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Active B12

Get the lowdown on what vitamin B12 is and what your B12 status could tell you about your health.

Author: Leanne Edermaniger

April 12, 2024

Reviewed by: Dr Thom Phillips

In this article:

What is Vitamin B12?

Vitamin B12, also known as cobalamin, is an essential vitamin naturally found in animal products. It’s one of the eight ‘essential’ B vitamins that our body is unable to produce, so needs to be absorbed from our diet. It is chemically one of the most complex molecules required by our bodies and is involved in the formation of red blood cells, DNA synthesis, and the normal function of the brain and nervous system[1].

What Is The Difference Between B12 and Active B12?

There are two types of vitamin B12:

  • Inactive: B12 is bound to haptocorrin (a protein that transports vitamin B12 in the bloodstream)
  • Active: B12 binds to the protein transcobalamin II, becoming biologically active and able to be transported into your cells

Approximately 80% of B12 in your blood is attached to haptocorrin[2] and is not available for use. Transcobalamin is responsible for delivering around 4 nanomoles (nmol) of B12 every day[3].

Total B12 measures both types of the vitamin, active (holotranscobalamin – holoTC) and inactive (haptocorrin). So, a total B12 blood test measures all the vitamin B12 in your blood. The results of a Total B12 test can be misleading, and in some cases even miss a deficiency, because it measures the B12 your body cannot use alongside the type it can.

An active B12 blood test, however, only measures the active version, the part your cells can take in and use, giving you a more accurate picture of your true B12 status.

What Are ‘Normal’ Active B12 Levels?

The healthy range of active vitamin B12 is between 38 and 188 pmol/L. However, there is data to suggest that levels below 70pmol/L are suboptimal and should be supplemented[4].

Data collected from 4,388 Forth customers in March 2024 has found:

  • Overall average active B12 level for adults in the UK is 96.5 pmol/L
  • The average level of active B12 in women is 96.1 pmol/L
  • The average for men is 97.1 pmol/L

What is Active B12 Good For?

Vitamin B12 has several important roles in the human body. It is required for the normal formation of red blood cells. If you are deficient in B12, it can result in megaloblastic anaemia, where your blood cells are larger than usual and in smaller circulating numbers.

Vitamin B12 is also important for the synthesis of DNA[5], where your genetic material is stored, acting as a cofactor for enzymes which are involved in its production.

B12 also supports cell metabolism and function, and deficiency can affect the body’s organ systems. Without adequate B12 levels, your body is at risk of developing chronic conditions, including dementia[6].

Does B12 Give You Energy?

In the fitness world, B12 has long been promoted for its presumed effect on energy and endurance.

B vitamins help your body to use nutrients, such as carbohydrates, fats, and proteins for fuel. When low, B12 can impair the synthesis of DNA and result in decreased red blood cell production and oxygen delivery.

Together, these may contribute to a lack of energy and exercise tolerance, fatigue and shortness of breath, common early signs of vitamin B12 deficiency. Studies have shown that even mildly reduced levels of Vitamin B12 can affect Haemoglobin concentrations in the blood of elite athletes which can have a direct impact on performance[7].

Does B12 Help With Mental Heath?

Vitamin B12 deficiency can affect your mood, contributing to depression, anxiety, dementia, and delirium[8]. Research has shown that lower B12 levels in adolescents are associated with a higher risk of severe anxiety and depression symptoms[9].

A recent study found that a high-dose B12 supplement could promote positive outcomes related to anxiety and visual processing in young adults with self-reported anxiety or depression[10].

Significant B12 deficiency can increase the risk of severe depression by two-fold. A study found that low B12 status in older adults increased the likelihood of developing depression symptoms over four years by 51%.
If you are experiencing depression, we recommend talking to your doctor.

What Causes Low Active B12?

The human body needs vitamin B12 for several important functions, and because it is an essential vitamin, it must be acquired from the diet. This can be problematic for vegetarians or vegans because B12 is naturally found in animal-based products.

Poor nutrition in the elderly community or amongst those who drink excessive amounts of alcohol can also be risk factors for vitamin B12 deficiency[11].

Other causes include autoimmune conditions, such as pernicious anaemia and malabsorption.

What is Vitamin B12 Deficiency?

Deficiency develops when there is not enough B12 in the body, causing anaemia and resulting in abnormally large red blood cells which do not function properly.

A vitamin B12 level below 70pmol/L would be considered low and would need to be investigated via blood tests.

Symptoms of Low B12?

Symptoms of low vitamin B12 include:

  • Fatigue
  • Anaemia
  • Sore tongue
  • Pins and needles
  • Muscle weakness
  • Vision issues
  • Depression
  • Psychological problems e.g., anxiety and depression

What is a Low Level?

According to the NHS, a low level of total B12 is below 150 nanograms per litre (ng/l) (or about 116 pmmol/L)[12].

At Forth, we suggest a low level of active B12 is below (38 pmol/L).

Can High Levels of Vitamin B12 Be Harmful?

Because vitamin B12 is a water-soluble vitamin it is generally considered safe, even at high doses. That’s because excess amounts of water-soluble vitamins are not stored by the body and are excreted in urine.

There is no maximum daily dose set for B12 because whatever your body doesn’t use, you’ll pee out. However, in rare cases, excessive B12 may cause acne, palpitations, anxiety, headaches, and insomnia[13].

However, elevated B12 levels could be a sign of excess supplementation, increased transport proteins, a haematological or autoimmune process, or chronic disorders such as renal failure, cancer or hepatic disease.

How to Check Active B12 Levels?

Blood tests are used to check active B12 levels.

Many of our nutrition and general health blood tests check your active B12 levels.

These can be done at home using our fingerprick kits.

What Other Markers Would Be Useful to Test Alongside B12?

Because vitamins B12 and B6 (folate) share the same purpose, a deficiency can cause similar symptoms, it is vital that you measure your folate levels alongside active B12.

If you’re both folic acid and B12 deficient it’s clinically vital that B12 is replaced first, ahead of folate.

Folate replacement can mask symptoms of a worsening B12 deficiency and result in long-term, irreversible damage to neurons. Our Advanced Health Check tests both B12 and folate, along with 37 other markers, giving you a comprehensive look at your well-being.

How to Improve Your B12 Levels

There are several ways you can improve your B12 levels, depending on your circumstances and preferences.

Diet: What foods are good for B12?

The following foods are rich in vitamin B12:

  • Animal liver and kidneys
  • Clams
  • Oysters
  • Salmon
  • Beef
  • Poultry
  • Milk and dairy products
  • Eggs

B12 Supplements

The foods that are rich in B12 are animal-derived, posing a challenge for anyone following a vegan or vegetarian diet. So, supplementation can be an alternative option for anyone who cannot naturally acquire vitamin B12.

There are several types of B12 supplements available. The most common is cyanocobalamin but other forms include:

  • Methylcobalamin
  • Hydroxycobalamin
  • Adenosylcobalamin

Cyanocobalamin is a synthetic form of vitamin B12 while Methylcobalamin is naturally found in food sources and used to make supplements.

How long do they take to work?

It can take a few weeks for your B12 levels to rise or for you to notice an improvement in symptoms when using supplements.

Vitamin B12 injections

Vitamin B12 injections contain a synthetic version of B12 called hydroxocobalamin.

B12 injections begin to work straight away but it can take up to a few weeks to notice your symptoms improving[14].

There is unlikely to be any benefit to having vitamin B12 injections without deficiency.

Written by Leanne Edermaniger

Based in the UK, Leanne specialises in writing about health, medicine, nutrition, and fitness.

She has over 5 years of experience in writing about health and lifestyle and has a BSc (hons) Biomedical Science and an MSc Science, Communication and Society.

- Health scores calculated


Article references

  1. Obeid, R. et al. (2006) ‘The cobalamin-binding proteins Transcobalamin and haptocorrin in maternal and Cord Blood Sera at birth’, Clinical Chemistry, 52(2), pp. 263–269. doi:10.1373/clinchem.2005.057810.

  2. Tan, Y. et al. (2023) ‘Vitamin B12, folate, homocysteine, inflammatory mediators (interleukin-6, tumor necrosis factor-α and C-reactive protein) levels in adolescents with anxiety or depressive symptoms’, Neuropsychiatric Disease and Treatment, Volume 19, pp. 785–800. doi:10.2147/ndt.s399378.

This article was written by Leanne Edermaniger

This information has been medically reviewed by Dr Thom Phillips

Thom works in NHS general practice and has a decade of experience working in both male and female elite sport. He has a background in exercise physiology and has published research into fatigue biomarkers.

Dr Thom Phillips

Dr Thom Phillips

Head of Clinical Services