Very good and balanced advice, as usual, Julian. It might interest the readers that aside from the threshold of 25(OH)D ≥ 20 ng/mL as the demarkation between sufficiency and insufficiency, there is the < 12 ng/mL to define deficiency. At concentrations inbetween an individual is considered "insufficient". Different medical associations may differ on their recommendations for supplementation in this grey zone of insufficiency.

My team and I have explored the effects of 25(OH)D sufficiency levels on sport performance in top athletes. But the results were not very strong and only affected the deficient athletes.