Correlates of prepubertal bone mineral density in cystic fibrosis.

Abstract

Aim--To examine early factors in bone mineral accretion in cystic fibrosis (CF). Methods--In 22 prepubertal children with CF and mild lung disease, the relation between total body bone mineral density (BMD) and measures of body composition, biochemistry, lung function, and physical activity was studied.

Results--There was a non-significant mild reduction in mean total body BMD. No relation was found between BMD and anthropometric indices, fat free soft tissue, degree of lung disease, degree of fat malabsorption, dietary energy intake, or level of physical activity. Significant impairments in physical growth were apparent in this population and were found to correlate with degree of lung disease.

Conclusion--A CF specific factor appears unlikely to be associated with the osteopenia commonly found in CF. Careful attention to general aspects of lifestyle and nutrition is recommended to maximise bone mineral accretion in this population.

(Arch Dis Child 2001;85:166-171)

Keywords: bone mineral density; cystic fibrosis; children; nutrition; body composition

With the dramatic improvements to life expectancy in cystic fibrosis (CF) over the past 25 years, attention has turned to complications which relate to older patients with CF. In particular, research has been directed towards factors which may maximise both survival and quality of life in the longer term. Osteopenia, commonly found in patients with CF, [1 2] has been the subject of considerable research interest in the past decade, and exacts a considerable toll through fractures, back pain, and kyphosis. Numerous authors have addressed the many reasons for the observed reduction in bone mineral density (BMD); poor nutritional intake, pancreatic insufficiency, calcium and vitamin D malabsorption, reduced levels of physical activity, corticosteroid usage, pubertal delay, chronic respiratory acidosis, and elevations in osteoclast activating cytokines have all been considered. It is probable that the aetiology of osteopenia in CF is multifactorial; new avenues for preventive management should follow the elaborat ion of the role of these factors.

The concept of peak bone mass has been widely used in developing strategies for the prevention of osteoporosis in the general population. By maximising peak bone mass in adolescence or young adulthood, along with strategies to reduce the rate of bone mineral loss, fracture risk and development of thoracic kyphosis may be substantially reduced. Bone strength is principally determined by bone density and bone volume, and several studies have shown the correlation between low bone mass and increased risk of fracture at, for example, the femoral neck. Bone mass accumulation during growth in childhood and adolescence is thought to be under the influence of a number of factors such as body weight, Tanner stage, and a variety of specific risk factors. [3-5] In CF, the other factors, both disease specific and iatrogenic, noted above may also come into play.

In normal children, total body BMD increases with age in a curve which resembles that for linear growth. [6] In puberty, there appear to be regional differences in bone mineral accretion. For example, the femur and spine have been found to undergo mineralisation at different rates. [7] In CF, it appears that bone mineralisation accretion begins to stray from normal developmental lines around the time of puberty. Failure to accrete bone mineral at an appropriate rate, as well as bone mineral loss have been considered. [8 9] Recent evidence has suggested that it is in fact inadequate accretion of bone mineral, rather than bone loss which plays the major role. [10]

Given that factors such as low body mass, gonadal dysfunction, and reduced physical activity have all been found to correlate with deficits in BMD, [1 11 12] "there would appear to be significant therapeutic potential in characterising the prepubertal correlates of BMD in order to maximise eventual peak bone mass. In this study we have sought therefore to consider pertinent factors in early bone mineral status.

The current study forms a part of a wider cross sectional characterisation of body composition in a cohort of Australian children with CF and mild or moderate lung disease, which included pulmonary and gastrointestinal status, biochemical markers of body composition, direct and indirect measures of body composition, and analyses of dietary intake, fat malabsorption, and physical activity. It was hypothesised that, in these prepubertal children, fat free soft tissue (FFST) content, dietary energy intake, illness severity, fat malabsorption, linear growth, and physical activity would correlate significantly with total body BMD.

Methods …