Too few people became depressed for the trial to answer it
A sub-study followed 720 people at risk of late-life depression. Only 4.7 percent became depressed, leaving an estimate spanning a 94 percent fall and a 28 percent rise.
What the study found
Whether it prevents late-life depression is still unanswered
A sub-study of the VITAL trial followed 720 people with mild symptoms or other risk factors for late-life depression. Vitamin D3 did not beat placebo, but only 4.7 percent became depressed, leaving an estimate compatible with a 94 percent reduction and with a 28 percent increase.
The caveat · Too small to answer, its authors say
The authors state plainly that the study lacked the statistical power to answer its own question. Depression was diagnosed against structured criteria at in-person assessments, which is a real strength, but too few cases arose for that rigour to help. A result this imprecise is an absence of evidence, not evidence of absence.