[tdburn]

My opinion, the two most important vitamin levels to check are vitamin D and B12. Many people are easily deficient of both.

Some say 60-80 blood serum level is the optimal (80+ is possibly toxic) while others point to 40-50 as healthy

[kstrauser]

The situation with B12 isn't so clear: https://www.health.harvard.edu/cancer/b-vitamins-may-raise-r...

[jackmott]

there was a recent post here about B12 supplementation causing cancer.

[dawnerd]

Not so much cause but increases the risk.