Use of body mass index percentile to identify fat-free mass depletion in children with cystic fibrosis

Summary

Background & aims

Nutritional failure in children with cystic fibrosis (CF) has a negative effect on their morbidity and survival. It is unknown if determination of fat-free mass is a better screening method for nutritional failure than the currently recommended body mass index (BMI) alone.

Methods

This cross-sectional study in 77 children with CF (age: 14.8 ± 2.9 y) measured fat-free mass, fat mass, bone mineral content and density using dual-energy X-ray absorptiometry. Nutritional failure was defined as BMI <10 percentile and/or fat-free mass index <5th percentile. Statistics were done using ANOVA and t-tests.

Results

Thirty-one percent (31%) of the patients with CF was characterized by nutritional failure, and 14% had low fat-free mass index with preserved values for BMI (hidden depletion). Only 52% of the patients with fat-free mass depletion was detected when using the criteria BMI <10 percentile. Patients with fat-free mass depletion had reduced values for forced expiratory volume in 1 s (FEV₁), independent of body mass index (P < 0.05), and lower values for bone mineral density in whole body, spine and hip, and spine bone mineral apparent density (P < 0.01). BMI ≤20 percentile was associated with a large drop in fat-free mass, a reduced FEV₁, and in bone mineral loss.

Conclusions

Depletion of fat-free mass enhances morbidity in children with CF and is undetected in many of these children when only BMI percentile is used as screening method. BMI percentile of 20 should be considered as the new critical threshold for nutritional failure in CF if body composition techniques are not available.

Introduction

Cystic fibrosis (CF) is one of the most common life-threatening inherited disorders in the Caucasian population. In the past decades, the median predicted survival of patients with CF has increased from 10–12 to over 37 years.¹ One of the major factors contributing to this increased survival has been the understanding of the role of optimal nutrition on the overall health status of individuals with CF. Studies showed that in children with CF, better nutritional status was associated with improved linear growth, better pulmonary functions, and exercise tolerance,2, 3, 4 and interventions to establish weight gain led to improvements in pulmonary functions.⁵ Since there is a strong association between BMI and overall health metrics, especially lung health,⁶ the Cystic Fibrosis Foundation recommends that children with CF maintain a body mass index (BMI) ≥50th percentile. However, malnutrition is still prevalent in children with CF.⁷

BMI is currently the universally accepted method to determine malnutrition in children. However, low body weight and BMI do not differentiate between fat mass (FM) and fat-free mass (FFM), and weight-for-height measurements underestimated the prevalence of malnutrition defined after body composition measurement using total body potassium⁸ or skinfold thickness measurements.⁹ FFM depletion commonly occurs in adults with CF.10, 11, 12, 13, 14 Previous studies showed that while FFM was low, FM was maintained in 40% of these patients (hidden depletion of FFM),10, 11 and 54% of the patients with a normal body weight for height had low values for lean body mass.¹⁵ Apparent or hidden FFM depletion rather than low body weight in CF was associated with overall disease severity as indicated by decreased lung function, respiratory muscle weakness, increased systemic inflammatory activity, low bone mineral density, and increased frequency of exacerbations.10, 11, 15

Studies in pediatric subjects with CF16, 17, 18 found differences in body composition between young and post-pubertal children with CF. Limited studies are available examining the prevalence of (hidden) FFM depletion in pediatric subjects with CF. A study in 22 children with CF reported that 45% of the patients with FFM had z-scores below −2 and half of the normal-weight patients (% ideal weight for height >85%) had FFM z-scores below −2.¹⁹ Furthermore, it is unclear at what age, changes in tissue distribution occur in CF and whether FFM depletion in children with CF is associated with increased morbidity (lung function, bone loss, disease severity) as previously observed in adults with CF.10, 11, 12, 13, 14

The hypotheses for this cross-sectional study were that FFM measurement in children with CF is a better method to screen for malnutrition than the use of BMI percentiles alone, and that depletion of FFM is associated with increased morbidity and worsening of lung health. In this study, we examined a group of randomly selected children with CF to determine (1) the prevalence of underweight and (hidden) FFM depletion, (2) whether (hidden) FFM depletion was associated with changes in body composition on whole body and subregional (trunk and extremities) level, and increased morbidity (reduced lung function, loss of bone mineral density), and (3) whether specific BMI percentiles can be used to predict FFM depletion and increased morbidity.