Methylmalonic acid (Methylmalonate) is an indicator of Vitamin B-12 deficiency in cancer.
Vitamin B-12 is required for Methylmalonate (Methylmalonic acid; MMA) to be converted to succinyl-CoA, and in combination with folic acid, for homocysteine (HC) to be converted to methionine. Therefore, Methylmalonate is clearly more specific to vitamin B-12 deficiency compared to HC[1].
References:
[1]. Vashi P, et al. Methylmalonic Acid and Homocysteine as Indicators of Vitamin B-12 Deficiency in Cancer. PLoS One. 2016 Jan 25;11(1):e0147843.