Body size and build differences may explain some of the variation in lung function between individuals with CF. We wondered if a blood test called "Serum creatinine" (which is done routinely in CF clinics as a way of assessing kidney function but which can also be used as an estimate of how much muscle a person has) and body mass index (BMI) are independently associated with lung function in CF.
Why is this important?
This is important as we might be able to use these simple tests in clinic to identify "at risk" groups of people with CF who would benefit from nutritional supplements and exercise programmes.
What did you do?
We employed a cross-sectional study design using UK CF registry data from 2096 clinically stable adults
What did you find?
Serum creatinine and BMI were positively and independently associated with FEV1 and FVC (measures of lung capacity). Compared to those with a normal BMI in the range of 20-25 Kg/m2, those with a BMI less than 20 Kg/m2 had lower FEV1
What does this mean and reasons for caution?
In this population of patients with CF a higher Serum creatinine was associated with better lung function. A low BMI was associated with poor lung function. The positive association between BMI and poorer lung function in cystic fibrosis is the opposite of that observed in healthy populations of adults, and explaining this may increase our understanding of the relationships between BMI and lung function generally. The relevance of BMI as a measure in CF has not been established.
One of the most important things for a reader to understand about the results of a study such as ours is that as we observed many people at the same point in time, we are unable to draw any conclusions regarding causality and can only speculate as to possible explanations for this association. Firstly, the observed associations may be a consequence of damaged lungs leading to reduced exercise capacity and hence resulting in lower muscle bulk, and this is consistent with the observation that skeletal muscle strength is reduced during an acute pulmonary exacerbation of CF. Secondly, lower skeletal muscle may impact on lung function by reducing respiratory muscle strength. Finally, the association may be added to by disease severity, probably via a combination of nutritional malabsorption, chest infections and inflammation.
Untangling these complex relations between body shape and lung function will require follow-up studies including observational studies and randomised controlled trials, ideally over an extended period of time. Such interventions should consider the impact of improving muscle mass on physical and quality of life related outcome measures and could include exercise based training programmes and dietary supplementation with amino acids. CF multidisciplinary teams typically include a specialist physiotherapist and specialist dietician and are ideally placed to provide a platform for physical and nutritional interventions based upon emerging supportive evidence.
If these associations have therapeutic potential in modifying disease progression it may be possible to "bulk up" a group of "at risk" patients selected upon them having a low serum creatinine and help improve their lung function with food supplements and exercises.