Somebody needs to read some Kuhn :-P
Vitamin D deficiency correlates strongly with time spent sitting around indoors. They control for physical activity, but not for time spent indoors as far as I can tell from the linked article. Separating time indoors from vitamin D levels would seem to be a tricky endeavor, so it's understandable that that's left uncontrolled.
A vitamin D-depression connection would make some sense. Given that humans largely evolved spending much more time outdoors than modern humans, deleterious effects of indoor life would not be surprising. +75% sounds like an extremely strong effect size deserving of some skepticism.
The original source: https://tilda.tcd.ie/news-events/2018/1813-vitd-depression/
You can take a vitamin D supplement and stay indoors or otherwise keep your routine and still receive the hormonal benefits if you are deficient.
If you really want to be anal about it, best move is to get your levels tested and then start supplementing and test levels again after a month, then up or down your dosage accordingly till you know how much you need to be taking daily. Most people depending on ethnicity, lifestyle/occupation, and locale just take like 500 - 5,000 IU D3 daily. If you're a white dude who goes surfing every day on the equator, you're probably good. If you're a black dude with a desk job, there's about a 90% chance you're deficient.
A doctor can diagnose extreme deficiency and might put you on like 50,000+ IU / week for a short while.
There really needs to be a lot more discussion over the specific scientific realms with lots more independent commentary on the study and results.
And as comes up frequently in human psychological issues repeatability is the gold standard and there isn't any in this specific case.
And humans and their behaviors are so complicated and so difficult to quantify that it seems so unlikely that one specific substance would be a "miracle cure" for human depression.
