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

July 15, 2008

Following a low-sodium diet does not appear to have any appreciable impact of asthma control as once thought, according to new research.

"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," wrote Zara E. K. Pogson, M.R.C.P., clinical research fellow at the University of Nottingham in England.

The results of the randomized, 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 and either received sodium supplements to approximate normal sodium intake of 80 millimoles per liter (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 better control 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."

Source: American Thoracic Society

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