[DavideNL]

> fixed that and I did not even get common cold for a long time, also no longer random days with no energy.

To state the obvious; Since there are so many things changing in our lives, it's impossible to know whether the "improvements" you mention were specifically caused (/only) by taking more vitamin D.

[tomalaci]

Sure thing. You should always be aware of anecdotal evidence.

In my particular case when I did initial blood test, vitamin D was the only thing critically low. There were few other micronutrients that were just slightly low. I was prescribed high-dosage D-vitamin supplements and was told to touch some grass while out in the sun (computer screen time did not count). In just a month after doing that my overall noticeably improved.

After the wake-up call with vitamin D efficiency I paid more attention to my diet and also having some regular exercise. Improving combination of those gave me my next jump in health improvement. Afterwards, I started using few supplements such as L-theanine with my usual cup of coffee in the morning, I'd like to believe it has given me just a slight boost in focus but it could still just be plain old caffeine. However, I know that some people respond to it a lot stronger so there is certainly variation and is quite anecdotal.

[kick_in_the_dor]

Sure, while you can't prove a direct cause-effect, the anecdotal evidence seems rather strong.

I also experienced this same effect after moving to London (from California) in the winter (very cloudy). I felt so low energy for a solid month, and was constantly feeling unwell. Moreover, I had the strangest "cravings" to bask in the sun (such a weird thing to explain, I've never had such an urge before).

The week that I started to figure out my Vitamin D deficiency, I made sure to get a lot of sun every day (it just started getting sunny in London). I noticed immediate improvements.

[amelius]

You can also see a doctor and get vitamin D injections.