Vitamin B12 deficiency raises homocysteine.

Vitamin B12 is a fundamental regulator of homocysteine levels, functioning as an indispensable cofactor within the one-carbon metabolism cycle.

Mechanistic evidence

The relationship between vitamin B12 and homocysteine is defined by a specific enzymatic reaction:

• The Methionine Synthase Reaction: Vitamin B12, specifically in the form of methylcobalamin (MeCbl), serves as the essential cofactor for the enzyme methionine synthase (MS). This enzyme facilitates the transfer of a methyl group from 5-methyltetrahydrofolate (5-MTHF) to homocysteine.
• The Catalytic Cycle: During this process, the cobalamin cofactor acts as an intermediate methyl carrier. It accepts a methyl group to become methylcobalamin and then transfers that group to the thiol group of homocysteine, producing the amino acid methionine and regenerating tetrahydrofolate.
• Metabolic Bottleneck: When B12 levels are insufficient, methionine synthase activity is impaired. This creates a metabolic "trap" where 5-MTHF cannot be utilized and homocysteine cannot be converted to methionine, leading to an accumulation of homocysteine in the blood.

Clinical effectiveness and biomarkers

The clinical manifestation of B12 deficiency is frequently identified through elevated total homocysteine (tHcy) levels.

• Functional Biomarkers: Serum homocysteine (typically >15 µmol/L) and methylmalonic acid (MMA) are established functional markers for B12 deficiency. Research across diverse populations, including NHANES cohorts, consistently shows a robust inverse correlation between serum B12 and homocysteine levels.
• Folate Interaction: While both B12 and folate are required for homocysteine remethylation, B12 is a mandatory cofactor. Evidence indicates that folate sufficiency cannot compensate for B12 deficiency; in some cases, high folate levels can actually exacerbate the metabolic markers (homocysteine and MMA) of a B12 deficiency.

Bottom line

Vitamin B12 is the essential cofactor for methionine synthase; its deficiency directly impairs the remethylation of homocysteine to methionine, leading to elevated serum homocysteine levels.