Low-sodium advice for asthmatics should be taken with a pinch of salt

Contrary to past studies — which have suggested a link between low-sodium diets and improved asthma control — a new study by researchers at The University of Nottingham found no evidence that cutting back on salt helps patients with their symptoms.

Dr Zara Pogson, clinical research fellow at The University of Nottingham, said: “Despite the clear benefit of a low-sodium diet on cardiovascular risk factors, there is no therapeutic benefit in the use of a low-sodium diet…on asthma control in our study population.”

The results of the randomised, double-blind, placebo-controlled trial were published in the second issue for July of the American Journal of Respiratory and Critical Care Medicine by the American Thoracic Society.

Nearly 200 subjects completed the study which compared the effect of changes in bronchial reactivity — a measure of asthma activity — on asthma patients who followed a strict low-sodium diet. Each subject either received sodium supplements to approximate normal sodium intake of 80 millimoles per litre (mmol) a day, or a placebo, for six weeks.

Dr Pogson and colleagues hypothesized that the subjects on the low sodium intake would show improved clinical control of asthma symptoms based on a test of asthma activity, measures of lung function, asthma symptoms and use of asthma medication. Contrary to their hypothesis, however, they detected no differences in any measures of asthma between the groups.

“We observed no difference in the outcome measures related to asthma activity in adults with asthma and bronchial reactivity who adopted a low-sodium diet for six weeks compared with those who did not, despite a final difference in daily sodium excretion of 50 mmol,” wrote Dr Pogson.

While past studies have suggested a link between low-sodium diets and improved asthma control, none were as large or tightly controlled as this study, suggesting that their findings may have been artifacts of study design rather than reflective of a true therapeutic benefit.

“We were disappointed that a simple measure, such as a decrease in sodium intake, does not result in improvements in asthma control,” said Dr Pogson. “We therefore cannot advise people with asthma to alter their sodium intake to improve control of their asthma, despite the fact that a low-sodium diet improves cardiovascular risk factors. This study suggests that further dietary research in asthma should be directed to factors other than sodium.”

According to Asthma UK, 5.2 million people in the UK have asthma — and 50 per cent of these people have severe asthma symptoms that have a major impact on their daily lives.