The review looks set to continue to put pressure on the food industry to reach the targets set out by an increasing number of food agencies to cut salt intake. In the USA, UK and Ireland, over 80 per cent of salt intake comes from processed food.
The authors of the new review, Timothy Mickleborough from Indiana University and Andrew Fogarty from the University of Nottingham, analyse both epidemiological and clinical evidence and conclude that, collectively, increased salt intake may increase the severity of asthma for people with the disease.
Salt is of course a vital nutrient and is necessary for the body to function. But salt reduction campaigners claim that in the western world, an average daily salt consumption of between 10 and 12g is far too high.
Epidemiological evidence has supported, in general, that lower salt consumption was associated with improved airway responses.
On the other hand, four studies with a total population of almost 4,000 subjects did not support this hypothesis, a discrepancy that was difficult to explain, said Mickleborough and Fogarty.
"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 reviewers. "Alternatively, dietary sodium may not have an effect on asthma in the general population."
However, for people with asthma, the science appears to be against salt. Mickleborough and Fogarty quote several clinical trials that focussed on salt intake for people with the condition a number that is on the rise in the Western world and the most common long-term condition in the UK today.
Indeed, according to the European Federation of Allergy and Airway Diseases Patients Association (EFA), over 30m Europeans suffer from asthma, costing Europe €17.7bn every year. The cost due to lost productivity is estimated to be around €9.8bn.
"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," they wrote.
The reviewers called for a large randomised-controlled clinical trial to fully address the issue.
"As a low sodium diet has other beneficial health effects, it can be considered as a therapeutic option for adults with asthma, although it should be considered as an adjunctive intervention to supplement optimal pharmacological management of asthma and not as an alternative," they said.
Further study is also required to look at the potential causal link between salt intake and asthma, suggested the reviewers.
"If the relationship between higher sodium intake and increased prevalence and severity of asthma is causal, then there are potential population benefits for asthma as well as cardiovascular disease to be derived from public health measures to reduce sodium consumption," concluded Mickleborough and Fogarty.
Dr Lyn Smurthwaite, research development manager at Asthma UK revealed to FoodNavigator.com that the charity are currently funding a clinical trial in this area, led by Dr Fogarty, with results expected at the end of 2007.
"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 UK is keen to explore. We are delighted to be currently funding a trial by Dr Fogarty into the longer-term effects of a reduced salt diet on asthma symptoms," 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."
Source: International Journal of Clinical Practice December 2006, Volume 60 Issue 12 Page 1616-1624 "Dietary sodium intake and asthma: an epidemiological and clinical review" Authors: T.D. Mickleborough, A. Fogarty