BMI, other nutrition markers weakly link with lung function in children

Nutritional status may be losing influence on patients' lung health

Lindsey Shapiro, PhD avatar

by Lindsey Shapiro, PhD |

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Surrogate markers of nutritional status, including body mass index (BMI) and fat-free mass index, only weakly associated with lung function among children and adolescents with cystic fibrosis (CF) in a recent study.

These young patients had different trajectories in nutritional status throughout childhood, with girls tending to maintain a higher age- and sex-adjusted BMI and fat-free mass than boys.

Findings indicate that, in a contemporary context, nutrition may be “less of an influencing factor on lung function that in previous decades,” the researchers wrote. “More studies are needed to confirm these findings and current BMI targets for children with CF may need revision.”

The study, “Body composition and body mass index measures from 8 to 18 years old in children with cystic fibrosis,” was published in the Journal of Cystic Fibrosis.

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The buildup of thick and sticky mucus due to CF-causing mutations can lead to a range of disease symptoms. These include the digestive problems that influence patients’ nutritional intake, affecting their ability to gain and maintain a healthy weight. A lack of proper nutrition also can further exacerbate lung function declines and other health issues.

BMI, a measure of body fat based on a person’s height and weight, long has been the gold standard for monitoring patients’ nutritional status, with a higher BMI linked to better lung health. The fat-free mass index measures primary components of body composition other than fat, like muscle.

While BMI remains the most commonly used metric for indexing nutritional status, it is not without limitations. Particularly, it is unable to directly distinguish between different components of body composition, such as fat or muscle.

Researchers noted, as examples, that a person with a healthy BMI still could have a disproportionately high fat composition and low muscle mass. Similarly, a person might be considered overweight by BMI due to considerable muscle mass.

Making this distinction is important for CF patients, they added, since excessive fat is linked to poorer outcomes and more muscle to better outcomes.

Body composition indexes have been developed to directly or indirectly examine fat (FMI) and fat-free mass (FFMI), which could serve as better alternatives to BMI in monitoring nutrition in CF patients.

A research team in Australia examined changes in body composition among 137 children with CF, ages 8 to 18, seen at Sydney Children’s Hospital between May 2007 and December 2020. None of these children were using CFTR modulator therapies, disease-modifying treatments that significantly can alter body composition, lung function, and overall health.

Body composition was assessed using a technique called dual energy x-ray absorptiometry, which measures three components: bone mineral content, fat mass, and fat-free mass. Some of the study’s children had only one such scan, whereas others had multiple across childhood, culminating in 339 total scans.

Weak, though significant, links between nutritional status, lung health

BMI, FMI, and FFMI often were below the expected average for both boys and girls. BMI tended to decrease with age in both sexes, although girls saw transient increases from ages 12.5 to 15, and they maintained a higher BMI for their sex than boys.

FMI declined in girls from around ages 9-12, increased from ages 12-14, and then declined again toward adulthood. For boys, this measure of fat mass rose from ages 8-12, then declined.

Fat-free mass generally increased between the ages of 8-18 for girls, but plateaued over the same period in boys. By the age of 18, girls still were showing increasing fat-free mass, whereas boys were on a downward trajectory.

These findings were opposite of what’s expected in the general population, where girls tend to maintain higher FMI than boys, while boys have a higher FFMI and BMI as they move toward adulthood.

“This may reflect the convergence of two energy demanding processes: boys entering puberty later in life alongside worsening lung function that occurs with age,” the scientists noted.

A weak, but statistically significant correlation was observed between BMI scores and lung function, as measured by percent predicted forced expiratory volume in one second (FEV1pp), with a higher BMI being associated with better lung function across the entire group of patients. FEV1pp assesses how much air a person can quickly and forcibly exhale.

Similarly, there was a weak but significant association between a higher FFMI and better lung function, which was maintained when boys and girls were analyzed separately.

More fat-free mass could be linked to stronger respiratory muscles that positively influence lung function, the scientists speculated.

Favorable treatment advances could affect value of nutritional status

The relatively weak associations observed between measures of nutritional status and lung function might mean that nutritional status is no longer as good at predicting lung function as it was once thought to be, the scientists stated.

They noted that this study spanned a 14-year period where the medical and nutritional management of CF underwent many “favorable” changes, which could have influenced its findings. The effects of puberty on body composition also were not formally accounted for in this work.

While CFTR modulators are becoming “a mainstay in [disease] management, this study provides insight into the natural course of CF and allows for comparison with future studies,” the researchers wrote.