Vitamin D and Cystic Fibrosis
Cystic fibrosis (CF) causes pancreatic exocrine insufficiency in about 85% of patients, which impairs absorption of fat-soluble vitamins A, D, E, and K. Vitamin D deficiency is nearly universal in untreated CF, and CF-related bone disease (fractures, low BMD) affects most adults with CF.
CF Foundation dosing targets
The Cystic Fibrosis Foundation recommends measuring 25(OH)D annually and maintaining a target of ≥ 30 ng/mL year-round. Standard dosing is 2–3× higher than the general population because of malabsorption:
- Infants: 400–500 IU/day; if 25(OH)D < 30, escalate.
- Children 1–10 years: 800–1,000 IU/day baseline; up to 4,000 IU/day for repletion.
- Adults: 800–2,000 IU/day baseline; 10,000 IU/day for correction of severe deficiency.
- Pancreatic enzyme replacement is essential; take vitamin D with a meal containing fat.
Water-miscible formulations
Standard oil-based cholecalciferol depends on bile and pancreatic lipase for absorption — the same processes that are impaired in CF. Water-miscible or micellar formulations (e.g. DEKA, ADEK, D3-Vi-Sol) bypass some of this and achieve better serum levels. If OTC cholecalciferol capsules fail to raise 25(OH)D despite adherence, switch to a CF-specific water-miscible product.
CF-related bone disease
Adults with CF have BMD 1–2 standard deviations below age-matched norms and elevated fracture risk. Causes are multifactorial: vitamin D and vitamin K malabsorption, chronic inflammation, chronic corticosteroid exposure, delayed puberty, hypogonadism, and reduced physical activity. Vitamin D + calcium is foundational; bisphosphonates are added in adults with osteoporosis.
Beyond bones — lung function
Small trials have examined vitamin D repletion in CF pulmonary exacerbations. Grossmann 2012 showed 250,000 IU cholecalciferol bolus in hospitalised CF adults trended toward reduced re-admission and shorter time to next exacerbation but the DISC pilot trial (2019) was underpowered. Larger trials are ongoing. Correcting deficiency to CFF targets is standard care regardless of the pulmonary question.
CFTR modulators — does the picture change?
Elexacaftor/tezacaftor/ivacaftor (Trikafta) has dramatically improved outcomes in eligible CF patients. Whether it reduces vitamin D requirement (through improved pancreatic function in some patients) is being studied. Continue standard CFF dosing until evidence changes practice.