Vitamin B12 – An every-day essential
Vitamin B12, also known as cobalamin, is an essential water-soluble vitamin. We cannot make vitamin B12 ourselves and are completely dependent on dietary sources of the vitamin.
It is primarily derived from food of animal origin, such as meat, dairy, poultry, eggs and fish. It is also found in fortified cereals, some fermented foods and fortified nutritional yeast. Vitamin B12 is stored in the liver and this stockpile can last for up to 4 years without replenishment.
A strict vegan diet, particularly for longer than 3 years, greatly increases the risk of low or marginal vitamin B12 status and vegans should seek out ways to ensure they consume adequate levels of the vitamin. However, the most common reason for vitamin B12 deficiency relates to malabsorption, particularly in older individuals.
Absorption of vitamin B12
To understand how vitamin B12 deficiency occurs in the majority of cases, we need to understand how vitamin B12 is absorbed by the body. Vitamin B12 is bound to proteins in the food we eat and free-form vitamin B12 is separated from protein in the stomach in a process involving hydrochloric acid. Vitamin B12 is already in its free form in fortified foods and supplements, so it may be easier to absorb vitamin B12 from these sources, particularly in individuals who have reduced amounts of gastric hydrochloric acid.
In the small intestine, vitamin B12 binds a molecule called ‘intrinsic factor’. Intrinsic factor protects vitamin B12 as it travels to the ileum (lower part of the small intestine), where it facilitates the absorption of vitamin B12 into the body. From there, vitamin B12 is transported to the liver, where it is either distributed to various organs and peripheral tissues or is stored for later use.
So, two important factors involved in the absorption of vitamin B12 are gastric hydrochloric acid and intrinsic factor.
Vitamin B12 insufficiency/deficiency
There are three main reasons as to why someone does not have sufficient levels of vitamin B12 in their body1.
- Difficulty absorbing vitamin B12 from food, possibly due to surgery or other changes that affect the lower part of the small intestine, such as coeliac disease.
- Prolonged use of certain medications, such as metformin (used to treat type 2 diabetes), which affects vitamin B12 absorption, and proton-pump inhibitors, which reduce the production of acid in the stomach.
- Decreased stomach acidity, which is common among older people, resulting in reduced levels of free-form vitamin B12 for absorption, even if recommended levels of the vitamin are consumed.
- An inability to make intrinsic factor, which can be due to gastric surgery, chronic gastritis or an autoimmune disease called pernicious anaemia, means your body is unable to absorb vitamin B12.
- Vegans are particularly at risk of developing vitamin B12 deficiency from a lack of dietary intake.
What does vitamin B12 do in the body?
Vitamin B12 works alongside vitamin B9 (more commonly known as folate/folic acid), and together they have a vital function in cell growth and development through their roles in fatty acid, amino acid and nucleic acid metabolic pathways.
The key functions of vitamin B12 include:
- The production of healthy red blood cells;
- The development and normal functioning of the central nervous system;
- The production of DNA, which forms the building blocks of every cell in the body.
Because vitamin B12 is needed for the production of healthy red blood cells and for the development and function of the central nervous system, vitamin B12 deficiency is most commonly associated with haematological and neurological symptoms2. However, the symptoms can vary greatly in presentation and severity. Also, because vitamin B12 and folate work in tandem, reduced levels of either result in an overlapping spectrum of symptoms associated with anaemia.
The symptoms of vitamin B12 insufficiency/deficiency can include:
- Fatigue, lethargy
- Megaloblastic anaemia
- Pale or yellow skin
- Mouth sores
- Mobility issues
- Pins and needles (paraesthesia)
- Reduced mental function; problems with memory, understanding and judgement
The symptoms of vitamin B12 insufficiency/deficiency usually develop gradually, but can worsen if the condition goes untreated.
Management and treatment
How vitamin B12 deficiency is treated, and the duration of that treatment, depends on the underlying cause and severity. In individuals with insufficient dietary intake, an oral supplement of vitamin B12 is usually adequate. In cases of malabsorption or pernicious anaemia, parenteral administration (an injection which bypasses the digestive tract) of vitamin B12 can replenish the stores in the liver.
Vitamin B12 deficiency and ageing
Older individuals are susceptible to developing vitamin B12 deficiency due to malabsorption4. The onset of symptoms tends to be very slow, as vitamin B12 stored in the liver can compensate for reduced absorption for several years, and the symptoms of fatigue, confusion and reduced mobility are often simply ascribed to the ageing process. Therefore, vitamin B12 deficiency is often overlooked in the elderly. Luckily, vitamin B12 insufficiency/deficiency is easily treatable if detected before permanent damage occurs.
So, it is important for individuals at higher risk of developing vitamin B12 deficiency, such as those with coeliac disease, intestinal surgery and advancing years, to have their vitamin B12 levels routinely checked.
Pernicious anaemia is a decrease in healthy red blood cells that develops when the intestine cannot properly absorb vitamin B12 due to a loss of intrinsic factor. Intrinsic factor is produced by cells in the stomach lining, and loss of intrinsic factor can occur by a weakened stomach lining (atrophic gastritis) or by an autoimmune condition in which the immune system attacks the intrinsic factor protein itself or the cells that produce it.
This results in a loss or absence of intrinsic factor, defective absorption of vitamin B12 and vitamin B12 deficiency. Pernicious anaemia is treated with vitamin B12 injections.
Anaemia is a condition caused by reduced numbers of healthy red blood cells or reduced levels of haemoglobin in each red blood cell, resulting in defective delivery of oxygen to organs and tissues throughout the body.
General symptoms of anaemia may include:
- feeling faint
- pale skin
- noticeable heartbeats (palpitations)
- loss of appetite and weight loss
A condition in which unusually large, structurally abnormal, immature red blood cells are produced, resulting in decreased oxygen delivery to organs and tissues (anaemia). The most common causes of megaloblastic anaemia are deficiencies of vitamin B12 or folate.
1Ankar A, Kumar A. Vitamin B12 Deficiency. [Updated 2021 Jun 7]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: Here
2Green, R. (2017). Vitamin B12 deficiency from the perspective of a practicing hematologist. Blood, The Journal of the American Society of Hematology, 129(19), 2603-2611.
3Khosravi M, Sotoudeh G, Amini M, Raisi F, Mansoori A, Hosseinzadeh M. The relationship between dietary patterns and depression mediated by serum levels of Folate and vitamin B12. BMC Psychiatry. 2020;20(1):63.
4Kernisan, L. How to Detect & Treat Vitamin B12 Deficiency in Older Adults. Webpage