Mrs. D's advice is dead-on. B12 and folate are two of the most critical nutrients involved with neurological health, dna synthesis and repair, and through the methylation cycle, critical to glutathione generation.

If you are not supplementing with the methyl form of b12, then i would suggest a urine mma test to measure functional sufficiency of b12 since serum b12 levels do not dinstinguish between active (methylB12 and adenolsylB12) and inactive (the commonly found cyanocblB12). Some people have a genetic inborn error that prevents efficient conversion of inactive to active forms, so serum B12 could be normal/high, but active/functional levels could be reduced. Even if you are taking the methyl form, the mma test that Mrs. D also mentioned will give you insight if your dosage is sufficient from a functional perspective.

The story is similar for folate, the form found in most common supplements is an inactive form. For folate, the FIGLU metabolite is the measure to check for functional sufficiency.

My mom who has PD has genetic polymorphisms on genes involved with B12 and folate metabolism. However, based on her MMA and FIGLU measures, she is only difficient in B12 and not folate. Her serum B12 was 900+ and she had recently (within about 4-6 weeks) started sublingual methylB12, yet her MMA was elevated indicating she still had a B12 deficiency. I suspect that her B12 deficiency was even higher before and is on its way down, but she nevertheless increased her methylB12 dosage to make sure .... We plan on etesting in about 4 months.