed that you can get all your daily water requirements from a combination of the water in food, and any non-alcoholic drink, implying that you don’t need to drink any pure water at all. I disagree with this, for many reasons.

It’s true, there is water in food, and I agree it can count towards the 2.5 litres of water a day you need to maintain ‘water balance’, ie – to replace the water you lose through urine, faeces, sweating, and breathing. Fruit and veg is around 90% water, and meat & fish are around 50% water. Nutritionists vary in their estimate of how much water you get from food, and it depends how fruit/veg rich your diet is, so the daily amount of water from food can range from 1/2 litre to 1 litre.


Where I disagree strongly is the contention that you can get all the rest of your water from drinks like coffee, tea, fruit juices, fizzy soft drinks. I was really surprised to hear The Food Hospital say this. Their argument was that only alcohol is sufficiently toxic and diuretic to be ruled out as an effective contribution to your hydration. But other nutritionists claim, and I agree, that strong coffee and caffeinated energy drinks do not contribute to your hydration, because of the caffeine’s diuretic effect, which means it forces your body to urinate it out, so the net effect is negative.

Again we get into murky water (excuse the pun) when it comes to caffeine, because different people react differently to caffeine, so it’s more diuretic for some than for others. Decaffinated coffee is ok, so is tea in moderation (much lower caffeine content than coffee), and herbal teas (even better). Milk can safely count towards your hydration too. Double expresso, Red Bull, Coca Cola, all these do not count towards your hydration in my view. A couple of regular coffees a day won’t do you much harm, but I don’t think they count because the net hydration effect is probably around zero.

Another thing about tea and coffee. The tannins in tea can impede iron absorbtion, and iron is a vital mineral for your health. So more than 2 cups a day is probably not ideal. And the caffeine in coffee can impede calcium absorbtion, another crucial mineral, so limit yourself to 2 cups a day (and not those massive cups in Costa or Starbucks which are the size of a goldfish bowl).

As for fruit juice, the problem is not that it won’t contribute to your hydration, but that the insulin reaction can lead to fat gain over time, and the acid in fruit juice attacks tooth enamel quite aggressively, so plain water is healthier. If you must have fruit juice, dilite it 50/50 with water, and don’t have more than a couple of small glasses a day. In any event, fruit juice only counts towards 1 of your recommended 5 fruit/veg a day, however much you drink, because it lacks fibre.

Fizzy soft drinks are a big no-no because of the huge amounts of sugar they contain, and even sugar-free versions are bad for you because the artificial sweeteners wreck your metabolism and lead to weight gain. Funny how the soft drink industry doesn’t mention that in their adverts.

And high caffeine energy drinks are definitely diuretic, not to mention the sugar (or artificial sweetners). They’re also implicated in raised pulse, blood stickiness, and an increased risk of blood clots if you have a heart or circulatory condition.

These are some of the reasons why plain water is a better option than most other drinks, both for your hydration and for other aspects of your health too. So going back to the 8 glasses of water a day ‘myth’, it’s not such a myth after all, is it? Agreed, it doesn’t all have to come from plain water, but once you’ve calculated in the water from food (1/2 litre), the water from moderate intake of tea/coffee (1 litre at most, and ideally less), that still leaves a litre of plain water you should drink, which is 4 x 250ml glasses.

If you’re overweight, pure water is the best liquid you can drink. Zero calories, and gives you that full feeling which prevents you over-eating.

When might you need more than 8 glasses of water a day? If you’re losing more water than usual, through sweating (very hot day, playing intensive sport etc), or if you’re suffering diarrhoea or vomiting. In these cases you should consider replenishing your electrolytes too, the 4 vital mineral salts (sodium, chloride, magnesium, potassium) that regulate the fluid balance throughout different parts of your body. You can get electrolyte sachets from the chemist for this purpose.

Is it possible to drink too much water? Yes, and there’s a medical term for it, hyponatraemia (water intoxication), where your electrolytes are dangerously diluted, which impairs the function of your brain, heart and muscles. 8 glasses of water a day is nowhere near this danger level, but if you need to drink more due to excessive sweating etc , then dissolve some electrolytes into the water to restore your electrolyte balance.

Finally it’s worth summarising just how important water is for your body and why. Without any fluids at all you’d be dead within days. Water is vital for your body to function, it transports nutrients to all the cells of your body, and transports waste products and toxins away from the cells and out of your body.

Every single biochemical process in your body needs water. It regulates your temperature, it lubricates and cushions all your joints including your spinal chord. And just a  5%  deficit in your water balance results in dehydration.

So the notion that you need 8 glasses of pure water a day is a myth, only to the extent that the more accurate advice is 4-6 glasses of pure water a day (for an average adult under normal conditions), plus a diet that includes plenty of fresh fruit and vegetables.