An Indiana University professor may have uncovered the mechanisms by which high-salt diets can trigger exercise-induced asthma, offering the most complete picture to date of how dietary factors can both aggravate and alleviate the symptoms of this common condition.
The study by exercise physiologist Timothy Mickleborough and his research team in IU Bloomington's Department of Kinesiology demonstrated for the first time that modifying salt intake for two weeks alters airway inflammation and the flow of oxygen into the bloodstream, termed the diffusion capacity of the lungs.
"These findings show that modifying your diet has the potential to modify a disease state," Mickleborough said. The findings appear in the June issue of Medicine & Science in Sports & Exercise, and Mickleborough presented them on June 3 at the annual conference of the American College of Sports Medicine in Nashville, Tenn.
Exercise-induced asthma, also called exercise-induced bronchoconstriction, is a condition in which vigorous physical activity triggers an acute narrowing of the airway afterward, making breathing difficult. Up to 90 percent of people with asthma have EIA, in addition to as much as 10 percent of the general population without asthma and more than 10 percent of elite athletes. EIA typically is treated with medications, some of which are banned in international competition.
The randomized, double-blind, crossover study involved 24 people with physician-diagnosed asthma and EIA. Study participants on the low-salt diet consumed 1,446 milligrams of sodium per day. Participants on the high-salt diet consumed 9,873 milligrams of sodium per day, an amount Mickleborough described as typical for many adults.
Participants on the high-salt diet showed a dramatic decline in lung function after physical activity. Twenty minutes after exercising, the forced expiratory volume in 1 second (FEV1) -- a measure of lung function -- of participants on the high-salt diet had dropped by 27.4 percent compared to just 7.9 percent for participants on the low-salt diet. Mickleborough attributed this to a combination of factors caused by the high-salt diet, including high blood pressure and increased blood volume. These factors can cause pulmonary edema, which can lead to airway obstruction. Mickleborough said a drop of 10 percent or more in post-exercise FEV1 is considered abnormal (EIA positive), so the participants with the low-salt diet essentially eliminated their EIA symptoms.
Mickleborough and his team also found a higher percentage of airway cells, which have been implicated in the pathogenesis of asthma and EIA, in the sputum of study participants on the high-salt diet, along with more proinflammatory mediators, which can cause constriction of the airways.
Mickleborough has been studying the impact of diet on exercise-induced asthma for eight years. His earlier research found that increased consumption of omega-3 polyunsaturated fatty acids, commonly found in fish oil, also could reduce EIA symptoms in asthmatics after just three weeks.