NEW YORK (Reuters Health) – In older adults, mean serum levels of 25-hydroxyvitamin D are somewhat higher after a year of taking cholecalciferol (vitamin D3) compared to ergocalciferol (vitamin D2), a study shows. Furthermore, the effect is the same whether dosing is daily or monthly, the researchers report in the Journal of Clinical Endocrinology and Metabolism, online February 2.

Dr. Neil Binkley and colleagues at the University of Wisconsin, Madison, conducted a study to see if the two forms of vitamin D were interchangeable, as is widely assumed, in terms of raising serum levels of 25-hydroxyvitamin D, or 25(OH)D.

The investigators assigned 64 community-dwelling adults over age 65 to receive vitamin D2 or D3 as 1600 IU daily or 50,000 IU monthly, along with daily or monthly placebos for blinding purposes. Initially, serum 25(OH)D levels were less than 30 ng/mL in 40% of the subjects.

Compliance rates in the four arms of the study ranged from 91.6% to 98.9%, according to the report.

“Total 25(OH)D increased from baseline to the 12- month follow-up with all regimens,” the researchers report.

The increases were 32% with D3 daily; 29% with D3 monthly; 21% with D2 daily; and 11% with D2 monthly. Furthermore, “The absolute increase at 12 months with D3 was greater than with D2 for both daily (9.2 vs. 6.1 ng/mL, respectively) and monthly (8.9 vs. 3.6 ng/mL, respectively) dosing,”

However, the response to the form of vitamin and the dosing schedule varied considerably between individuals. In fact, serum 25(OH)D levels remained below 30 ng/mL in about 20% of the subjects. “This warrants measurement of 25(OH)D concentration when vitamin D supplementation is used in clinical practice,” Dr. Binkley and colleagues advise.

“In conclusion,” they write, “vitamin D supplementation with 1,600 IU daily or the equivalent amount once per month (50,000 IU) does not ensure a serum 25(OH)D concentration of more than 30 ng/mL in all people.”

They add, “Vitamin D3 is slightly, but significantly, more effective than vitamin D2 at increasing circulating 25(OH)D. The physiological importance of this, if any, remains to be determined.”

Reference:

Evaluation of Ergocalciferol or Cholecalciferol Dosing, 1,600 IU Daily or 50,000 IU Monthly in Older Adults

J Clin Endocrinol Metab 2011;96.