What is Active B12?

Cells need B12 to copy their DNA before they divide, so the tissues that feel a shortage first are the ones dividing fastest. The cells that go on to become red blood cells are among them: when B12 runs short they divide more slowly, and the cells that form are larger than usual.

MCV is the average size of a red blood cell, so it registers that enlargement but cannot name its cause. Measured alongside MCV, Active B12 ties the change in the cells to the supply behind it.

B12 comes only from animal foods and fortified products, so a diet without them supplies none. Absorption is a separate step that can fail on its own: it depends on stomach acid and a carrier system in the small intestine, and conditions or surgery affecting either can block it despite a good diet. The liver holds a store built up over years, which covers a gap in either diet or absorption.

Intake and absorption can both stop completely and a shortage still takes one to two years to appear. A yearly measurement is frequent against that pace, close enough to catch a supply that has started to drain before it has run down. This is Do Health's monitoring schedule, not a general recommendation for everyone.