10/18/2026
As the daylight gets shorter, your skin makes less vitamin D, and food covers relatively few people.
A 2024 meta-analysis pooled 43 trials and more than 49,000 people. Across the whole group, no meaningful effect on respiratory infections. In the subgroups, a modest effect for daily dosing during the darker months.
So the useful question is not whether to take it. It is where your levels actually sit.