Low socioeconomic status and lung function

From the authors:

We thank P.H. Quanjer for his comments regarding our paper, in which we apply a novel modelling approach to lung function data from the unique Danish cystic fibrosis registry and show that low socioeconomic status (SES), as measured by parental education level, is associated with an increased rate of decline of % predicted forced expiratory volume in 1 s in the Danish cystic fibrosis population [1]. We agree that it is important to develop a better understanding of the pathways to social inequalities in cystic fibrosis outcomes.

P.H. Quanjer points out that chest circumference may be an important mediator of the association between SES at birth and later lung function. Malnutrition and poor growth are major concerns in children with cystic fibrosis and optimising nutritional status is clearly important for subsequent lung health. Our analyses have shown social gradients in growth outcomes in children with cystic fibrosis in the UK, evident from around the time of diagnosis [2]. These findings point to important effects of SES in utero or in the initial period prior to diagnosis. Both are plausible, but a limitation of studies thus far has been a lack of data on social gradients in birth weight, which would complete the picture. Further data on SES gradients in birth weight in cystic fibrosis would clarify the extent to which the early growth differentials are simply a reflection of the broader SES effects on birth weight in the general population.

On a related point, height has been independently related to survival in people with cystic fibrosis and it has been suggested that this may be mediated through greater lung capacity in taller people [3]. Furthermore, Fogarty and co-workers [4, 5] have suggested that measures of body habitus may partially explain the male survival advantage in cystic fibrosis. In the UK, SES has an important effect on height from the outset in children with cystic fibrosis and this difference tracks through to adulthood [2]. Further longitudinal analyses are required to understand how inequalities in birth length, and the factors that influence it, may influence chest capacity and lung function in adulthood, and ultimately survival, in cystic fibrosis.

P.H. Quanjer also points to the complex interaction between measures of SES and health over the life-course. The Diderichsen model of pathways to health inequalities has informed our analyses in cystic fibrosis [6, 7]. A key feature of the Diderichsen model is that it incorporates both social causation and social selection mechanisms within a common framework, across the life-course. We encourage further studies that use this approach, in order to better unpick the complex causal chains that link social inequalities and health.