[thewebguyd]

> Maybe following their example for this winter could be my next 'nutrition experiment'

Anecdotal and a sample size of 1, but I tried supplementing Vitamin D last year in the winter months. I live in the PNW, which between October and March, the sun is too low to trigger vitamin D synthesis in the skin to see if it had any effect on my energy levels and mood, I suffer from seasonal affective disorder pretty severely.

Taking 5,000 IU daily had no noticeable effect for me. A slight increase in energy levels but not significant enough that I'd be confident in attributing it to supplementation. I was hesitant to supplement more without medical advice and a blood test.

That's not to say Vitamin D isn't important (it is), and the scientists in Antartica definitely know what they're doing, but it's more to say YMMV.

For me, just making an effort to do more physical activity outdoors during the dark months had more of an impact

[seba_dos1]

~5000 IU daily between February and May was barely enough to raise 25(OH) D level in my blood from 9 to 30 ng/ml.

Depending on who you ask, 30 is either the bound between "deficient" and "insufficient", or between "insufficient" and "sufficient". Regardless of who you ask, there's plenty of headroom until "excess".

[rcxdude]

Yes, it's possible but relatively hard to overdose on vitamin D, and due to a cock-up in some calculations in a study, until recently the recommended supplement amounts were about an order of magnitude too low: https://pmc.ncbi.nlm.nih.gov/articles/PMC5541280/