D3 raises blood levels more than D2, but not when taken daily
At the same weekly dose for 12 weeks, vitamin D3 raised blood levels 45 nanograms per millilitre against 24 for vitamin D2. Taken daily, the gap vanished.
What the study found
D3 beats D2 on blood levels, and only when dosed occasionally
At the same weekly dose for 12 weeks, vitamin D3 raised blood 25-hydroxyvitamin D by 45 nanograms per millilitre against 24 for vitamin D2, about 87 percent more potent. Pooling head-to-head trials, that gap held for large occasional doses and vanished when the two were taken daily.
The caveat · Blood levels, not how anyone felt
The outcome in both studies is a blood level; neither measured whether anyone felt or functioned better. The 87 percent figure comes from 33 people on a single-blind protocol at a weekly dose, which makes it a statement about that regimen rather than about a daily D3 tablet against a daily D2 one. The pooled comparison searched the literature only to July 2011.