Thanks so much for your clear response, MrsD. I recently had testing with these results: Vitamin B12 was 908 pg/mL and Folate RBC was 658 ng/mL. My G.P. doctor says these are fine, but 23andMe forum participants say I may still need supplementation, esp. for folate, due to my MTHFR "compound heterozygous" status, but type, source, dosage were unclear. Sounds like I should also request homocysteine testing. Thanks again!