The proper claim is that raising people from deficient (<20ng/ml) to a sufficient level (>30ng/ml ideally) should improve outcomes. Sometimes looking at a subset of those who started deficient is good enough to find such an effect, but sometimes it isn't, such as if the dose used was not high enough to raise people up enough, particularly common if the subjects weights were high which requires higher intakes to move the needle a lot, so other subset analysis look at norm-weight subjects. Proper study guidelines were published in the mid 2010s and meta-analyses like this one that aggregate many studies that don't conform to these guidelines are just as invalid as individual studies that don't.
Nonesense. It's well established that dietary D3 (or D2, but D3 is better) clearly incleases 25(OH)D serum levels. It is that serum biomarker whose deficiency (levels below 20ng/ml or 30ng/ml) that has the largest association with higher risk of dozens of diseases as well as all-cause mortality. You can defend your phrasing by saying that "not well" just means one has to take a lot of it. Yes, there is a wide variety of dose response, which is why it's best to test blood levels to titrate supplementation amount.
The academic paper this story is based on is here: https://nature.com/articles/s43587-024-00793-y
Despite the linked story title only mentioning omega3, the paper was about 3 interventions, that, vitamin D, exercise and found reduced epigenetic age from the interventionts, with bigger anti-aging benefit as they were combined.
With respect to vitamin D specifically, this isn't by itself good evidence vitamin D reverses aging. But it is consistent with the totality of evidence that being vitamin D deficient probably (causally) speeds aging. And it mildly increases the overall weight of evidence of a connection.
As with much vitamin D research, it would have been better for the intervention to have been titrate vitamin D supplement amount to achieve an optimal target range (eg 30-60ng/ml) rather than using a fixed relative moderate dose (2000IU).
The problem with a fixed dose is that surely some subjects started severely deficient, some mildly, and some had sufficient levels. The moderate but not very high fixed dose will have helped many in the middle group climb out of deficiency, not been enough for many in the 1st group to bring them fully out of deficiency, and been unnecessary for those in the 3rd group, so the study ends up seeing a weaker overall effect averaged over all subjects. This is well known within the vitamin D world, eg see https://x.com/KarlPfleger/status/1732514710715514883, but somehow has not permeated to be widely enough understood, even within the scientific community (or the Hacker News community).
No, ample evidence shows that vitamin D (or vitamin D deficiency at least) is causally related to multiple diseases. The evidence is pretty overwhelming. Eg here's a just published review on D deficiency causality for Covid based on applying Hill's Criteria for causality https://www.mdpi.com/2072-6643/17/3/599 and a prior 2022 paper on the same subject https://pubmed.ncbi.nlm.nih.gov/35308241/
Some vitamin D skeptics claim tat there's insufficient evidence from RCTs but few people question the causality of smoking's influence on cancer despite no RCTs for that---that relationship was established also via Hill's Criteria.
For detailed treatment of this see this thread on X: https://x.com/KarlPfleger/status/1732514710715514883