How do they net out the idea that people with healthy lifestyles would be less likely to have a vitamin D deficiency? (Better diet, more exposure to sunlight)
There was an actual randomized trial that came out recently [1] where hospitalized patients were given extremely large doses of Vitamin D (explanation of equvalent doses [2]) and it dramatically reduced ICU admission.
This supports the idea that Vitamin D helping with COVID is probably causal rather than just a correlation.
One should not leave out the important part "pilot study". As said by the authors in the conclusions themselves, it still requires a larger one to give definite answer.
I agree that a larger N would be preferable, but the effect size was enormous, and so the P value was <.001. Therefore, IMO, it's very likely that Vitamin D has a causal effect. Given that Vitamin D has at least some causal effect, it's much more likely that the observational studies that show a correlation are due to causation.
They measure obesity and find it 0.02 vs 0.01 for vitamin d, which seems a decent enough proxy for lifestyle for this kind of study so your underlying point is valid here. Actually i think the biggest missing control is race.
I went in skeptical even though intuitively being deficient in vitamin d makes sense, this isn’t convincing but I agree with with their conclusion that it’s worth a bigger study.
If someone reading is wondering whether to maintain a healthy level of vitamin D to help if you get covid, yes you should. Should have done that before this study ofc.
Race can be a proxy for the amount of melatonin, which influences vitamin D. It can also be related to many other things. How would you use race to interpret the data?