Which metabolic marker is elevated in a case of vitamin B12 deficiency, even when vitamin B12 levels are borderline low?

In cases of vitamin B12 deficiency, one of the key metabolic markers that becomes elevated is methylmalonic acid. When vitamin B12 levels are low, even if they are only borderline low, the body struggles to convert methylmalonic acid into succinyl-CoA due to insufficient B12 coenzyme activity. This leads to an accumulation of methylmalonic acid in the blood, making it a specific and sensitive marker for vitamin B12 deficiency.

While homocysteine can also be elevated in vitamin B12 deficiency, it is not as specific since it can increase in other conditions as well, such as folate deficiency. Vitamin B12 and folate are involved in the same metabolic pathways, but the distinction lies in the fact that methylmalonic acid directly reflects the impairment caused by vitamin B12 deficiency.

Folic acid and serum iron do not provide relevant information regarding vitamin B12 status. Folic acid levels do not typically rise in response to vitamin B12 deficiency, and serum iron is unrelated to B12 metabolism. Therefore, the elevation of methylmalonic acid serves as a reliable indicator of vitamin B12 deficiency, highlighting its diagnostic significance.