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What is the difference between 'Active vitamin B12' and 'Total vitamin B12'?

There are several ways to measure your vitamin B12 level - how can you be sure you're checking the right thing?

Vitamin B12 has received a lot of attention recently. This may be due to the increased popularity of plant-based diets, which, although healthier in many ways, can often lack enough vitamin B12, as it is not naturally present in plant foods.

Vegans and vegetarians are not the only people who may be concerned about their vitamin B12 levels. Between 10 and 15% of people aged over 60 years poorly absorb vitamin B12 from their foods, because of inflammation in the digestive tract (also known as atrophic gastritis), often leading to a deficiency [1]. Low absorption of vitamin B12 can also occur due to conditions such as Crohn’s disease or coeliac disease.

Measuring your blood vitamin B12 levels, using our Vitamin B12 (Active) Blood Test, is recommended as a valuable way to assess whether your body is getting enough vitamin B12 [2]. But with several ways to test vitamin B12 available, how can you be sure you are checking the right thing?

What is vitamin B12?

Vitamin B12, also known as cobalamin, is an essential nutrient which we need to get from our food. It has roles in many important functions in the body, including keeping your body’s nerve and blood cells healthy and making DNA, the genetic material in your cells [3].

Without getting sufficient vitamin B12 from your food, the body cannot work optimally. Vitamin B12 deficiency can cause symptoms such as:

Extreme tiredness (fatigue) Feeling faint Headaches Pale skin Mood changes such as depression and/or irritability Noticeable heartbeats Pins and needles in your arms, hands, legs or feet Mouth ulcers Memory problems [4]

What are the different types of vitamin B12?

Once inside the body, there are two proteins which vitamin B12 can attach to, and each protein affects how the vitamin works:

Haptocorrin: scientists have not fully figured out the role of haptocorrin, but it may protect or transport vitamin B12 in our blood. When attached to this protein, vitamin B12 becomes inactive. Transcobalamin II: this is the protein which transports vitamin B12 into our cells where the vitamin can be biologically active. When bound to this protein, vitamin B12 becomes ‘Active Vitamin B12’. [1]

What is Total Vitamin B12?

Total vitamin B12 refers to all the vitamin B12 in your blood. Total vitamin B12, therefore, includes the vitamin attached to both proteins Haptocorrin and Transcobalamin II [2].

Why should you measure Active vitamin B12, not Total vitamin B12?

Surely it would be better to measure the total vitamin B12 in your blood? Not quite.

About 85-90% of the vitamin B12 in your blood is not available for your body to use because it is strongly attached to the protein Haptocorrin. When attached to this protein, vitamin B12 cannot be taken in and used by your body cells.

Therefore, a normal total vitamin B12 level could mislead you into thinking that you have healthy vitamin B12 levels - even when your body cannot use most of it. In other words, a normal total vitamin B12 level could hide a deficiency. Scientists often call this a false normal result.

On the other hand, active vitamin B12 (the quantity bound to Transcobalamin II) makes up only 10-15% of vitamin B12 in your blood [1], but it is the most important kind. This is the biologically active form of vitamin B12, and therefore takes part in all your body’s essential biological reactions – this is the one to measure!

Pregnancy may cause lower than expected levels of total vitamin B12, even though there is no problem with absorption and no vitamin B12 deficiency. Active vitamin B12 does not have the same issue. Women who are taking the combined contraceptive pill may see a similar effect.

You can measure your active vitamin B12 blood levels using our Vitamin B12 (Active) Blood Test. Our team of expert doctors will interpret your results. If your levels are lower than they should be, our doctors may recommend appropriate lifestyle changes to get your vitamin B12 levels back into a healthy range. They may also recommend tests for pernicious anaemia, an autoimmune cause of low vitamin B12.

Vitamin B12 Blood Test

Who may be at risk of a vitamin B12 deficiency?

You are at higher risk of a vitamin B12 deficiency if you: have an autoimmune disease including pernicious anaemia have a medical condition that affects your absorption of nutrients, including coeliac disease and Crohn’s disease have undergone gastric bypass surgery follow a vegetarian or vegan diet and don’t consume foods or drinks fortified with vitamin B12 have undergone radiotherapy to the abdomen or pelvic area have pancreatic disease [2]

So, measuring your active vitamin B12 level, rather than your total vitamin B12 level, using a test such as Vitamin B12 (Active) Blood Test, is the most reliable and informative way to find out whether your vitamin B12 levels are in the healthy range.



1. Human Nutrition, 12th Edition (2011). Edited by Catherine A. Geissler and Hilary J. Powers. 2. 3. 4.