ORLANDO – People with an excessive blood level of vitamin D from overdosing with supplements had a 2.5-fold increased incidence of atrial fibrillation, based on a study of 132,000 residents of Utah and southeastern Idaho.
The finding "suggests the need for caution with vitamin D supplementation and the need for careful assessment of serum levels if high doses [of vitamin D] are used," Megan B. Smith said at the annual scientific sessions of the American Heart Association.
The finding also suggests that patients identified with new-onset atrial fibrillation should be evaluated for a possible extremely high vitamin D level, said Ms. Smith, although in the results she reported, the high blood level of vitamin D linked with a significantly elevated incidence of atrial fibrillation, greater than 100 ng/dL, was extremely unusual, occurring in just 291 of the 132,000 people (0.2%) included in the study.
Although the mechanism linking such an extremely elevated blood level of vitamin D to a markedly increased rate of new-onset atrial fibrillation remains unclear, a likely explanation is the hypercalcemia that vitamin D toxicity can cause. Hypercalcemia can, in turn, reduce cardiac conduction velocity and shorten cardiac refractory time, said Ms. Smith, a dietician at Utah State University in Logan.
"Utah [residents have] tremendous use of supplements. From what we’ve seen in the charts we have, excessive use of vitamin D supplements is the primary driver" of the high levels seen, said Dr. T. Jared Bunch, director of electrophysiology research at the Intermountain Medical Group in Murray, Utah, and lead investigator for the study. "The few patients [with very high vitamin D levels] who I have seen got vitamin D in their milk, from a multivitamin, and from vitamin D pills. They get it from multiple sources," but added that the low prevalence of levels above 100 ng/dL also showed that it is a difficult level for a person to reach.
"Utah has an enormous problem with vitamin D deficiency, so we had this large group of people" who were members of Intermountain Healthcare, and had their vitamin D level measured once as part of their routine care. A survey by Dr. Bunch and his associates showed that unless asked, people don’t usually tell their physician that they take a vitamin D supplement, and that physicians at Intermountain Health do not usually ask patients about their vitamin D intake.
The measurement numbers documented the extent of the vitamin D deficiency problem, with 38,000 of the 132,000 people measured (29%) having a blood level below 20 ng/dL. This group with vitamin D deficiency showed significantly elevated prevalence rates of diabetes, hypertension, coronary artery disease, heart failure, and depression, compared with people in the designated "normal" vitamin D range of 41-60 ng/dL. But notably the incidence of atrial fibrillation in the deficiency group was not significantly different than the rate in the reference group with a normal vitamin D level at baseline.
"There is something unique" about the excess, toxic level, for atrial fibrillation incidence, Dr. Bunch said in an interview.
To better examine the potential role of vitamin D in elevating atrial fibrillation risk, Dr. Bunch and his associates are now regularly measuring blood vitamin D levels in Intermountain Healthcare members and prospectively tracking their atrial fibrillation incidence.
The results reported by Ms. Smith came from a retrospective analysis of the one-time vitamin D measurement by an immunoassay, and atrial fibrillation incidence tallied over an average 584 days of follow-up based on ECG testing and ICD-9 codes in each person’s medical record. The most common vitamin D level measured was 21-40 ng/dL, in 73,547 people (56%). Another 17,234 people (13%) had a level of 41-60 ng/dL, which the researchers considered normal and which they used as the reference group.
During follow-up, the incidence of new-onset atrial fibrillation was about 1.5% in all subgroups based on their baseline vitamin D level, except for those with a level above 100 ng/dL, who had an incidence of about 4%. A multivariate analysis that controlled for baseline differences in demographics identified a significantly elevated atrial fibrillation rate only in people with a baseline vitamin D level greater than 100 ng/dL.
Ms. Smith and Dr. Bunch said that they had no disclosures.