In patients with cystic fibrosis (CF) the commonest cause of death is respiratory failure. Respiratory failure can have many causes. However, in patients with CF a major contributor is the impairment of the muscles required for breathing (respiratory muscles). Respiratory muscle impairment can result from poor nutrition. Lung function declines particularly during adolescence whilst body composition also changes at the same time. Thus, the investigators plan to study the relationship of nutrition and body composition to respiratory muscle strength and lung function in children and young people with CF aged between 12-18 years. The body mass index (BMI) is currently used in the clinical setting to measure nutritional status in CF. At King's College Hospital (KCH) there are portable devices to assess both respiratory muscle function and lung function. The research team will use a Bioelectrical Impedance Analysis (BIA) device to assess body composition, including BMI and lean body mass (LBM). The aim of the study is primarily to assess whether measurements of LBM impairment may better relate to poor lung function compared to BMI and secondly to examine whether lung and respiratory muscle function correlates with exercise tolerance.

In patients with Cystic Fibrosis (CF) the commonest cause of death is respiratory failure. Respiratory failure can have many causes. However, in patients with CF a major contributor is the impairment of the muscles required for breathing (respiratory muscles). Respiratory muscle impairment can result from severe narrowing of the airways, poor nutrition, chronic infection and inflammation, lack of aerobic exercise and use of steroids (Dassios, 2015). The most rapid decline in lung function is seen during adolescence and coincides with a change in body composition (Loomba-Albrecht, 2009). The body mass index (BMI) is currently used in the clinical setting to quantify nutritional status in CF. However, measurements of the proportion of lean muscle, such as lean body mass (LBM), may better describe nutritional impairment in CF (Pedreira, 2005, Ionescu, 1998). LBM and BMI have been measured using dual x-ray absorptiometry (DXA) in children and young adults with CF showing a stronger association of LBM rather than BMI with pulmonary function especially in the undernourished adolescent (Sheikh, 2014). Assessment of respiratory muscle function and body composition has previously required specialised equipment, such as DXA, which is not readily available in many CF clinics. In addition, DXA involves radiation which may have unwanted side effects if used routinely for body composition monitoring. At King's College Hospital there are now, however, portable devices to assess both respiratory muscle function and body composition. The investigators will use a Bioelectrical Impedance Analysis (BIA) device to calculate body composition, including BMI and LBM. The aim of the study is primarily to assess whether LBM rather than BMI better predicts both pulmonary and respiratory muscle function using portable equipment that avoids use of avoidable radiation. Secondly, the research team aims to examine whether pulmonary and respiratory muscle function correlates to exercise capacity. These may yield useful information about targeting nutritional support and exercise to improve respiratory muscle and pulmonary function.

A cross-sectional study will be undertaken. Age, height, and weight will be recorded. Spirometry, impulse oscillometry and body plethysmography will be measured with a pneumotachograph based system (Jaeger Masterscreen PFT, Carefusion Ltd, Basingstoke UK) according to the American Thoracic Society and the European Respiratory Society guidelines. The highest value of forced vital capacity (FVC), forced expiratory volume in one second (FEV1), ratio of FEV1 to VC (FEV1/VC), forced expiratory flow between 25 and 75% of VC (FEF 25-75%), functional residual capacity (FRC), residual volume (RV), total lung capacity (TLC), respiratory system resistance at 5Hz and 20Hz (Rrs5, Rrs20) will be recorded following at least three technically acceptable measurements. Respiratory muscle function data will be obtained from the Micro RPM Respiratory Muscle Analyser (CareFusion, San Diego, California, USA): maximum inspiratory pressure (MIP), maximum expiratory pressure (MEP), sniff nasal inspiratory pressure (SNIP), maximum relaxation rate (MRR), time constant of relaxation (τ, tau), and maximum rate of pressure development (MRPD). A respiratory health questionnaire will be completed. Body composition information will be obtained with the Inbody S10 Body Composition Analyzer (Inbody Ltd, Cerritos, California, USA): Body mass index (BMI), BMI-z scores, fat free mass (FFM), segmental lean mass (LM) [LM-right arm (LMRA), LM-left arm (LMLA), LM-trunk (LMTR), LM-right leg (LMRL), LM-left leg (LMLL)], visceral fat area (VFA), body cell mass (BCM). A field test to assess exercise tolerance test will be performed and the level of habitual activity will be assessed using a questionnaire. Information will be collected on genetic mutations, chronic infection status, use of systemic corticosteroids, and co-morbidities.

Inclusion Criteria:

• Confirmed diagnosis of CF
• Ages 12-19

Exclusion Criteria:

• Acute illness or hospitalisation that would render the participants unable to undertake the assessment, including pulmonary exacerbation in past 2-weeks,

Children and young people with confirmed CF, between 12 years and 18 will be studied.