Except that in this case you can just go to your doctor and get your vitamin D level tested. Then you adjust your dosage as needed to reach the recommended blood concentration. Adequate vitamin D levels are the subject of a little controversy [0], but I don't think it is a fad to consider that people need a certain level of vitamin D.

[0]https://www.health.harvard.edu/blog/vitamin-d-whats-right-le...

Alternative thoughts on adequate vitamin D: https://www.vitamindcouncil.org/i-tested-my-vitamin-d-level-...