www.lesswrong.com/posts/sF5gAxnmifQe2TBNt/the-worthlessness-of-vitamin-d-is-mild...
2 corrections found
VITAL carefully stratified the assignment to vitamin D or placebo based on baseline vitamin D levels, which should give more statistical power from a given sample size.
VITAL randomization was not stratified by baseline vitamin D levels. The trial papers say randomization was stratified by sex, race, and age group, while baseline blood sampling was optional and available for only about two-thirds of participants.
Full reasoning
This description of the VITAL randomization is contradicted by the trial publications.
The VITAL baseline paper states that randomization was computer generated within sex, race, and 5-year age groups. It does not list baseline vitamin D level as a randomization stratum.
A later VITAL biomarker paper also explains that baseline blood collection was voluntary and that 16,903 participants (65.3% of randomized) provided a pre-randomization blood sample; 25(OH)D was measured in 15,804 participants. Because measured baseline vitamin D was not available for the full cohort, the trial could not have "carefully stratified" assignment on that variable across all participants.
So the trial did analyze outcomes by baseline vitamin D in subgroups, but the assignment itself was not stratified on baseline vitamin D level.
2 sources
- Baseline characteristics of participants in the VITamin D and OmegA-3 TriaL (VITAL)
Randomization, which was computer generated within sex, race (African American vs. not) and 5-year age groups (age 50-54, 55-59, 60-64, 65-69, 70-74, 75-79, ≥80 years) in blocks of eight...
- Serum 25-hydroxyvitamin D in the VITamin D and OmegA-3 TriaL (VITAL)
A total of 16,903 (65.3 % of randomized) participants provided a (pre-randomization) blood sample at baseline ... Total 25(OH)D was measured in 15,804 baseline ... samples.
Among the major trials, only VITAL, ViDA, and FIND measured it for more than a tiny number of subjects.
This omits D2d. In the D2d trial, all 2,423 participants had vitamin D levels measured at baseline, so VITAL, ViDA, and FIND were not the only major trials with substantial baseline measurement.
Full reasoning
This claim is incorrect because D2d—which the article itself includes in its table of major trials—also measured baseline vitamin D in essentially the whole cohort.
The NIH summary of D2d says: "All study participants had their vitamin D levels measured at the start of the study." The NEJM paper likewise reports that participants were randomized regardless of baseline serum 25-hydroxyvitamin D level, and the trial's subgroup analyses used those baseline measurements.
Since D2d enrolled 2,423 participants, that is plainly more than a "tiny number of subjects." So VITAL, ViDA, and FIND were not the only major trials with broad baseline vitamin D measurement.
2 sources
- NIH-funded trial finds vitamin D does not prevent type 2 diabetes in people at high risk
All study participants had their vitamin D levels measured at the start of the study.
- Vitamin D Supplementation and Prevention of Type 2 Diabetes
We randomly assigned adults ... to receive 4000 IU per day of vitamin D3 or placebo, regardless of the baseline serum 25-hydroxyvitamin D level ... Categories of serum 25-hydroxyvitamin D are based on the 2010 Dietary Reference Intakes ...