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The Greenhouse

A Focus on B12

Vitamin B12 is a complex compound with some quite complex activities and functions as you can see from the diagram below. I’ll try not to bore you too much with the science, but there are some important things for you to understand about B12 when on a plant based diet.

Why is B12 important?

A unique and complex vitamin, B12 is often referred to as cobalamin due to the fact that it contains a metal ion cobalt. There are various forms of cobalamins, with only two being active as co-enzymes in the human body methylcobalamin and adenosylcobalamin. Most supplemental B12 is supplied as cyanocobalamin due to its stability and this must be converted to a biologically active form in the body.

Vitamin B12 is essential for normal blood formation, neural function and growth and is involved in cells’ metabolism, DNA synthesis and regulation.

The diagram below shows how B12 is required for the synthesis of the amino acid methionine which is part of a biological process called methylation. Dysfunction in methylation can affect gene expression and has been linked to diseases such as cancer. Inadequacies in nutrients involved in this metabolic cycle can lead to excess homocysteine which may be implicated in cardiovascular disease and dementia.

Another of B12’s co-enzyme activities involves the body’s energy production cycle and the synthesis of haemoglobin the oxygen carrier in red blood cells.

Diagram courtesy of Oregon State University

Why your stomach is important

Absorption of B12 from food requires an optimal working digestive system. Stomach acid and enzymes are needed to free the B12 from food to allow it to bind to intrinsic factor which is secreted by the stomach cells. It is then transported to the last section of the small intestine where it can be absorbed.

Stomach acid may be low ((hypochlorhydria) due to various reasons such as older age, certain medications, dietary and lifestyle factors and this can affect B12 absorption.

Possible consequences of B12 deficiency

Pernicious anaemia can result from antibodies attacking the stomach cells and intrinsic factor, which stops B12 from being absorbed and causes deficiency. A common infection called Helicobacter Pylori may also be implicated in the condition, as the antibodies may react to this also.

Megaloblastic anaemia can be caused when B12 deficiency affects folate availability. The resulting impairment of DNA synthesis causes large dysfunctional red blood cells.

Symptoms of B12 deficiency may include tingling in the hands and feet, fatigue, shortness of breath, swollen red tongue and gastrointestinal symptoms such as constipation and loss of appetite. Deficiency has also been associated with depression. It is important to see your doctor about any concerns and to rule out any other conditions.

Other causes of B12 deficiency may include genetic disorders, coeliac disease, surgical removal of the stomach or infections. Plant based diets can also be a cause of deficiency.

Foods, supplements and tests

Whilst some people on purely plant based diets claim they are getting enough vitamin B12 from their diet, it is important to remember that although some studies have shown plant foods such as nori and purple laver to be good sources, unless you are eating these foods every day it is very unlikely that you will be getting enough B12 without using fortified foods or supplements.

B12 is synthesised from bacteria and is present in animal foods, so vegetarians who eat dairy and eggs will consume it in their diet but may also need to include a supplement. Some types of B12 found in plant foods such as algae may not be biologically active. You also need to consider any absorption issues as discussed above.

There are many B12 supplements available in different forms such as methylcobalamin, cyanocobalamin and hydroxycobalamin and in various types such as sublingual, chewable and sprays. The research is not conclusive about which form is best and much depends on your personal health history so it may be advisable to consult a nutrition practitioner if you’re not sure.

GP’s offer standard serum total B12 tests but this type of test is unable to discriminate between active and inactive forms. Declines in active B12 in a deficiency state may be masked by the more abundant inactive form and show up on a total B12 test as being within reference range. Active B12 ((holotranscobalamin or holoTC), Methylmalonic acid (MMA) and homocysteine tests are offered by private laboratories and may be a more reliable way to establish B12 levels.

Useful resources

Biolab UK offer private B12 tests

If you would like to discuss any aspect of your diet with a Registered Nutritional Therapist, please email

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