Vitamin B12

Vitamin B12 plays an important role in the body, contributing to the maintenance of healthy red bloods cells and a healthy immune system, nerve function, and digestion, therefore it is important to ensure we have enough in our diet.

The NHS recommends that adults aged 19-64 years have an intake of 1.5mcg a day of Vitamin B12, and according to the National Diet and Nutrition survey this is easily being met, with men having an average intake of 5.7 mcg per day and women an average of 4.6 mcg per day.

What are vitamin B12 dietary sources?

The main dietary contributors of Vitamin B12 are milk, milk products, meat, and meat products, which is important to consider given the current trend for a reduction in meat and dairy in many groups, with plant based food sales increasing significantly year on year.

All animal products and yeast extract are a source of Vitamin B12, though particularly good sources include meat, salmon, cod, cheese, and some fortified breakfast cereals.

Bioavailability, how much of the Vitamin B12 is absorbed from the food, is a key consideration when identifying sources, which is discussed in length here for example eggs may be a good source of Vitamin B12 but it is shown to be poorly absorbed.

As plants do not have the required enzymes, they are unable to synthesise Vitamin B12.

This is not the same case for bacteria, with some algae providing a rich source, which is important for supplementation of vegan and vegetarian diets.

Who is at risk of vitamin B12 deficiency?

Dietary Vitamin B12 is found in 3 forms, known collectively as cobalamins, with the synthetic form known as cyanocobalamin.

The absorption of B12 is a complex process, with several opportunities for malabsorption to occur, increasing the likelihood of deficiency.

Though stores in the liver will prevent deficiency for 3 to 5 years, deficiency can still occur for various reasons. One of which could be a result of inadequate dietary intake, most likely to be observed in those adhering to a vegan or vegetarian diet.

In a population with increasing prevalence of diabetes, it is also important to note there is a well documented increase in B12 deficiency observed in those taking metformin.

In individuals with pernicious anaemia, a condition that increases in prevalence with age, hydrochloric acid (HCL) and other important co factors in the absorption of B12 are not synthesised, resulting in it not being absorbed and subsequently excreted in bile.

Malabsorption can also occur because of surgical complications such as blind loop syndrome, which is characterised by bacterial overgrowth in the intestine. The bacteria utilise B12, preventing absorption.

A gastrectomy procedure, most commonly a treatment for stomach cancer but also carried out as weight loss surgery in obese patients, can also have implications. This is due to the reduction of HCL synthesis in the stomach, which is key in allowing the disassociation of B12 from its food matrix.

Similarly, achlorhydria, or a reduced ability of cells to secrete HCL, has been observed in approximately 25% of elderly individuals, making them more likely to suffer from B12 malabsorption. It is important to note that this does not interfere with the absorption of B12 from supplements or fortified foods, making a case for B12 supplementation in the elderly.

B12 deficiency in the elderly is associated with reduced cognitive function, however paradoxically the evidence showing supplementation improves function is mixed and inconclusive, with some studies showing no improvement.

Consequences of vitamin B12 Deficiency

Deficiency in B12 results in impaired cell division leading to abnormal gut mucosa, neuropathy, a sore tongue, and megaloblastic anaemia, which is characterised fewer, larger red blood cells than normal.

These symptoms would also present as a result of folate deficiency, however specific to B12 deficiency is the demyelination of the spinal cord, optic nerves, and peripheral nerves.

Detecting vitamin B12 Deficiency

There are several biomarkers for deficiency including serum B12, whereby a result of less than 180 ng per litre indicates deficiency. Others include methymeonic acid, plasma homocysteine, and transcobalamin, which is the form of B12 that enters the cells and therefore may be more accurate and useful than serum B12 alone. An enzyme that is dependent on B12 is methylmalonyl CoA mutase, providing another potential biomarker for deficiency.

Suboptimal vitamin B12: A cause for concern?

Whilst the consequences of deficiency are well defined, increasingly research is suggesting there is an impact of suboptimal status as well.
The effects of this include neural tube defects, cardiovascular disease, cancer, and neuropsychiatric disorders, all of which are now thought to be of increased risk even in the absence of abnormal biomarkers or clinical symptoms. Understanding how to target individuals with a suboptimal status is a public health challenge.


For those eating a healthy diet including meat and dairy, vitamin B12 deficiency is very unlikely to be a relevant concern. However, supplementation is a useful and important tool for those following a vegan diet, or identified at risk groups such as the elderly.

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