Being underweight is linked to poorer lung function in people with bronchiectasis, but not to increased risk of hospitalization or death, a new U.S. study indicates.
One of the challenges in bronchiectasis is a substantial person-to-person variation in how the disease manifests, progresses, and responds to treatment. As such, finding useful ways to classify people with the disorder is important to help clinicians make informed prognoses and design clinical trials.
Previous research suggested that people with bronchiectasis who are underweight — defined by the World Health Organization as having a body mass index (BMI) below 18.5 kg per square meter (kg/m2) — have more severe disease and a higher mortality rate. BMI, a ratio of weight to height, is affected by a variety of factors, including nutritional status.
In the new study, researchers analyzed data from the U.S. Bronchiectasis and Nontuberculous Mycobacterial Research Registry, a database that collects clinical information on people with bronchiectasis in the U.S. In total, the scientists assessed data for people who had at least five years of follow-up data available.
Of the 469 people analyzed, most (83.3%) were women, and the mean age was 65 years. The majority (92%) of the patients were white, 27 (5.4%) were Asian, and the remaining patients identified as Black or other races.
Most patients, 317 (63.9%), had a BMI within the normal range (18.5–24.9 kg/m2), with 61 (12.3%) considered underweight based on BMI.
“Interestingly and surprisingly, fewer patients than expected were underweight,” the researchers wrote.
Compared to other weight groups, underweight patients had significantly poorer lung function, as measured by forced vital capacity and forced expiratory volume. However, no statistically significant differences were seen among the BMI groups in exacerbation frequency, hospitalizations, or lung surgery.
Statistical analyses — which controlled for demographic factors, lung function, and co-existing conditions such as asthma — indicated that women were 3.19 times more likely to be underweight than men.
In addition, Asian patients were 4.78 times more likely to be underweight compared to whites. Notably, although Asian patients made up 5.4% of the total group, they constituted 16.4% of all underweight patients.
Other studies have shown that Asian individuals tend to have lower BMI than other racial groups. In theory, this could be due to biological differences (e.g., genetics) or other factors, such as diet or lifestyle. Prior research suggested that different BMI cutoffs could be used to define “underweight” in different racial groups, but, “there has been much debate and controversy over implementing ethnicity-specific BMI parameters to define being overweight and obese,” the scientists wrote.
They added that the results of this study “suggest that the Asian patients with bronchiectasis might be distinctive within the U.S. population.”
Over the five years analyzed, nearly all patients had a stable BMI — that is, patients in any given weight category usually stayed in that same category over the whole period analyzed.
“Surprisingly, our data revealed that, for the majority of patients, BMI remained stable by absolute BMI number and by BMI category. This suggests that either patients with bronchiectasis have a more stable nutritional status that one might expect, or that a longer period of study and/or larger sample size may be required to see differences in nutritional status and disease outcome measures,” the researchers wrote.