Do you lose your train of thought mid-sentence, have trouble concentrating, or feel mentally tired every morning—despite getting a good night’s sleep? What we call brain fog is becoming increasingly common in a daily life that is moving faster, with many hours spent in front of screens and more impressions than the brain has time to process. But sometimes brain fog can also be caused by something entirely different: a deficiency of an essential vitamin.
Vitamin B12 is needed, among other things, for the body to produce red blood cells, for the nervous system to function, and to produce and repair DNA. Since the body cannot produce B12 itself, we need to get it through food or supplements.
But how much B12 you eat is only part of the equation. For the brain and the rest of the body to be able to use the vitamin, it also needs to be absorbed through the gastrointestinal tract, transported onward, and ultimately made available to the cells.
Why B12 deficiency can cause brain fog
The brain and nervous system depend on B12 to function properly. Among other things, the vitamin is needed to produce and maintain myelin—the protective sheath around our nerve fibers that helps nerve signals travel efficiently.
If the nervous system does not get the amount of B12 it needs, this may show up as poorer memory, difficulty concentrating and solving problems, slower thinking, and mental fatigue. These symptoms are often collectively described as brain fog.
None of these symptoms are pleasant to deal with, but it is all the more important to take them seriously if they persist. Prolonged and untreated B12 deficiency can have significantly more serious consequences, including neurological damage and cognitive decline.
And there is one detail that is easy to miss: neurological symptoms of B12 deficiency can occur even without anemia, which is otherwise something many people associate with low B12 levels.
You can eat B12 and still get too little
If B12 is so important, it is easy to think that the solution is simply to eat more B12-rich food. But the vitamin has a rather complicated journey to make before the body can actually use it.
First, B12 needs to be released from food in the stomach. It then needs to bind to a specific protein, intrinsic factor, so that it can ultimately be absorbed in the final part of the small intestine.
This means that the amount of B12 you eat does not necessarily indicate how much the body actually manages to absorb.
Low stomach acid, pernicious anemia (an autoimmune disease that prevents the body from absorbing B12 from food), celiac disease, Crohn’s disease, gastrointestinal surgery, and certain medications can all make it more difficult for the body to absorb B12. A damaged or more permeable intestinal barrier can also affect how well the intestine is able to absorb nutrients.
Would you like to understand more about what happens when the intestinal barrier becomes more permeable? Read about leaky gut here.
A normal B12 level does not always tell the whole story
So how do you know whether the body actually has enough B12?
If a deficiency is suspected, B12 status can be assessed with a blood test. But there is another thing worth knowing here: the total amount of B12 in the blood and the amount that is actually available to the cells are not quite the same thing.
When total B12 is measured, all the B12 circulating in the blood is counted. But not all of it can be absorbed and used directly by the cells.
Some of the vitamin is bound to the transport protein transcobalamin. This portion is called holotranscobalamin, or active B12, and it is this form that can deliver B12 to the body’s cells. Therefore, active B12 can provide additional information about how much of the vitamin is actually biologically available.
Active B12 can be measured both at private testing clinics and through regular healthcare services, but in regular healthcare it is not a given that active B12 will be measured immediately. Total B12 is often measured first, and if the results are uncertain or borderline, healthcare providers may also measure MMA (methylmalonic acid), which can rise when the cells do not have enough functioning B12.
This means that an individual B12 value sometimes needs to be considered in a broader context, especially if symptoms are present even though the test result does not clearly indicate a deficiency.
When your brain feels sluggish—look beyond your intake
If you recognize these symptoms, B12 deficiency is of course not necessarily the cause. Lack of sleep, stress, iron deficiency, thyroid problems, and several other factors can cause similar issues.
But B12 deficiency is one possible cause that can be investigated, and it is important not to overlook it simply because you eat meat, fish, or other B12-rich foods.
Because when it comes to B12, it is entirely different to get the vitamin into your body, successfully absorb it through the gastrointestinal tract, and ultimately have enough available to the cells. And it is only when B12 reaches the cells that the brain and nervous system can actually use it.
