Fat-free Mass Linked With Liver Function, Physical Activity in Teens

Exercise has a 'beneficial effect' on adolescents with cystic fibrosis, study shows

Patricia Inácio, PhD avatar

by Patricia Inácio, PhD |

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An adult and a child go running together.

Physical activity increases fat-free mass — the lean mass in muscles, organs, bones, ligaments and tendons — in adolescents with cystic fibrosis (CF), according to a recent study.

The findings also showed that fat-free mass was significantly lower in those with CF-related liver disease.

The study, “Physical activity and liver disease affect the fat-free mass in adolescents with cystic fibrosis,” was published in the European Journal of Pediatrics.

CF is caused by mutations in the CFTR gene, which provides the instructions to produce a protein channel of the same name that controls the flow of water and salts through cells.

Defects in CFTR result in thick mucus being produced that accumulates in multiple organs, including the pancreas. This impairs the release of enzymes needed to properly digest food, particularly fats.

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Study Advocates Physical Activity as Routine Care

Assessing certain body parameters, including body mass index (BMI) — a measure of body fat — is commonly used to determine pulmonary function in CF children and adolescents.

“BMI, however, cannot inform on body composition which is known to be globally affected in CF,” the researchers wrote.

The typical decrease in fat-free mass observed in CF patients corresponds to a lower muscle composition and muscle wasting. Most importantly, CF mortality has been shown to be more strongly linked to low fat-free mass rather than a low BMI.

Patients with impairments in lung muscles often show overall muscle abnormalities. Also, the CFTR protein is present in the skeletal muscles — those used for movement — which suggests that CF has a relevant impact on them.

What affects fat-free mass in teens with CF?

To learn which factors drive fat-free mass in adolescents with CF, a team led by researchers at the Kings’s College London, U.K. studied 28 CF patients (median age 15, range 12–19; 11 males). F508del, the most common CF-causing mutation, was seen in 25 patients — 16 had the mutation in both copies of the CFTR gene and nine had it in only one copy. Three had other CFTR mutations.

Fat-free mass was assessed using bioelectrical impedance analysis, a technique that estimates body composition by calculating the resistance from body tissues to the passage of an electrical current. One participant was excluded because fat-free mass results were not collected.

Lung function was determined by forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and functional residual capacity (FRC). FVC is is the amount of air a person can exhale after a deep breath and FEV1 represents that volume in one second. FRC is a measure of the volume of air left in the lungs after a normal exhalation. Maximal inspiratory and maximal expiratory pressure were measured by a handheld manometer and expressed as percentage predicted.

BMI, as well as FVC, FEV1 and FRC were expressed as z-scores, which shows how far a data point is from the reference value in a healthy population.

To evaluate exercise tolerance, the participants completed the modified shuttle walk, where they walked or ran at increasing speeds between two cones placed on a flat corridor. The test was terminated either when participants completed 15 levels, were too unable to breathe to maintain their speed, or failed to reach the target on time.

Participants were also asked to complete the Habitual Activity Estimation Scale that measures the percentage of inactivity (lying down), somewhat inactive (sitting) somewhat active (walking) and active (activities that increase heart rate). With this scale, the researchers calculated the number of hours spent on each activity level.

The median fat free mass was 37.7 kg and the median fat-free mass index (FFMI) — calculated as fat-free mass divided by square height (kg/m2) — was 13.5.

Higher FFMI correlates with lung function, exercise

A higher FFMI significantly correlated with higher age, number of shuttles, reported hours of activity a day, FVC z-score, and maximum inspiratory and expiratory pressure. FFMI showed no correlation with BMI z-score, FEV1 z-score, and FRC z-score, however.

Median FFMI was significantly higher in males than females (15.1 vs. 12.7 kg/m2). Patients with liver disease had a significantly lower FFMI than those without liver disease (11.9 vs. 14.4 kg/m2).

“The finding of a more reduced fat-free mass in CF patients with liver disease is a novel finding in the whole CF population and not only in CF adolescents,” the researchers wrote.

No significant differences in the median FFMI were seen between participants with or without chronic infections (12.2 vs. 13.9 kg/m2) as well as between those with CF-related diabetes and those without diabetes (12.2 vs. 14 kg/m2).

Fat-free mass was independently associated with male sex, number of shuttles, and CF-related liver disease, statistical analyses showed. Lung function measured by FVC z-score was not a determinant factor of fat-free mass.

Researchers concluded “physical activity exerts a beneficial effect on fat-free mass in adolescents” with CF, but “CF-related liver disease negatively affects fat-free mass.”

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