Introduction

Vitamin B12 testing remains the most common vitamin investigation in clinical practice and is often included in the investigation of common problems such as anaemia and dementia.
The assessment of Vitamin B12 status using blood tests is imperfect and although a variety of other tests can be used to improve assessment, this can lead to complexity and confusion. In this discussion I hope to share the insights from thousands of analyses and hundreds of clinician’s questions.

Sources of Vitamin B12

Vitamin B12 is a unique cobalt-containing molecule naturally synthesised by bacteria. Some animals, especially herbivores, absorb it from their intestinal microbiome, and build up a store.

Other animals, particularly carnivores, can obtain B12 by eating animals that store B12, or animal-based products such as eggs and milk. Vegetarians consuming milk products and eggs may have low B12 levels, as the B12 content of milk is often low (1mg/L) and even lower if ultra-heat treated. Non-animal sources of B12 are extremely limited, with Nori seaweed containing small amounts and B12 levels in mushrooms and most other plant-based sources reflecting bacterial exposure (eg manure/compost).(1)

Only strict vegetarians are considered at serious risk of dietary B12 deficiency, and even then only after some years. However, vegetarian and vegan diets are becoming increasingly popular. Similarly, breast-fed infants of vegan mothers, if not supplemented, may also be at risk of B12 deficiency.