Dehydration: how much should we drink per day?

By April Cashin-Garbutt, MA (Editor)May 30 2012

As the northern hemisphere approaches the summer months, many of us begin to think about increasing our fluid intake to prevent dehydration. But exactly how much should we be drinking a day? Does the amount vary depending on what we are drinking? And what happens if we don’t drink enough fluid?

How much should you drink per day?

The Department of Health estimates that we should drink around 1.2 litres of fluid per day, according to the NHS. (1)This equates to eight 150ml (around 5-oz) glasses.

The literature on fluid intake can be confusing. This is because some sources refer to the amount of fluid we should intake through beverages alone; whereas other sources refer to the total water intake.

This difference may be partly responsible for the confusion over how much water it is necessary to consume on top of the water found in your food.

Twin experiment on consumption on water in addition to that provided by food. Source BBC

According to a study carried out in the U.S. and reported by the Food and Nutrition Board of the Institute of Medicine of the National Academies, drinking beverages made up 81 percent of total water intake on average. Thus, water in food provided approximately 19 percent of water intake for those surveyed. (2)

The NHS reports an even higher amount of water coming from foods – around 1 litre per day. (1) The explanation for this difference probably depends on what foods the statistic was calculated on. The comparison of the water percentage provided by food is going to depend on your diet.

Your recommended fluid intake actually depends on how much food you consume. On average, people need 1-1.5ml of water for each calorie they consume. (3) For a person consuming 2000 calories this corresponds to 2-3 Litres of fluid per day.

The amount you should drink will also depend on how much exercise you are doing. The NHS recommends that, in addition to your normal daily intake, you should consume up to 1 litre (or 2 pints) of fluid for each hour of exercise you carry out. (4)

Water intake should also be increased during pregnancy. This is partly due to pregnant women being advised to increase their calorie intake. It is also to maintain fetal circulation, amniotic fluid and a higher blood volume. (3)

Essentially then, how much you should drink per day will vary from person to person. It will depend on a number of factors including what food you are eating and how much exercise you are doing.

How do different fluids vary the amount we should drink?

Caffeine is a diuretic which means that it makes your body lose water, i.e. it makes you urinate more frequently. Consequently, drinks that contain caffeine, such as coffee, tea, cola and energy drinks may have a negative effect upon your water intake. (5)

Specifically, a study published in the European Journal of Epidemiology found that water losses due to caffeine corresponded to 1.17ml per mg of caffeine. (6) This works out at a loss of 163.8ml per mug of filter coffee (based on it containing 140mg caffeine). (7)

The study also found that alcohol caused water losses of 10ml per gram of alcohol. (6)

Based on an average glass of wine, the alcohol content will cause you to lose more fluid than is provided by the glass itself!

This is estimated for a large (250ml) glass of wine of average strength (13%).(8) The alcohol content of the glass is 32.5ml, which equates to 25.6g of alcohol (based on 100% ethanol having a density of 0.789g/ml).(9)This means that the body will lose 256ml of fluid, which is a higher volume than the glass of wine itself!

What happens if we don’t drink enough fluid?

In essence, not drinking enough fluid leads the body to become dehydrated. One may initially have only mild dehydration and feel thirsty and uncomfortable, but dehydration can escalate and become very serious. In severe cases, dehydration can even result in death. (10)

Dehydration can have a range of effects upon the body including:

• Headaches
• Light-headedness or dizziness
• Lower volume of urine output
• Dark-coloured , strong-smelling urine
• Lack of energy
• Muscle weakness
• Dry mouth, eyes and lips
• Fast heartbeat (1, 10, 11)

There may be other longer term health problems that are caused by dehydration:-

• Constipation
• Kidney Stones
• Fatigue (10)

A recent study, published in the British Journal of Urology, stated that increasing water intake can reduce risk of kidney stones. (12)