15 Causes of Low Vitamin B12 Status

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Do you struggle with persistent fatigue or unexplained low energy? Are you having trouble maintaining your strength at the gym? Are you constantly getting mouth ulcers or dealing with low mood? While there are many potential causes of these symptoms, vitamin B12 deficiency could be part of the problem holding you back from feeling and performing your best.,

Why Do I Need Vitamin B12?

Vitamin B12, biochemical name cobalamin, is an essential B-vitamin that helps to convert the carbs you eat into fuel. In fact, b-vitamins work as a team and also help to metabolize proteins and fats (to a lesser degree). Vitamin B12 is particularly important for your nervous system, producing RNA and DNA (your genetic building blocks) for growth and repair, methylation pathways, red blood cell production (which carries oxygen to all of your cells so you can perform and thrive), and much more.(1) It also teams up with folate (i.e. vitamin B9) and vitamin B6 to metabolize homocysteine in the body, where high levels are associated with inflammation and increased risk of heart disease.(2) B12 also plays a role in melatonin production, the circadian hormone that helps to initiate sleep so you can recover effectively.

As I mentioned at the start of this article, common symptoms of low B12 include fatigue, shortness of breath, weakness, as well as anxiousness, poor sleep, inability to concentrate, subpar memory, loose stools, numbness and tingling sensations in your toes and fingers.(3) These are all quite general symptoms and common in a variety of possible conditions. Let's take a look at who might be at risk of low vitamin B12 levels. 

Who Is At Risk of Low B12?

A dietary insufficiency reflects a level too low to best support your daily demands, while a frank deficiency is a lack of minimal intake required to sustain health (that will eventually lead to adverse symptoms) It can take some time for symptoms to present and it's actually quite difficult to assess how much of the population is truly vitamin B12 deficient, but the research suggests your risk of deficiency increases with age.(4) All of that said, who is most likely at risk of low B12? Let's do a quick rundown...

  • Elderly - As you age you're more likely to be deficient, likely due to poorer absorption, reduced intake of dark meats (i.e. more difficult to chew) and medications.

  • Diabetes - Clients with type-2 diabetes are at greater risk of deficiency and complications like loss of nerve function in the fingers, toes and eyes is a serious possibility if left uncontrolled.

  • Vegans - If you don't consume any dairy or animal protein, you're definitely at high risk of low B12. If you're a female vegan, this will be even more pronounced due to blood loss via monthly menses.

  • Digestive Conditions - if you suffer from Crohn's or colitis, short bowel syndrome (i.e. portion of your small intestine removed), pancreatic disease or bariatric weight loss surgery (i.e. stomach stapling) your ability to absorb B12 will be compromised.

  • Vegetarians - Also at higher risk, but if you eat eggs or dairy, (or don't mind adding more shellfish or fish in your diet occasionally) then you may be alright.

  • H. pylori infection - a common cause of stomach ulcers that damages cells, reducing intrinsic factor output, a substance the body needs to absorb B12.

  • People or athletes with eating disorders.

Other Common Causes of Low B12?

On a day-to-day basis, let's look at three common causes that lead to low vitamin B12 in folks. Digging a little deeper can help you identify clients who may need more B12 to upgrade their performance in the gym, at the office, or at home. 

Low Stomach Acid 

Optimal levels of hydrochloric acid in the stomach are essential to producing a key protein called intrinsic factor (IF) that is required for the absorption of B12. Vegans, vegetarians and older clients are all at higher risk of low stomach acid (as well as clients always "on the go" and pushing their sympathetic nervous system), increasing the likelihood of low B12 status (especially when combined with a busy schedule or regular/intense training). Medications for low stomach acid - called protein pump inhibitors or H2 blockers, drugs ending with the suffix "-azole,” or “-tidine” - or regular antacid use can also lead to low stomach acid levels and thus low IF and B12. 

High Activity, High Stress

Are you training intensely for an event or competitive season? Are you really busy and on the go with work and/or kids at home? The greater stress load on your body, the greater the demand for vitamin B12 which can put drain your levels, especially when combined with a diet that doesn't contain enough of the essential vitamin. 

Genetic SNiPs

Your DNA may play a role in reduced B12 status. Genetic single nucleotide polymorphisms (SNiPs, pronounced “snips”) are small genetic variants or blips that occur in your DNA that can lead to inadequate production of key enzymes required to absorb specific vitamins.New research shows the genetic SNiP FUT-2 gene may strongly contribute to low levels of B12.(5) However, it’s important to remember that even if you test positive for a SNiP on a genetic test, it doesn’t mean you’ll have lifelong deficiency. You may need to take extra steps to ensure your diet is rich in B12 foods.


There are a lot of other drugs that can impair vitamin B12 status; metformin (to treat diabetes), antibiotics, methotrexate (used in chemotherapy), colchicine (used to treat gout), anti-seizure medications (i.e. phenytoin, pirimidone), and a group of cholesterol-lowering drugs (i.e. cholestyramine). If you’re currently taking any of these medications, talk to your doctor about supporting your B12 status.

Lab Testing For Low B12

The traditional method for assessing low vitamin B12 status is to measure total serum cobalamin via blood draw. If your levels are below 200pg/mL you're typically considered to have a frank deficiency (depending on the lab). You may have a megaloblastic anemia, where low B12 causes enlarged the red blood cells (e.g. as if they've been "baking in the oven" too long due to low B12 levels) or a pernicious anemia due to your stomach cells inability to produce intrinsic factor (this is typically flagged by your doctor). If your levels fall between 200-400pg/mL "insufficient" from a functional medicine perspective.

The problem is a total serum blood draw is neither sensitive, nor specific for cobalamin deficiency.(6) In short, it stinks as a stand alone test. You really need a variety of markers to be able to more accurately assess a client's need.

The reality is the absorption of B12 in the body is tremendously complex, involving an array of different proteins: haptocorrin, intrinsic factor and transcobalamin (TC). The overwhelming majority of your B12 (about 80%) is bound to haptocorrin, however the portion bound to TC is called holotranscobalamin (holoTC), the metabolically active form of vitamin B12.(7) Thus, a lab test for for serum holotranscobalamin (holoTC)  is an earlier marker of low serum cobalamin, more sensitive to insufficient intake.  

Another biomarker for vitamin B12 status is methylmalonic acid (MMA), as well as homocysteine, whose levels rise in vitamin B12 deficient patients. These markers help to give you a clearer "functional picture" of what's going on with your client and thus combining holoTC and MMA together is typically your best bet. (This is particularly important in clients with type-2 diabetes who are at higher risk of irreversible nerve damage). 

Adding more to the story, if you're taking high doses of supplemental folic acid it can mask low vitamin B12 status. Genetic testing can also be useful to identify SNiPs in your DNA that may predispose you to insufficiency or deficiency.

The Best Foods To Increase B12 Status

Food is hands down the best way to restore adequate and optimal vitamin B12 status. Often, low B12 status reveals a diet that is lacking in animal protein and thus nutrient density. Prioritizing animal protein - in particular darker cuts of meat and organ meats - is crucial for correcting the root cause of most insufficiencies and deficiencies. Traditional diets are a great foundation for correcting low B12, try adding more of the following to your nutritional arsenal (listed per 100g serving);

  • clams - 49mcg

  • chicken liver - 17mcg

  • sardines - 9mcg

  • mackerel - 8.7mcg

  • trout - 7.8mcg

  • salmon - 3.2mcg

  • beef - 3.0mcg

  • eggs - 1.3mcg

Aim for 2-5 servings (3oz) per day of these nutrient-dense foods, depending on your level of deficiency and activity.

What About B12 Shots?

If you're very deficient in B12 and need to restore your levels quickly, then a series of B12 shots can be a good option. The injection is given intra-muscularly (IM), normally in the back of your shoulder, and bypasses your digestive system dramatically increasing absorption. It's most often used in clients with digestive conditions impairing their capacity to absorb B12. 

It’s important to note that the typical form of B12 used in doctors offices is cyanocobalamin, a cheaper and subpar form of B12. Opt for methylcobalamin form, this “active” form of vitamin B12 does not require conversion in the body. B12 shots are typically given in 1,000mcg doses, weekly for 4-8 weeks depending on your levels and clinical picture. (Ask your doctor for more information)


The Bottom Line: If you're vegan, elderly, have a chronic digestive condition or struggle with consistent low energy then you may have low vitamin B12 status. Use a variety of biomarkers to assess your B12 status, focus on B12 rich foods from animal sources to correct insufficiency/deficiency and talk to your doctor or nutritionist if about supplements or B12 injections.

Low energy, fatigue and persistent brain fog are a major performance roadblock. Upgrade your diet and ramp up B12 intake and support your capacity to perform and recover at your best. If you suspect you may be low, find a practitioner in your area to help identify the root cause and support you with an individualize and personalized approach.

Dr. Marc Bubbs ND, CISSN, CSCS


Want o learn more? Listen to Dr. Ben Lynch talk about genetic SNiPs, folate and B12 in Episode #45 Dr. Bubbs Performance Podcast...