Absorption efficacy comes in here for sure. Doesn’t matter how much one swallows if the system cannot absorb it one proves deficient. Was reading very recently Vitamin D should be taken with fat to aid absorption. Be good to find out quantities. More research.
No doubt genetics plays a part with absorption as does overall vitamin balance within the body and, in ‘our’ cases on here the hypersensitivity associated with the condition. Our associated GI issues don’t help either.
Migraine management for Otolaryngologists
A lot of it must come down to individuality because if you search this forum you will find there are people with extremely low Vitamin D (as low as 9) yet others who have extremely high levels yet according to discussion all had been relatively housebound for longish periods of time.