Vitamin B12 & Brain Function

What does Vitamin B12 have to do with brain function? Plenty. In fact, Vitamin B12 deficiency can cause “brain fog”, depression, and more.

One of the eight B vitamins, B12 is involved in the metabolism of every cell in your body.

B12 deficiencies affect three major systems in your body:

– Blood
– Nervous system
– Gastrointestinal tract (less often)

These three systems are targets because their cells either have a high turnover rate (blood and intestinal lining) or need a lot of B12 to function smoothly (nervous system). The symptoms of low B12 levels are related to each of these areas.


B12 deficiency can result in a specific type of anemia called megaloblastic anemia. With this type of anemia, red blood cells don’t develop normally. Instead, they are very large and shaped like an oval, not round like healthy red blood cells.

General symptoms of anemia may include:

– extreme tiredness (fatigue)
– lack of energy (lethargy)
– breathlessness
– feeling faint
– headaches
– pale skin
– noticeable heartbeats (palpitations)
– hearing sounds coming from inside the body, rather than from an outside source
– loss of appetite and weight loss

If you have anemia caused by a Vitamin B12 deficiency, you may have other symptoms, such as:

– a pale-yellow tinge to your skin
– a sore and red tongue
– mouth ulcers
– pins and needles
– changes in the way that you walk and move around
– disturbed vision
– irritability
– depression
– changes in the way you think, feel, and behave
– a decline in your mental abilities, such as memory, understanding, and judgment (dementia)

Some of these symptoms can also happen in people who have a Vitamin B12 deficiency but have not developed anemia.

Nervous System

B12 deficiency causes symptoms affecting your:

– Nerves (numbness, tingling, tremors, balance problems)
– Mind (depression, brain fog, mood swings, and, in rare cases, hallucinations and psychosis)

Gastrointestinal Tract

You might experience digestive symptoms and weight loss because you’re not absorbing food efficiently.

The irony is that with all of these, there’s usually just one predominant symptom, causing the connection to low B12 to be delayed until other symptoms start to appear. For example, if your only symptom is tingling in your hands, you might undergo all sorts of diagnostic tests before your doctor thinks, “Maybe we should check her B12 level”.

What Causes B12 Deficiency?

The list of causes is lengthy, but by far the most frequent culprit is a dietary one. Vegetarians who aren’t paying attention to the B12 in their food choices will have downward-drifting B12 levels, and virtually all vegans not taking Vitamin B12 supplements ultimately develop deficiencies. Even various vegan organizations acknowledge that it’s not possible to get adequate B12 while following a strictly vegan diet because the richest sources are animal products.

Other causes of B12 deficiency include pernicious anemia, an uncommon autoimmune disease that destroys parietal stomach cells. These cells produce a substance called intrinsic factor, necessary for B12 absorption.

Also, since you need stomach acid to absorb B12, long-term use of acid-suppressing proton pump inhibitors (Nexium, etc.) can lead to B12 deficiency. So can chronic intestinal conditions such as Crohn’s disease, celiac disease, and intestinal parasites.

Four Reasons This Diagnosis Is Missed

There are four reasons why this diagnosis is missed:

– B12 isn’t measured during routine blood tests.

It’s not an expensive test. Yet, doctors generally don’t order a B12 evaluation if there’s no evidence of anemia (which would be picked up on a routine blood test). However, the fatigue, nervous system, and gastrointestinal symptoms can precede anemia by months.

– Doctors rarely ask patients about their eating habits.

And patients rarely volunteer this information. For example, vegetarians and vegans should be regularly tested for possible B12 deficiency. But if your doctor doesn’t know your eating habits, you won’t be tested.

– There’s disagreement about normal levels of B12.

Most labs test B12 levels between 200 pg/ml and 800 pg/ml. Someone at 250 pg/ml would be classified as normal, but symptoms can start appearing at 350 pg/ml. In Japan, normal B12 was recently raised to 500 pg/ml and higher.

– Folic acid or folate-containing foods without B12 can mask a developing B12 deficiency.

Folate will keep your blood count normal even if your B12 is falling. But the folate will not protect your brain and nervous system.

For example, you’ll have no evidence of megaloblastic anemia (because of the folate). But your symptoms of numbness, tingling, balance problems, and emotional issues will continue unchecked. By eating so many folate-containing green vegetables, beans, and lentils, vegetarians and vegans inadvertently mask their own slowly developing B12 deficiency.


It’s virtually impossible to take too much B12, as any excess of this water-soluble vitamin is eliminated via urination. 

Foods high in B12 are these animal products: meat, poultry, seafood, dairy, and eggs (with eggs having the least). Because all animals store B12 in their livers, eating liver is an excellent (though not particularly popular) treatment for B12 deficiency. Your grandmother or great-grandmother likely remembers a time when her doctor told someone in the family to eat more liver.

People with low B12 are likely to also have gastrointestinal symptoms that interfere with B12 absorption. The best way to quickly increase (and maintain) B12 levels are:

– B12 injections
– Chewable tablets
– Nasal sprays and skin patches

In our own practice, we suggest a series of at least four B12 injections spread out over the course of 1 month and a blood draw to determine your B12 levels.