In a study of healthy, older urban residents living in India, vitamin B12 blood levels were found to be low in 16% of the 175 subjects tested. But when the blood was tested for methylmalonic acid (MMA), 55% of the population were found to have a functional vitamin B12 deficiency. MMA is elevated when there is not enough B12 available in the cells for the energy pathway. It is therefor a highly sensitive indicator of cellular vitamin B12 deficiency even in otherwise healthy people, and can be easily measured in the blood or urine.