When it comes to the optimum amount of water to drink each day, everyone seems to have a different opinion. So should we be drinking 6-8 glasses, two litres, or even more? Do other liquids besides water count? And what are the warning signs that you're drinking too little water? We asked registered nutritionist Kerry Torrens for her expert opinion.

What are the benefits of drinking water?

When you stop to think that more than two-thirds of your body is water, it's obvious how important it is to stay well hydrated. Hydration is needed for digestion, for our heart and circulation, for temperature control and for our brain to work well. Water is, without doubt, the single most essential component of the human body.

Drinking water may boost mental performance

Research suggests that losing as little as 1% of your body weight in fluid may reduce mental performance, as well as potentially inducing fatigue and headache. This mild level of dehydration can easily occur over the course of a normal day’s activities, which highlights how important drinking little and often is for your health.

Drinking water may boost mood

Being dehydrated can also affect our mood and mental wellbeing, with studies suggesting that energy levels, cognition and emotions may all be affected.

Drinking water may boost physical performance

If you exercise, some studies suggest that as little as a 2% loss in your body’s water content may impact how well you perform physically. Dehydration may compromise your body’s ability to control its temperature, increase feelings of tiredness and unsurprisingly, make exercise more difficult. However, temperature, increase feelings of tiredness and unsurprisingly, make exercise more difficult. However, research in this area is conflicting. One small study which kept athletes in the dark about their hydration status showed that dehydration made no difference to their performance. Clearly other factors besides temperature, climate and endurance also play an important role here.
 

What are the symptoms of dehydration?

Thirst and passing dark-coloured urine are key signs that you may be dehydrated, as well as feeling lethargic, dizzy or having a dry mouth and lips. If you’ve been ill with diarrhoea and vomiting or fever, you can become dehydrated very quickly unless you replace the extra water lost from the body. In certain circumstances rehydration solutions can be useful because they help to replace the water, salts and minerals that your body has lost. If you are experiencing this, the NHS recommends that you consult a pharmacist who may recommend oral hydration sachets, and speak to your GP if your symptoms don't improve with treatment.

Checking the colour of your urine is widely considered to be the easiest and most practical way to assess your hydration needs – aim to pass urine which is light yellow to clear in colour.

Babies, children and the elderly are the most at risk of dehydration. 

How much water should you drink a day?

Each individual's needs are unique to them and depend on their health, age, size and weight as well as activity levels, the type of job they do and the climate they live in. Drinking little and often is the best way to stay hydrated. In the UK, the Eatwell Guide suggests you should aim for 6-8 glasses of water and other liquids each day to replace normal water loss – around 1.2 to 1.5 litres. Water, lower fat milk and sugar-free drinks, including tea and coffee, all count.

In March 2010, the European Food Safety Authority (EFSA) issued a report suggesting an adequate total daily intake of 2 litres of fluids for women and 2.5 litres for men. This quantity includes drinking water, drinks of all kinds and the moisture available from the food we eat. On average our food is thought to contribute about 20% of our fluid intake which, therefore, suggests a woman should aim to drink about 1.6 litres and a man should aim for 2 litres.

However, there is controversy surrounding our hydration needs. Some argue that there’s a lack of scientific evidence to support the perceived health benefits of drinking the often touted 2 litres a day, especially when it comes to those of us who live in temperate climates and who lead a largely sedentary lifestyle. However, the NHS still recommends that we consume around 6-8 glasses, with more required in hot weather or if exercising.
 

Do liquids other than water count?

Water, milk, sugar-free drinks and tea and coffee all count, but remember that caffeinated drinks like tea and coffee can make the body produce urine more quickly. Fruit juice and smoothies also count, but because they contain ‘free’ sugars (the type we are encouraged to cut back on), you should limit these to a combined total of 150ml per day.

Many of the foods we eat contribute to our fluid intake – for example, dishes like soup, ice cream and jelly, as well as fruit and veg with a high water content, such as melon, courgette or cucumber.

Can you drink too much water?

It is possible to go the other way and drink too much, although for most people with healthy kidneys this will be managed by urinating more frequently. Hyponatraemia is a condition caused by too much water which causes sodium levels to fall dangerously low. Athletes who participate in endurance events and take on too much fluid may be at risk of this condition.

When should I refer to my GP?

If you have not passed urine for more than 8 hours, you feel light-headed or lethargic, are suffering confusion or have a rapid pulse rate, refer to your GP or healthcare practitioner. You may also wish to consult your GP if you have been experiencing consistent thirst, because this can be a symptom of a chronic health condition, such as diabetes.