(204) Should DEXA form part of routine nutrition assessments in children with CF?

Background: Optimising nutritional status in children with cystic fibrosis (CF) is associated with improved lung function and ultimately, survival. Body Mass Index (BMI) has traditionally represented the cornerstone of nutrition assessments with a BMI > 50th percentile universally accepted as optimal. BMI is quick and easy to perform but it cannot discern between fat-free mass and fat mass, with the former considered physiologically more important than BMI. Hence, alternative measures such as fat-free mass index (FFMI) derived from dual-energy x-ray absorptiometry (DEXA) scans may better predict lung function in the clinical setting, but this has not been evaluated longitudinally. This study aimed to (1) describe longitudinal changes in body composition indices with age and gender and (2) determine whether FFMI is a superior tool for assessing nutritional status.

Methods: This was a longitudinal, retrospective study of children with CF (8 to 18 years) who attended the CF annual review clinic at the Sydney Children’s Hospital, Australia between 2007-2020. Standard centre practice included a DEXA scan every second year until transition. Height-squared indices of body composition including FFMI and fat-mass index (FMI) were determined from DEXA results. Z-scores for BMI, FFMI and FMI were derived from Well’s reference population [1]. Forced expiratory volume in one second predicted (FEV1%) was calculated using Knudson values and was a numerical surrogate of lung function. Repeated measures correlation analyses were performed to assess the association between FFMI, FMI, and BMI with FEV1%.

Results: A total of 350 DEXA reports were analysed from 137 patients (median: 3 scans, interquartile range: 3). Seen in Figure 1, BMI-z and FMI-z decreased with age while FFMI-z increased with age in our cohort. FFMI-z was the only measure that remained below 0 throughout childhood. Gender analyses showed comparable z-scores for all three measures although females tended to be higher from 12.5 years old, except in FMI-z.

There was a weak, but statistically significant, positive correlation between FEV1% and both BMI-z (r = 0.14, p = 0.04) and FFMI-z (r = 0.24, p < 0.001). FMI-z had a negligible negative correlation with FEV1% (r = -0.05, p = 0.50). The correlation between FEV1% and BMI-z was comparable between males and females (males: r = 0.17, females: r = 0.12). This was also seen between FEV1% and FFMI-z (males: r = 0.24, females: r = 0.26). The Pearson correlation coefficient (r) suggests that whilst FFMI-z had a stronger association than BMI-z, both have a limited strength of association with FEV1% (r < 0.3).

Conclusions: Deficits in FFMI exist despite age and gender. FMI has no correlation with FEV1%. FFMI-z derived from DEXA did not have a superior relationship with FEV1% when compared to BMI-z. Furthermore, both measures accounted for a very small component of an individual’s lung function. Based on these findings, there is little evidence to suggest the routine use of DEXA in the nutritional assessment of children with CF.

Acknowledgements:

References: [1] Wells JCK, Williams JE, Chomtho S, Darch T, Grijalva-Eternod, C, Kennedy, K, et al. Body-composition reference data for simple and reference techniques and a 4-component model: a new UK reference child. Am . Clin Nutr. 2012; Oct 17;96(6):1316-26. https://doi.org/10.3945/ajcn.112.036970.