Which vitamin deficiency leads to increased levels of homocysteine and urinary methylmalonic acid?

The deficiency of cobalamin, also known as vitamin B12, is associated with increased levels of homocysteine and urinary methylmalonic acid. This is due to the crucial role that vitamin B12 plays in two key metabolic processes: the conversion of homocysteine to methionine and the conversion of methylmalonyl-CoA to succinyl-CoA.

In cases of vitamin B12 deficiency, the enzyme methionine synthase, which relies on cobalamin as a cofactor, becomes impaired. This leads to the accumulation of homocysteine. Elevated levels of homocysteine are significant because they are associated with an increased risk of cardiovascular diseases.

Additionally, vitamin B12 is involved in the metabolism of certain fatty acids and amino acids, particularly in the conversion of methylmalonyl-CoA to succinyl-CoA via the enzyme methylmalonyl-CoA mutase. When vitamin B12 is deficient, this pathway is disrupted, leading to the buildup of methylmalonic acid, which is then excreted in the urine.

Thus, both elevated homocysteine levels and increased urinary excretion of methylmalonic acid are clinical markers indicative of vitamin B12 deficiency. This makes c