Vitamin B12 (cobalamin) and folate (vitamin B9) are essential water-soluble vitamins that function as critical cofactors in one-carbon metabolism, DNA synthesis, and methylation reactions. Both are measured together clinically because their deficiencies produce overlapping hematological manifestations, particularly megaloblastic anemia, and their metabolic pathways are interdependent. Accurate interpretation requires evaluating both markers simultaneously, as isolated deficiency of one can mask or mimic deficiency of the other. Vitamin B12 is absorbed in the terminal ileum via intrinsic factor produced by gastric parietal cells, stored primarily in the liver, and serves as a cofactor for methionine synthase and methylmalonyl-CoA mutase. Folate is absorbed in the proximal small intestine, converted to its active form (5-methyltetrahydrofolate), and is essential for purine and thymidylate synthesis as well as homocysteine remethylation. The two vitamins converge in the methionine cycle, where 5-methylTHF donates a methyl group to homocysteine in a B12-dependent reaction, linking their deficiencies to elevated homocysteine levels.
Vitamin B12 and folate are two important B vitamins that your body needs to make healthy red blood cells, support your nervous system, and help cells divide properly. Low levels of either vitamin can cause a type of anemia where red blood cells become abnormally large and don't work well, leading to fatigue, weakness, and shortness of breath. B12 deficiency is especially important to catch early because it can cause nerve damage and memory problems that may not fully reverse if treatment is delayed. Folate is particularly critical during early pregnancy to protect the developing baby's brain and spinal cord. Your doctor measures both vitamins together because they work closely with each other, and a problem with one can affect how the other behaves in your body.
When elevated: Elevated serum B12 levels may indicate hepatic disease, myeloproliferative disorders, or solid tumors releasing B12-binding proteins, and should prompt clinical investigation rather than reassurance. Elevated folate is generally not associated with significant clinical toxicity, though very high supplemental intake may mask B12 deficiency. Supraphysiologic B12 levels in the absence of supplementation are considered a potential biomarker for occult malignancy in some clinical contexts. When low: Low vitamin B12 is associated with megaloblastic anemia, irreversible neurological damage, elevated homocysteine, and adverse pregnancy outcomes including neural tube defects. Low folate is strongly associated with megaloblastic anemia, hyperhomocysteinemia, and significantly increased risk of fetal neural tube defects when deficient periconceptionally. Both deficiencies are associated with increased cardiovascular risk through homocysteine elevation, though homocysteine-lowering therapy has not consistently demonstrated cardiovascular event reduction in clinical trials.
B12 and folate are critical for red blood cell formation and oxygen transport, both essential for endurance performance and recovery. Athletes with restricted diets (plant-based, low-calorie) or high training volume may have increased demand; adequate levels support energy metabolism, reduce homocysteine (a cardiovascular risk factor), and optimize mitochondrial function for sustained performance.
Turnaround Time
3 days (up to 12 days)
Fasting Required
No
Order online
Choose your lab and check out. We send your lab requisition automatically — no doctor visit needed.
Get your sample collected
Visit a lab service center near you for a quick blood draw (or book at-home phlebotomy where available).
See your results
Your results land in your Insider portal, ready to review and act on — that easy.
Your price
$17.97$60
3.3× less than retail
Versus the typical direct-to-consumer retail price for this test (illustrative — consumer prices vary by provider and region).
$17.97
$18.00
$37.90
Peer-reviewed sources supporting the educational content on this page.
