Vitamin D in High Daily Doses Doesn’t Appear to Help – or Do Harm

Higher doses of vitamin D supplements do not appear to lead to better clinical outcomes, either in lung function or bone health, in people with cystic fibrosis (CF), a systematic review reported.

Because this crucial vitamin is lacking in patients, however, further studies evaluating the long-term effects of vitamin D supplements — those going beyond the recommended daily background dose for these people — are needed, the research team wrote.

The review study, “Vitamin D supplementation in patients with cystic fibrosis: A systematic review and meta-analysis,” was published in the Journal of Cystic Fibrosis.

Pancreatic insufficiency, due to a lack of the pancreatic enzymes necessary to help digest food, is a common digestive symptom of CF. Vitamin D’s lack can be a consequence of this insufficiency, driven by the buildup of thick mucus that clogs the pancreatic ducts.

Vitamin D is particularly important for bone health, as it stimulates calcium absorption by the gastrointestinal tract. This vitamin is also needed for the immune system to work properly, with some studies suggesting that sufficient levels of vitamin D can lower the risk of lung infections and possibly prolong survival for CF patients.

“However, clinical benefits and harms of vitamin D supplementation in this population are poorly documented,” the researchers wrote.

Scientists in Hungary conducted a systematic review to gather and summarize evidence from clinical studies exploring the potential benefits and side effects of vitamin D supplements in CF patients.

After a search in four online databases — Embase, Medline, Cochrane Central Register of Controlled Trials, and Web of Science — they identified 2,738 relevant studies.

Duplicates and studies that did not match certain eligibility criteria were then removed, including those not part of a randomized controlled trial (RCT) or reporting no original data.

In the end, researchers selected a total of eight RCTs for their systematic review. These eight studies compared the safety and efficacy of vitamin D supplements given CF patients to those of a placebo. Trials lasted between three and 24 months, and were highly variable in the supplemental doses given — typically, an additional daily vitamin D supplement of 1,600 to 5,000 international units.

Patients on these supplements were seen in analyses to have much higher levels of 25-hydroxyvitamin D — a vitamin D precursor — circulating in their bloodstream than those in a placebo group.

Still, no significant differences were found between the two groups regarding clinical outcomes that included bone health, lung function, and immunological indicators.

No significant differences were evident, either, in the incidence of side effects between both groups.

Based on these findings, the researchers argued that “additional vitamin D does not seem to influence clinical and laboratory outcomes.”

However, “the lack of significant differences between groups should still not be considered as definite ineffectiveness, the quality of evidence being very low,” they added. “The main reason behind this is the low number of participants in each study and the heterogeneous choice of reported outcomes.”

Additional controlled and randomized trials with longer follow-up periods — those lasting five or more years — are needed to evaluate vitamin D supplements in these patients, the team concluded.