Link between asthma and sweating

“Sweaty people 'less asthma prone'”, is the headline on the BBC News website. Researchers suggest that the ability to sweat may do more than keep the body cool, it may lower the chance of exercise-related asthma. People “who make less sweat, tears and saliva when exercising may have more breathing problems”, the BBC says.

The study is from research that looked at people who were suspected of having exercise-induced asthma. They were tested separately with two drugs, one to mimic the effects of asthma and one to induce sweating. The people who showed the greatest response to the drug that induced asthma symptoms also responded to the other drug with the lowest sweat secretion. The study did not investigate sweaty people, as implied by the headline, and although the link is interesting and has some possible underlying mechanisms that might explain how the connection occurs, it is too early to say that a cause has been found or what the research might mean to people with other types of asthma.

Where did the story come from?

Dr Chan Park and colleagues from the Naval Medical Center, San Diego in California conducted the research. The sources of funding are not declared. It was published in the peer-reviewed medical journal: Chest .

What kind of scientific study was this?

This was an observational study of 56 young US Navy or Marine Corps volunteers, men and women between 18 and 32 years old. They had all been referred to a naval medical centre with possible exercise-induced asthma. The study did not have a control group, instead it looked at the strength of association (correlation) between two different groups. The first group were thought to have exercise-induced asthma and tested positive to the methacholine challenge test and the second were thought to have exercise-induced asthma and tested negative on the challenge test (a cut-off was set by the researchers).

The methacholine challenge test is a standard test used to help diagnose asthma. A fine mist of the chemical methacholine is created by a “nebuliser” machine and this is breathed in by the patient. Methacholine is a synthetic chemical (a non-selective muscarinic receptor agonist) that stimulates part of the nervous system and causes a narrowing of the airways (bronchoconstriction). Airway narrowing is measured using the FEV1 – a test of how fast and hard a patient can breathe out. People with asthma react to a lower dose of inhaled methacholine than those without asthma; and those who had a 20% or more fall in their FEV1 over baseline after breathing in the methacholine, were classified as having a positive result.

Sweating was stimulated on the skin by applying another drug, pilocarpine (also a muscarinic receptor agonist) on gel patches that also acted as electrodes. After a low current was applied across the electrodes, sweat was collected from the skin, and the sodium concentration was measured and the samples weighed.

In a follow-up experiment, the researchers also tested the production of saliva and tears in an additional 58 healthy volunteers. They analysed the correlation between these measures and sweat secretion rates.

The researchers divided the subjects into those whose maximum reduction in FEV1 was less than 20% and those in whom the reduction was greater. They assessed the statistical significance of the differences in sweat secretion rates and sodium secretion rates between the two groups. They also analysed the "correlation coefficient” between the total sweat volume and the maximum fall in FEV1 on a continuous scale. This coefficient indicates the strength and direction of a linear relationship between two random variables.

What were the results of the study?

The researchers say that the 56 volunteers showing excessive airway reactivity, as shown by the reductions in FEV1 during the methacholine challenge test, also had diminished values for pilocarpine-induced sweat secretion. The r statistic (Pearson’s test) for this – a measure of the strength of the association – was -0.59 (p < 0.0001) suggesting a large correlation.

The rate of pilocarpine-stimulated sweat secretion in the healthy volunteers also correlated highly with salivary flow rate and tearing rate.

What interpretations did the researchers draw from these results?

The researchers conclude that sweating (hyperhidrosis), excess saliva (sialorrhea) and excessive tearing are traits that may indicate a phenotype (an individual physical characteristic), that “predicts resistance to hyperactive airway diseases such as exercise-induced asthma in humans.”

What does the NHS Knowledge Service make of this study?

While there is speculation by the researchers that low sweating might mean less fluid in the airways and that it is possible that this could be a mechanism protecting people from exercise-induced asthma, it is important to see this study in context.

• The study was observational, so by design it will not be possible to imply that there is a causal link from this study. 
• The tests both used drugs that work at the same receptor (muscarinic receptor agonists) to stimulate physiological responses and therefore the link between the results is not altogether surprising.
• Men and women were included in the study but the proportions of each and any differences in the recordings are not reported.
• The application of these findings to people outside of the population group studied is not clear. For example, as they were all naval recruits between the ages of 18 and 32 years, who had suspicions that their asthma was triggered by exercise, these results cannot be extrapolated to the more common forms of asthma that occur in childhood.

Overall, this study suggests a theory that dry airways may contribute to the symptoms of asthma in people who are known to have asthma triggered by exercise, but more research will be needed to investigate the mechanisms.

Sir Muir Gray adds...

Whatever the truth, people with asthma should not be put off exercise.