There's definitely something else there because it's been shown to improve things for people at extreme latitudes during their respective winters. But supplemen

There's definitely something else there because it's been shown to improve things for people at extreme latitudes during their respective winters. But supplements are probably not necessary for most people in regions where you can get at least some access to sunlight every few days. So while there is a decent bit of established evidence from more extreme environments that Vitamin D is important, there's only now really been enough research covering whether a supplement is necessary for people who can source it through "normal" pathways.