The review -- published in the December issue of the International Journal of Clinical Practice --analyzed both epidemiological and clinical data. According to researchers Timothy Mickleborough from Indiana University and Andrew Fogarty from the University of Nottingham, "Collectively, the studies to date investigating the potential relationship between dietary sodium and the severity of asthma or airway hyper-responsiveness have provided support for the hypothesis that increased dietary intake of sodium may increase the severity of disease in those with asthma."
It is widely accepted that salt is vital for normal body function in humans, but the average salt consumption of Westerners is between 10 and 12 grams a day; way too high, say experts.
Evidence gathered in the review also suggested that lowered salt consumption was linked to improved airway responses in non-asthmatics, but the researchers said at least four studies stood in contrast to this theory.
"Possible explanations for the inconsistency of the data are that dietary factors may have a different effect in children and young adults, as the age of the individual is an important factor in determining the sensitivity to sodium," said the scientists in their report. "Alternatively, dietary sodium may not have an effect on asthma in the general population."
Fogarty and Mickleborough said that a randomized and controlled clinical study would be needed to get more reliable data and prove whether or not there was a causal link between asthma and salt intake. Such a study is already underway, said Dr. Lyn Smurthwaite, research development manager for the charity Asthma U.K.'s, adding that Fogerty would be heading up the trial.
"Reducing salt in our diets is thought to be beneficial for many reasons and the possibility that it may improve asthma symptoms is something Asthma U.K. is keen to explore," said Dr Smurthwaite. "For several reasons, previous research has shown that eating lots of fresh foods can help to reduce symptoms in some people with asthma and we also know that many processed foods contain high levels of salt. With this in mind we would encourage people with asthma to eat a healthy diet of fresh foods that are likely to contain lower levels of salt."
The European Federation of Allergy and Airway Diseases Patients Association reports that more than 30 million Europeans have asthma, and the annual health care costs to Europe translates to about $23.2 billion (USD) and about $12.8 billion (USD) in lost productivity.