In articles in print and on the web, and on television and radio, we are told to drink at least two litres of water a day, or eight cups of water a day, or some other amount.
But the main message is always that we should drink more water.
Since water makes up between three quarters and just over half of our body weight, depending on age, this water has to come from somewhere.
There is now a “Drink Up” campaign, that encourages us to drink more water, with a website that I really can’t understand. Maybe it’s just saying drinking water is cool (nothing wrong with that, of course), with it’s own #spreadthewater hashtag.
I have no reason not to believe that drinking more water is good for us. After all, everyone seems to be in agreement. But I do wonder what evidence there is to support the idea that we need to drink more water.
Health benefits of drinking more water
The suggestion in all the advice to drink more water is that we currently don’t drink enough. So how much water is enough?
Enough could mean the volume of water we need to drink to avoid dehydration. Or, enough could mean drinking the average amount for other people.
In either case, does enough mean two litres (or eight cups, depending on the source of the advice), and who says so?
Research on drinking more water
From the research literature it seems that other people have been interested in this question
There are articles with titles such as, “Human water needs”, “Water, hydration and health”, and “Drink at least eight glasses of water a day. Really?”, as well as others with heavy scientific titles.
The thing that surprised me right away is that there seems to be much less agreement about the health benefits of water than I expected. Any magazine, web article or TV programme I had seen all presented the drink more water message as an undisputed fact.
How much water is enough?
The first thing to say is that the amount of water we need to drink is related to how physically active we are.
As you might expect, when we are very physically active we lose a lot of water through sweating and this needs to be replaced.
Also, the temperature around us has an effect on this. Our water requirements can increase three-fold as the air temperature goes up from 20º C to 40º C.
This means that it’s not really possible to give a volume of water that everybody needs to drink all the time.
And this doesn’t even include special situations like pregnant or lactating women, who need even more water over and above their normal requirements.
We also tend to forget that the foods we eat can contain water. Fruit, for example, has a high water content. This means we could underestimate our water intake, depending on the foods we eat.
The term that’s used to describe whether we are getting enough water is “Adequate intake (AI)”.
Most of the scientific articles seem to say that we need enough to avoid dehydration and drinking more than this “adequate amount” doesn’t really bring many health benefits.
Adverse effects of dehydration
There are several ways to define dehydration. One is where the concentration of the blood increases by a specific amount. Another is where a person loses a certain percentage of their body weight.
The thirst centres in our brain is very sensitive to blood concentration, and will make us thirsty before we get to the stage of dehydration.
This means it is unlikely that someone with access to clean drinking water will become dehydrated under normal conditions.
For example, dehydration can be described where the blood concentration has increased by 5%. Our thirst response is stimulated when the blood concentration increases by less than 2%, so we should start taking on water before dehydration can happen.
The result? We start feeling thirsty when our blood concentration is still “normal”.
People doing very vigorous exercise can lost 6-10% of their body weight through sweating, so it’s clearly important that this fluid is replaced as soon as possible.
Sports and academic performance
From a sports performance point of view, athletic performance starts to deteriorate as soon as the dehydration level reaches 2%, and goes downhill from there.
Types of athletic performance that deteriorate due to dehydration include reduced endurance, increased fatigue, altered body temperature control, reduced motivation and an increase in perceived effort, making everything seem more difficult.
Thinking power is also reduced by dehydration. For example, concentration, alertness and short-term memory are affected by mild to moderate dehydration.
We hear most about the effects of dehydration in school children, who are encouraged to drink water regularly through the school day to help them avoid this.
Health benefits of adequate water intake
There is scientific evidence that suggests adequate water intake can help to avoid a variety of health problems. Here are some examples.
- Kidney stones – increased urine volume from increased water intake reduces development of kidney stones
- Exercise-related asthma – this can be made worse by lack of fluid intake when exercising
- Heart disease – increased water intake can reduce the risk of fatal heart disease
- Thromboembolism and strokes – this is the formation of clots inside the blood vessels and dehydration can increase the risk
- Urinary tract infection – urine can help wash bacteria out of the urinary tract, and increased water intake leads to increased urine production
So it seems to be that drinking enough water protects us from the problems caused by dehydration, but drinking more than enough doesn’t help us any more than that.
Doesn’t drinking more water help our skin?
Of all the so called “facts” about the benefits of drinking more water, the one that comes up most of the time is that it helps to keep our skin young and beautiful.
The benefits often touted for drinking water and our skin include making it more moisturised, making it smoother, flushing toxins from the skin, reducing wrinkles and slowing ageing.
There’s no doubt that dehydration reduces the elasticity of the skin, and this is used an an indicator in of hydration status healthcare. But there doesn’t seem to be much evidence that drinking more than an “adequate” amount of water produces the benefits above.
How much water is enough?
Studies have looked at water intake in various countries over the entire age range. Water intake seems to increase from 1300ml per day in very young children, up to 2700ml per day for adult women and 3700ml per day for adult men.
This is total water intake from all food and drinks
Estimating water requirements needs assumptions about body size and physical activity. One way of looking at water requirements is in relation to energy requirements.
This brings us back to the fact that people who are more physically active need to drink more water.
One interesting fact about water intake is that comparisons between different countries show much lower water intake in Europe compared the the USA.
Adverse health effects of increased water intake
Under normal conditions there aren’t really any risks associated with increased water intake. The water balance systems in the body increase the production of urine, so any excess fluid is lost quite quickly.
People with kidney disease have to be more careful since the kidneys are the main organs responsible for ridding the body of excess water.
The main danger from excess water is blood and body fluids that are too dilute. This causes water to build up in various organs, which can damage them.
There is quite a well know example of this that you might have heard about. A few years ago a radio station in California held a competition to win a Nintendo Wii called, “Hold your wee for a Wii”.
The winner would be the person who managed to drink the most water without going to the bathroom.
A 28 year old woman drank nearly two gallons of water and died several hours later of water intoxication. The cause of death was cerebral oedema – a build up of water in the brain, causing it to swell
So, should we drink more water?
From the above it would seem that a number of short term, and more serious long term problems can be caused by drinking too little water leading to dehydration.
Drinking more water helps us to avoid dehydration and the problems that can go with it. For most healthy people it seems that increasing water intake (within limits) is safe, and although the benefits of drinking water above “adequate intake” are not really proven, it shouldn’t do us any harm.