Women on placebo reached 57% at the vitamin D target around summer, then slipped. Weekly calcifediol held above 93%, in a drug-maker's analysis of 261 women that didn't test it against ordinary vitamin D

Calcifediol is the form of vitamin D the liver makes from the vitamin before it circulates in the blood, so taking it skips one step. A randomised, double-blind phase II/III trial tested a weekly 100 µg capsule across Europe, with centres in countries including Spain, France, Italy and the Czech Republic. This new paper looks back at the women only: 192 who took calcifediol and 69 who took placebo for a year, all starting with low blood levels of 25-hydroxyvitamin D, or 25(OH)D. A little over a third, 100 women, had severe deficiency, at 10 ng/mL or less.

The blood results are clear. In the calcifediol group, average levels began at 12.21 ng/mL, reached 23.68 by week 4 and 39.47 by week 16, then stayed above 41 ng/mL. At week 4, 68.2% of women on calcifediol had reached at least 20 ng/mL, against 5.8% on placebo. From week 16, more than 93% stayed there. For the higher 30 ng/mL target, the share was above 76% from week 16 to week 52, while placebo peaked at 6.2%.

The placebo arm is the most interesting line on the chart. With no active treatment, 57.4% reached 20 ng/mL by week 24, and the average climbed to 20.36 ng/mL, then slid to 16.68 by week 52. Those higher readings coincided with more visits falling in August or September, and the authors point to the seasons and sunshine. Diet might have played a part too, since vitamin D intake was never measured. The capsule group showed no such dip.

Safety looked unremarkable over the year, with adverse events of any cause in 35.9% on calcifediol and 33.3% on placebo. Readings of 80 ng/mL or more, a level the study tracked as elevated, turned up only with calcifediol, in 1.6% of women, none of whom had a related adverse event. Raised calcium was about equally rare (2.1% against 2.9%). Women with raised parathyroid hormone, a typical result of low vitamin D, fell from 19.7% to 4.6% on the capsule and from 22.1% to 17.0% on placebo.

Then the caveats. This was a post hoc analysis, done after the main trial, and it left out anyone who needed rescue vitamin D during the study. The authors acknowledge that this may have selected the placebo women who were recovering by themselves, which if anything would make the capsule's advantage look smaller. The trial was funded by the manufacturer, FAES FARMA, three of the authors are its employees, and two others were paid as investigators. Almost all participants were White, and no formal test compared the treatment effect across body-weight or menopause groups, so the similar results there are only suggestive.

Above all, the comparison was with a dummy pill. This paper can't say whether calcifediol works better than an ordinary vitamin D3 supplement, and blood levels aren't the same as stronger bones or fewer falls. Those weren't what this analysis measured. For a woman with a confirmed deficiency, it is decent evidence that this regimen corrects the number and holds it through winter. Whether it deserves a place over standard vitamin D3 is a separate question.