Vitamin B12, also known as cobalamin, is one of eight B-vitamins required by the body that we cannot produce on our own and must get through our diets (1). Vitamin B12 is the most structurally complicated and largest vitamin of all the B vitamins. Vitamin B12 is shaped like a ring, and this ring is made out of something called corrin. In the centre of the ring, sits the biochemically rare element, cobalt.
All the substrates from which B12 is made must be synthesised by bacteria. This means that any one person’s vitamin B12 status is partially dependent on the health of their gastrointestinal tract and gut microbiome. After bacterial synthesis, the human body converts any form of B12 into an active form that it can use.
Main Roles of Vitamin B12
Vitamin B12 has an important role in cellular metabolism, as one of two "active” forms – methylcobalamin and 5-deoxyadenosylcobalamin. The function of vitamin B12 is closely linked with vitamin B9 (folate). You could think of these two vitamins as being very good friends because they do almost everything together! Adequate levels of vitamin B12 and folate are required to convert homocysteine to methionine (2). A deficiency in either B12 or folate can cause this cycle to become “jammed” and homocysteine can build up in the blood. Elevated levels of homocysteine are associated with increased risk of stroke, cardiovascular disease, neurological conditions, infertility, and birth defects (13-16).
Vitamin B12, again working alongside folate, is essential for the synthesis of DNA and the production of red blood cells, which carry oxygen around the body (1). Vitamin B12 also has a critical role in the nervous system to help produce the myelin sheath surrounding nerve cells (the myelin sheath insulates nerves to protect them and increase the speed of communication) (1). Given that you have nerves supplying every part of your body, a deficiency in B12 can lead to system wide consequences!
Symptoms and Consequences of Nutrient Deficiency
The absorption of vitamin B12 in the human body is by far the most complicated out of all vitamins and minerals. It relies on proper enzymatic function at the stomach, the intestine and the cellular level. A dysfunction at any one of these critical points can lead to a deficiency in B12, irrespective of how much vitamin B12 someone is getting in their diet.
Deficiencies in B12 are usually caused by malabsorption, as suggested above. However, as animal sources of food (animal proteins) are the main source of vitamin B12, vegans and vegetarians are at significantly greater risk of vitamin B12 deficiency from inadequate dietary intake (4). Older adults are also at an increased risk due to a reduction in the production of hydrochloric (stomach) acid as a result of ageing. Additionally, certain conditions such as pernicious anaemia, lead to massive dysfunction at one of the critical B12 absorption points, causing a deficiency. Finally, there are many commonly prescribed medications that interfere with the absorption of B12, including antacids (for reflux), proton pump inhibitors (hypertensive medications), metformin (for diabetes or weight management), nitrous oxide anaesthesia, and some epileptic medications. If you can identify with any of the higher risk categories for vitamin B12 deficiency outlined here, it is recommended that you speak to an Ellipse Health Dietitian about preventing or managing a deficiency.
Vitamin B12 deficiency symptoms are often mistaken for other conditions because they tend to be common and may even feel “normal” given that a deficiency in B12 generally takes a long time to develop. Symptoms include weakness, fatigue, loss of appetite, hazy memory, slow digestion/constipation, and difficulty concentrating (5, 6). The liver is responsible for being able to store relatively large amounts of B12. It can store enough to last for several years, such that the onset of a vitamin B12 deficiency is usually quite gradual and frequently misdiagnosed. Due to its utter importance in nerve function, an unmanaged B12 deficiency can lead to neurological disorders, including peripheral neuropathy (numbness, pain and/or tingling in hands and feet), mild cognitive impairment, Dementia, and loss of vision (7, 19-20). Low vitamin B12 is also associated with depression, and B12 supplementation has been shown to improve depressive symptoms (17-19).
With consideration that vitamin B12 and folate do almost everything together, an excessive intake of folic acid (the synthetic form of folate often found in fortified foods and supplements) can mask a B12 deficiency. When B12 concentrations become low, folic acid can “takeover” to prevent anaemia and mask many symptoms that would otherwise indicate a deficiency in B12. This may sound like folic acid is picking up the slack for its good friend B12, but what it really means is that B12 concentrations continue to fall lower and lower without the person affected being aware. While this is happening, homocysteine concentrations rise and neurological damage can begin.
One way to “catch” this is with a simple blood test. Ask your medical doctor to test for serum homocysteine levels. If your serum homocysteine levels are elevated, your doctor should suspect a potential vitamin B12 deficiency at the cellular level (in combination with your signs and symptoms) even if your serum vitamin B12 is within “normal” range. Useful blood tests for assessing B12 status include: haematology, vitamin B12 (active and serum), folate, and homocysteine.
Some studies have suggested that low vitamin B12 levels are associated with obesity and being overweight (9). Other studies have also shown that adequate vitamin B12 status can improve insulin resistance (10). Nevertheless, all individuals should aim for at least the recommended daily intake (RDI) of vitamin B12 everyday as an absolute minimum. The idea that certain clinical populations may benefit from increased amounts of vitamin B12 for therapeutic benefit is a promising area of science that requires more research.
Recommended Daily Intake (RDI)
The RDI for B12 has been set to prevent megaloblastic anaemia and to maintain adequate serum vitamin B12 concentrations. For both men and women, the RDI is 2.4 micrograms per day, increasing to 2.6 micrograms per day for pregnant women and 2.8 micrograms per day for lactating women (11). Pregnant women require more B12 for foetal and placental needs, while lactating women secrete B12 in their milk to support the growing infant and so need more to reach their own requirements.
The minimum daily requirement for adults can be achieved by eating 100 g of salmon or tuna OR 100 g of lamb OR 4 whole eggs. However, the uniqueness of B12 absorption and activation in the body means that even people who consume plenty of meat, fish, eggs and dairy foods can still be at risk of a B12 deficiency and may require supplementation to prevent or manage a deficiency, at least during some periods of their life (e.g., while taking certain medications).
Vegans or vegetarians who consume little to no foods of animal origin should speak to their doctor about testing their vitamin B12 status and have a discussion with a savvy dietitian about nutritional supplementation and/or fortified food products.
Dietary Sources of B12
Vitamin B12 is synthesised by bacteria in the gastrointestinal tract of animals and is then absorbed by the host animal. It is found concentrated in animal tissues, hence why vitamin B12 is found only in foods of animal origin.
Food - B12 content (micrograms / 100 g)
Liver - 18
Sardines - 8.9
Mackerel - 6.9
Salmon - 4.3
Tuna - 3.0
Lamb - 2.6
Chicken - 0.4
Egg (whole) - 0.8
Milk - 1.4
Cheese, brie - 1.7
Cheese, parmesan - 1.4
Cheese, blue - 1.2
Cheese, cheddar - 1.1
Yoghurt, plain, unsweetened - 0.4
Butter - 0.2
Mushrooms - 0.04
Hemp milk, unsweetened (fortified) - 0.6
By Jessica Turton & Steffani Clayton
Accredited Practising Dietitians and Qualified Nutritionists
This article provides general information from the current scientific evidence base and clinical judgement of the author. It is designed for educational purposes only and should not be substituted for medical advice. The author recommends you seek personally tailored support from a qualified healthcare practitioner before undertaking any major lifestyle change.
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