Vitamin B12 is an essential nutrient integral to the healthy functioning of the brain and nervous system. It is also involved in DNA synthesis and the formation of red blood cells. Because vitamin B12 is produced by bacteria in the large intestines of animals, plant-based foods are not a reliable source of vitamin B12 making people who follow a vegan diet a key at-risk group for deficiency. In this podcast, I’ll look at the roles of vitamin B12 and the main causes and consequences of deficiency. 

Vitamin B12 (also called cobalamin) is a crucial B-group vitamin. The human body needs vitamin B12 to make red blood cells, DNA, and carry out other functions such as nerve transmission. And for this, vitamin B12 doesn’t act alone as folate metabolism is tightly linked to the regeneration of the active form of B12 in the body.

Vitamin B12 is important in red blood cell production and is a key reason why deficiency leads to anaemia. Vitamin B12 also maintains the conductive sheath that surrounds and protects nerve fibres and promotes their normal growth. Bone cell activity and metabolism also depend on vitamin B12.

Because vitamin B12 is required to convert folate to its active form, one of the most obvious vitamin B12-deficiency symptoms is the anaemia of folate deficiency. This anaemia is characterised by large, immature red blood cells, which indicate slow DNA synthesis and an inability to divide.

How we absorb vitamin B12 is a little complex but is relevant to know about as it explains some of the risk factors for deficiency. In the stomach, hydrochloric acid and the digestive enzyme pepsin release vitamin B12 from the proteins to which it is attached in foods. The stomach also secretes a molecule called intrinsic factor. As vitamin B12 passes to the small intestine, it binds with intrinsic factor. Bound together, intrinsic factor and vitamin B12 travel to the end of the small intestine, where receptors recognise the complex. There the intrinsic factor is degraded, and the vitamin is gradually absorbed into the bloodstream.

A special type of anaemia called pernicious anaemia is a disease that affects the lining of the stomach with the dual consequences of a failure to produce intrinsic factor and reduced hydrochloric acid, resulting in vitamin B12 malabsorption. If pernicious anaemia is left untreated, it causes vitamin B12 deficiency, leading to anaemia and neurological disorders, even in the presence of adequate dietary intake of vitamin B12.

Vitamin B12 deficiency can be caused by either inadequate absorption or a poor intake; sometimes a combination of both. Inadequate absorption typically occurs for one of two reasons: a lack of hydrochloric acid or a lack of intrinsic factor. Without hydrochloric acid, the vitamin is not released from the dietary proteins and thus is not available for binding with intrinsic factor. Without intrinsic factor, the vitamin cannot be absorbed.

Many people, especially those over 50, develop atrophic gastritis which is a process of chronic inflammation of the lining of the stomach. Without healthy stomach cells, production of hydrochloric acid and intrinsic factor diminishes. Even with an adequate intake from foods, vitamin B12 status suffers.

Surgical procedures in the gastrointestinal tract, such as weight loss surgery or surgery to remove all or part of the stomach, often result in a loss of cells that secrete hydrochloric acid and intrinsic factor. This reduces the amount of vitamin B12, particularly food-bound vitamin B12, that the body releases and absorbs.

In otherwise healthy people, vitamin B12 deficiency can still be an issue. A prolonged inadequate intake of vitamin B12 can occur with a vegan diet because natural food sources of vitamin B12 are limited to animal foods. People who stop eating animal-derived foods containing vitamin B12 may take several years to develop deficiency symptoms because the body recycles much of its vitamin B12, reabsorbing it over and over again. Even when the body fails to absorb vitamin B12, deficiency may take up to three years to develop because the body conserves its supply. Yet, B12 deficiency is an issue still with some reports of up to half of all people following a vegan diet long-term classed as vitamin B12 deficient. If you want to know more about this, there was a nice review paper on vitamin B12 and vegetarian diets published in the Medical Journal of Australia which I’ll link to in the show notes https://www.mja.com.au/journal/2013/199/4/vitamin-b12-and-vegetarian-diets 

Vitamin B12 protects the nervous system and without it, permanent damage can result. Vitamin B12 deficiency can be slow to develop, causing symptoms to appear gradually and intensify over time. It can also come on relatively quickly. Given the array of symptoms a vitamin B12 deficiency can cause, the condition can be overlooked or confused with something else.

Vitamin B12 deficiency is characterised by anaemia, fatigue, weakness, constipation, loss of appetite, and weight loss. Neurological changes, such as numbness and tingling in the hands and feet, can also occur. Additional symptoms of vitamin B12 deficiency include difficulty maintaining balance, depression, confusion, dementia, poor memory, and soreness of the mouth or tongue

Typically, vitamin B12 deficiency is treated with vitamin B12 injections, since this method bypasses potential barriers to absorption. However, high doses of oral vitamin B12 may also be effective.

Food sources

Vitamin B12 is unique among the vitamins in being found almost exclusively in foods derived from animals so that includes fish, meat, poultry, eggs, milk, and milk products. Vitamin B12 is generally not present in plant foods. Anyone who eats reasonable amounts of animal-based food is assured of an adequate intake, and vegetarians who consume milk products or eggs are also protected from deficiency.

People who follow a vegan diet need a reliable source, such as soy milk fortified with vitamin B12 or vitamin B12 supplements. With few exceptions, vitamin B12 detected in plant foods is likely to be the inactive analogue, which has no biological activity, and can interfere with the absorption of the active form of this vitamin.

Spirulina which is a biomass of cyanobacteria is sometimes promoted as a good source of vitamin B12, but what you’re not being told is that it is in the form mostly as inactive B12 analogues that may even have the ability to interfere with B12 absorption.

More popular foods that have gained recent attention in vegetarian-based diets are fermented foods such as tempeh, kimchi, sauerkraut and kombucha. Unfortunately, these fermented foods only contain trace amounts and do not hold a sustainable source of vitamin B12. Extensive research shows that the amounts of B12 claimed to be in these plant products is inaccurate and misleading because the vitamin B12 is in an inactive, unavailable form.

Mushrooms do contain a small amount of active vitamin B12, but the quantity of mushrooms needed to supply adequate amounts of B12 is impractical to make this a useful food source.

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.