Vitamin D deficiency in Cystic Fibrosis Despite routine supplementation, the prevalence of vitamin D deficiency in cystic fibrosis (CF) is common ranging from 20 to over 80%, depending on the definition used to categorize the normal range.15-20 The etiology of low vitamin D in CF is poorly understood but contributing factors include decreased gastrointestinal absorption, impaired hydroxylation of vitamin D21, reduced sun exposure combined with increased use of sunscreens, seasonal influence as well as geographical location. Low bone mineral density has been well documented in CF and although bone disease in CF is likely multifactorial, vitamin D deficiency has been implicated as an etiologic factor15;17;19;22. Although few studies show a clear relationship between vitamin D and low bone density, it is plausible that sub-optimal serum vitamin D levels may aggravate bone disease in this population and that normalizing levels will maximize bone health. Furthermore, fracture risk appears to be increased in the CF population17;19;23. Vitamin D supplementation in elderly and osteoporotic subjects has been shown to reduce fractures, improve BMD and decrease falls11 but there are few studies examining the effect of vitamin D supplementation in CF16;18;24.