Body composition in individuals with cystic fibrosis

Abstract

Because nutritional status is intimately linked with pulmonary function and survival, nutrition has been at the mainstay of cystic fibrosis (CF) care. Body Mass Index (BMI) is traditionally used to define nutritional status because of the ease with which it can be calculated, but it has a number of limitations including its inability to differentiate fat mass (FM) from lean body mass (LBM), the latter thought to confer health advantage. A number of tools are available to quantify body composition including dual-energy x-ray absorptiometry (DXA), bioelectrical impedance, MRI, CT, air displacement plethysmography, and stable isotopes, and these have been used to varying degrees in studies of CF. In CF, LBM tends to be lower for a given BMI, particularly at lower BMI. In adults, lower fat-free mass (FFM) correlates with greater CF disease severity, lower pulmonary function and higher inflammatory markers. FFM is also positively associated with greater bone mineral density, while greater FM is associated with greater loss of lumbar spine bone mineral density over 2 years. In youth, LBM is positively associated with pulmonary function. The predictive value of body composition for functional and clinical outcomes and the role of improving LBM on these outcomes remain undefined. With improvements in BMI accompanying highly-effective modulator therapy, closer evaluations of body composition may inform risk for more traditional, non-CF adult outcomes in CF.

Keywords: %BF, Percentage body fat; BC, Body composition; BIA, Bioelectrical impedance; BMI, Body mass index; Bioelectrical impedance; Body composition; CF, Cystic fibrosis; CFTR, Cystic fibrosis transmembrane conductance regulator; CT, Computed tomography; Cystic fibrosis; DXA, Dual-energy X-ray absorptiometry; Dual-energy X-ray absorptiometry; FFM, Fat-free mass; FFMI, Fat-free mass index; FM, Fat mass; FMI, Fat mass index; Fat mass; HEMT, Highly Effective Modulator Therapy; LBM, Lean body mass; Lean body mass; MRI, Magnetic resonance imaging.

Fig. 1

An example of a post-processed whole body DXA scan. Fat is represented in yellow, bone in blue, and muscle in red . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

The relationship between age and (a) FFMI and (b) fat mass index (FMI; FM divided by height in m²) during the life course, using data from the reference child and adolescent. During early life, boys and girls have similar fatness, but boys have more LBM and are therefore heavier. During childhood, both sexes lose fatness, but boys gain relatively more LBM. During adolescence, boys gained primarily LBM and little FM, whereas girls gain little LBM but substantial FM. In adulthood, men show substantially less FM and greater relative LBM than women in all human populations .

Fig. 3

The association between FEV1%-predicted and LBMI-Z adjusted for age among males and female youth with CF. LBMI-Z is positively associated with FEV1%-predicted in males (p < 0.0001) and females (p < 0.0001) .