"When two sources of inactive B12 were eaten together, B12 levels became worse. But when algae (higher inactive B12), fish (higher in active B12) and milk (higher in active B12) were eaten together, B12 levels improved. This suggests that two sources of active B12 may outweigh the negative effects of its inactive counterpart.
On the other hand, when one food source of inactive B12 (nori seaweed) was eaten with one food source of active B12 (fish), B12 levels worsened. This suggests that if a food source contains a higher level of inactive B12 vs. a food that contains active B12, it can cancel out the active B12 of a food and prevent the body from using it at all— hence being referred to as “useless.”