Original ArticleBone Mineral Density in Cystic Fibrosis: Benefit of Exercise Capacity
Introduction
Over 2 decades ago in a young adult cystic fibrosis (CF) population, we reported low peripheral bone mineral density (BMD), hypovitaminosis D, and malabsorption of vitamin D (1). Numerous studies have reported a high prevalence of low BMD in patients with CF 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Reported rates vary between 12% and 50%, depending on patient numbers 2, 5, 9, 10, differences in patient populations 9, 11, and in the definitions of bone disease (2). Clinical correlates of low BMD in CF include forced expiratory volume in 1 s (FEV1) (2), markers of bone turnover 5, 13, steroid usage (6), hypogonadism (14), markers of calcium metabolism including 25-hydroxyvitamin D (25(OH)D) and parathyroid hormone (PTH) 5, 8, chronic inflammation (15), and genotype (16). Several studies have also evaluated the relationship between exercise activity and BMD in CF and have shown a weak correlation between physical activity, as assessed with questionnaires, and BMD 2, 5, 6, 12, 17. However, such studies may be prone to subjective evaluation and suffer from recall bias.
No study has evaluated objective measurements of exercise capacity as contributors to total proximal femur and lumbar spine BMD in CF, despite evidence that exercise plays a strong preventative role in healthy adults (18). A previous study evaluating objective exercise measurements and BMD in CF demonstrated exercise capacity and body mass index (BMI) to be independent predictors of a composite BMD score (19). We hypothesized that exercise may influence different parts of the skeleton in adults with CF. The aim of this study was to evaluate a comprehensive set of objective measurements of gas exchange and ventilatory mechanics during maximal exercise cycle ergometry in adults with CF, and correlate these with measurements of BMD in the lumbar spine and total proximal femur.
Section snippets
Patient Population
Since 1999, all adults attending the Irish National Referral Centre for Adult CF patients undergo elective BMD evaluation with bone densitometry every 18–24 mo as part of scheduled long-term assessment. Patients aged 16 yr and older from the entire country are referred for multidisciplinary care, and the number attending our center has steadily increased from 170 in 1997 to 250 in 2005 as treatment has become more centralized and life expectancy continues to improve. From July 1999 to July 2002,
Baseline Data
Ninety-four male and 75 female patients had BMD evaluated (Table 1). Genotyping was available for 110 patients; 96% carried the ▵F508 CFTR mutation, with 69 patients being homozygous and 37 patients being heterozygous. Mean BMI was 20.6 kg/m2 with 52 patients (31%) being malnourished (BMI < 18.5 kg/m2). Mean FEV1 was 47.7% predicted consistent with moderate lung impairment, but there was a wide spectrum of disease, 31 patients (18%) having an FEV1 ≤ 1.0 L indicating a subgroup with severe disease. BMD
Discussion
The major findings from our study in a young adult CF group are 3-fold: firstly, objective exercise measurements correlated significantly with BMD; secondly, VO2 uptake was an independent predictor of BMD; and thirdly, exercise measurements demonstrated a stronger relationship with total proximal femur Z-scores compared to lumbar spine Z-scores.
We found maximal O2 uptake to be a strong correlate of BMD. Our findings corroborate those of a previous study evaluating objective exercise
Acknowledgments
Funding was supplied by The Health Research Board of Ireland and the Cystic Fibrosis Research Trust Fund of Ireland.
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