Doctors should be cautious about advising patients with respiratory infections to drink plenty of fluids, researchers have warned.
Colds trigger water conservation
Respiratory infections such as a cold or bronchitis stimulate the release of large amounts of a water-conserving hormone, a BMJ study reports.
Therefore giving extra fluids may run a risk of fluid overload and salt loss.
However, University of Queensland researchers have not been able to prove their theory categorically.
They searched the scientific literature, but were unable to find any trials providing definitive evidence that giving increased fluids to patients with respiratory infections may cause harm.
But they did undercover several cases in which patients with respiratory infections showed signs of salt loss.
They say that until it can be proved that taking extra fluids is safe, doctors should be cautious about recommending the practice to all patients - particularly those with infections of the lower respiratory tract.
Doctors often advise patients to drink extra fluids when they have a respiratory infection to replace fluid lost through sweating, sneezing, heavy breathing and having a runny nose.
It is also thought that drinking plenty of water helps to reduce congestion by keeping mucus secretions thin and runny.
However, the body automatically aids this process by producing anti-diuretic hormones that stimulate the kidneys to take as much water out of urine as possible.
Increased production of anti-diuretic hormones has been recorded in people with infections of the lower respiratory tract. However, it is uncertain that a similar process takes place in those who have upper respiratory tract infections.
The researchers argue that if the body is already conserving water, then topping up supplies still further runs a risk of upsetting the body's delicate salt balance.
In theory, this could lead to a condition called hyponatraemia, or excessive salt loss, which can cause irritability, confusion and tiredness. In extreme cases it may lead to convulsions or a coma.
Dr Kevin Gruffydd-Jones, a GP in Bath and a member of the General Practice Airways Group, told BBC News Online that he had doubts about the theory.
He said: "People with respiratory infections often tend to get breathless, and lose a lot of water vapour when they breathe out.
"Therefore, it seems a bit counter-intuitive not to take more fluid in."
Professor Andrew Peacock, of the British Thoracic Society, said: "This is an interesting observation that needs to be followed up with larger-scale research.
"There is no medical reason for people with a cold or bronchitis to drink any extra water in the absence of fever."