With many tests available on the market, it can be confusing as to which one to use to get an accurate reading of your B12 levels. To understand the difference, you must know a little more about B12 first… 

What is B12 and why is it important? 

Also known as cobalamin, B12 is an essential vitamin for our nervous system, the maintenance of bones, nails, hair and skin, the conversion of food into energy and the production of red and white blood cells and platelets. B12 plays such a fundamental role in our body that it’s vital we keep an eye on our levels.  

How do we obtain it? 

Vitamin B12 is found mainly in animal products, particularly seafood and organ meats. It can also be found in smaller amounts in eggs and dairy, and some fortified foods such as breakfast cereals and plant milks. According to recent research,1 a very limited number of plant foods, such as Shitake mushrooms and nori seaweed, may contain minimal amounts of B12. However, most plant foods contain what is commonly known as ‘false B12’, which is an inactive form and can displace ‘true B12’, potentially causing someone to become deficient in it.  

When we eat a food containing B12, substances called gastric acid and pepsin are released in the stomach, enabling the B12 to be liberated from the food. B12 is then attached to a transport protein called haptocorrin, which is produced by the salivary glands to protect B12 whilst it moves through the acidic environment of the stomach to the small intestine. Without haptocorrin, the majority of B12 would be destroyed and only a tiny amount would reach the small intestine.  

When in the small intestine B12 is bound to another protein called intrinsic factor, which helps it to move into cells. Once safely into the cells, the intrinsic factor is broken down, releasing the B12 so it can be transported into the bloodstream. Once in the bloodstream, the majority of B12 is bound to haptocorrin so it can be stored, and the remainder is bound to transcobalamin (TC), which as the name suggests, transports B12 into cells so it can be utilised. When B12 is bound to transcobalamin, it is known as holoTC. If haptocorrin levels are too high, then it can result in insufficient levels of B12 being available for cells.  

As you can see, it is not easy to obtain B12, particularly if there are any health conditions that affect production of the various substances that assist the absorption of B12. 

Testing for B12

Knowing how B12 gets from our food to our cells is vitally important when we look at the different methods of testing our B12 levels. Having this wider detail enables us to understand why some types of tests are better than others. If you suspect you have a deficiency or are just keen to avoid one, we hope the following information will help you to get a proper result. Here is a list of the older and more commonly used tests to the newer tests now on the market: 

• Total B12 (serum) 

This is the most widely available B12 test. However, whilst it demonstrates if someone has enough B12 in their blood, it does not show if B12 is active, i.e. whether it is being utilised by cells. Research has shown that someone who has a serum B12 level within the normal range may still show signs of B12 deficiency.2 Also, one thing to be aware of with the total B12 test is that there is no differentiation between ‘false B12’ and ‘true B12’, so whilst someone may be within a normal range, they may not have enough ‘true B12’ to support their health. 

• Active B12 (serum) 

Also known as the holoTC test. This test measures levels of B12 that are bound to transcobalamin in the serum, ready to be transported into cells. Therefore, not only does it show whether someone has a B12 deficiency, but it also indicates that B12 is already in its active form. However, whilst some studies have proposed that this is a better test than the total B12 test because it is able to diagnosis a deficiency earlier, due to low levels of active B12 occurring before those of total B12, other studies disagree.3 

• Homocysteine 

B12 together with B6 and folate (B9) are involved in a process called the methylation cycle, which in very simple terms involves the conversion of one substance to another. If someone does not have enough of any of the nutrients needed for this process, it can interrupt it, and this can have a significant impact on many vital functions. These include DNA production, detoxification, oestrogen metabolism, and energy production, and this can increase the risk of conditions such as cardiovascular disease, neurological disorders and infertility.  

Homocysteine is one of the substances that can increase if the methylation cycle is not functioning efficiently. Therefore, testing levels of homocysteine can help to determine if someone has a B12 deficiency, which is impairing this cycle. However, homocysteine can also be elevated if a person has a folate, B2 or B6 deficiency, or is of an older age, or has kidney dysfunction. Therefore, if homocysteine is high, further testing should be done to determine if low B12 is contributing to this issue, or if in fact it is caused by something else. This is where an MMA test can prove useful.  

• MMA (plasma) 

Studies suggest that an MMA test is more accurate than a homocysteine test, and significantly more accurate than the total B12 and active B12 tests.2 MMA, which stands for methylmalonic acid, is a by-product of a process in the body vital for energy production. Vitamin B12 is necessary for the conversion of MMA to another substance called succinyl-CoA, so if someone has a B12 deficiency, this can prevent this conversion from taking place and levels of MMA can rise.  

A significant advantage of this test is that it not only does it indicate a deficiency, but it can also show low levels of B12, enabling someone to rectify their B12 status before a deficiency occurs. It can also identify absorption issues, as MMA only reduces when B12 has been fully absorbed, so usually, the higher the MMA levels, the lower the B12 levels will be. 

However, MMA in plasma can also be elevated if a person has kidney dysfunction or dehydration, so these factors should be ruled out if a test shows MMA levels to be high. 

• MMA (urine) 

Although this essentially has the same benefits as the MMA plasma test, it may have a couple of added advantages! A 2020 study comparing plasma homocysteine, plasma MMA, and urine MMA, found that the urine MMA test appeared to be more accurate than the other two tests,  plus high levels were less likely to be linked to kidney issues.2 A urine test is also particularly useful if someone dislikes having blood tests, as many of us do! 

Of course, to determine if there are any underlying issues that could be causing a B12 deficiency, more testing may be necessary once someone has received a diagnosis. However, as suggested by the current evidence, using a urine MMA test to provide this diagnosis may be the best method available, not only giving us an increased chance of good health but also greater peace of mind. 

References 

1. Marques de Brito B, Campos VM, Neves FJ, Ramos LR, Tomita LY. Vitamin B12 sources in non-animal foods: a systematic review. Crit Rev Food Sci Nutr. 2023;63(26):7853-7867. 

1. Supakul S, Chabrun F, Genebrier S, N’Guyen M, Valarche G, Derieppe A, Villoteau A, Lacombe V, Urbanski G. Diagnostic Performances of Urinary Methylmalonic Acid/Creatinine Ratio in Vitamin B12 Deficiency. J Clin Med. 2020 Jul 22;9(8):2335. doi: 10.3390/jcm9082335. 

1. Golding PH. Holotranscobalamin (HoloTC, Active-B12) and Herbert’s model for the development of vitamin B12 deficiency: a review and alternative hypothesis. Springerplus. 2016 May 20;5(1):668. doi: 10.1186/s40064-016-2252-z.