Can A High-Salt Diet Aggravate Asthma?

Can eating too much salt take your breath away?
Ever since the 1930s, when a provocative study found a high-salt diet worsened asthma symptoms in children, researchers have speculated that our national appetite for salt is one reason asthma leaves more and more of us breathless. Thanks to recent work by Tim Mickleborough, Ph.D., a physiologist at Indiana University’s Department of Kinesiology, we’re closer to knowing how.
Mickleborough and his colleagues started with 24 young men and women with exercise-induced asthma. Twelve were put on a low-salt diet that allowed them no more than 3,750 mg of salt per day (that’s 1,600 mg sodium, salt being 40 percent sodium). The other 12 were put on a high-salt diet: they ate the same foods as the first group plus a daily capsule containing 10,000 mg of extra salt. The low-salt group were given placebo capsules. (Government guidelines recommend no more than 2,300 mg of sodium per day—about 1 teaspoon salt.)
At the end of the two-week test period, the subjects in both groups ran on a treadmill until they were exhausted, and the team then tested how much air they could exhale.
The researchers found that the people on the low-salt diet could exhale far more air than those on the high-salt diet, and they needed their bronchodilator drugs less often. They also had far fewer markers of inflammation in their sputum, indicating their airways were less constricted.
The findings also showed that when people followed the high-salt diet, the oxygen they breathed was less able to move from their lungs into their bloodstreams. One reason, suggests Mickleborough, is that salt increases the volume of blood in the tiny vessels in the lungs. That forces fluid into the space between the blood vessel and the airway, making it harder for oxygen to travel across that space into the bloodstream.
No one knows precisely how salt creates such havoc, but for his part, Mickleborough thinks using the information to reduce breathless episodes is most important. “In the Western world food is full of salt,” he notes. “Certainly, if you’re asthmatic or an exercised-induced asthmatic, you’ve got to stay away from fast foods and the salt shaker at the table.”
Bottom line: If you have asthma, you might breathe easier if you watch salt and avoid processed food (where over three-quarters of the salt we eat comes from).
—Rachael Moeller Gorman