Drinking too much water could overload your system when you have a cold. The
doctor's advice to "drink plenty of fluids" when you have a respiratory
infection is not based on sound evidence, and could even be dangerous,
Australian researchers warn. Professor Chris Del Mar and colleagues from the
Centre for General Practice at the University of Queensland in Brisbane
report their findings in today's British Medical Journal.
"[The advice to drink water] doesn't have any empirical evidence to support
it and has some theoretical reasons why it might not be a good idea," Del
Mar told ABC Science Online. But the advice had been around for a long time.
"We don't really know where it comes from. It's from time immemorial,
ancient times; we think even Roman times it may have even been used."
Theoretically, drinking more when you are sick replaces fluid lost by
evaporation when the body has a temperature. It also helps to break down and
drain away thick mucus. But when Del Mar's team examined the literature they
found no studies that showed drinking more fluids made people better. And
they found some evidence suggesting the contrary.
"We found data to suggest that giving increased fluids to patients with
respiratory infections may cause harm," the researchers reported. They found
some evidence that people with lower respiratory infections, so-called
"chesty colds", who drink extra fluids were more at risk at having fits due
to low sodium levels. Del Mar said researchers were not certain why these
people had low sodium but it was possibly due to secretion of antidiuretic
hormone (ADH). This is a hormone the body releases when it is under stress
and makes the body retain fluid.
"It's obviously a protective mechanism designed to protect animals when
they're very sick and are unable to go and get food and drink. They then
shut down fluid loss and retain fluid until they get well again and go and
get fluid again," said Del Mar.
If a person secreting ADH is given additional fluid, he said, their body
could become overloaded with water: "And if you get too much water in the
body you get a thing called cerebral oedema which is waterlogging of the
brain and that can cause fits." Less extreme symptoms of low sodium are
irritability, confusion and lethargy. Secretion of ADH had not been observed
in people with upper respiratory tract infections, like the common cold. "To
date there are no randomised controlled trials to provide definitive
evidence, and these need to be done," the researchers wrote. "Until we have
this evidence, we should be cautious about universally recommending
increased fluids to patients, especially those with infections of the lower
respiratory tract."
Del Mar admitted it would be "tricky" to design a randomised controlled
trial to provide the evidence; it would require a lot of people, a lot of
money and was not in anyone's particular interest to do it. He could not
advise how much water would be too much to drink when you were sick. But he
thinks people with respiratory infections should not be encouraged to drink
more than they feel like drinking: "Just be guided by your thirst. Your body
will tell you," he said.