That is absolutely the right question to ask. If the results are not adjusted for vitamin D levels, and it does not appear that they are, then the results are largely useless because vitamin D is very easy to supplement.
Note: Adjusting by vitamin D levels is not the same as adjusting by supplemental intake. Even adjusting for supplemental intake must be a linear adjustment, not a binary one.
Note: Adjusting by vitamin D levels is not the same as adjusting by supplemental intake. Even adjusting for supplemental intake must be a linear adjustment, not a binary one.