08/06/2025
B12 is one of the most misunderstood nutrients when it comes to lab testing and symptom evaluation. Many people are told their B12 is “fine” because the blood test looks normal or even high, while they continue to experience neurological issues, fatigue, brain fog, or mood changes that mimic pernicious anemia.
The reason this happens is because B12 must first be transported into the cell to do its job, and that process depends on a protein called transcobalamin. If this transport system is impaired, your serum B12 may look normal or even elevated, but your cells are still starving. This is called a functional or cellular deficiency, and it can cause all the symptoms of low B12 without the classic lab changes.
Environmental triggers like mold toxins and heavy metals can silently disrupt this handoff. Genetic variants such as MTHFR, MTR, MTRR, TCN2, and FUT2 can make some people more vulnerable to these blocks. Even trace lithium status can influence the handoff of B12 into the cell, meaning that suboptimal lithium can worsen cellular deficiency even if your blood looks perfect.
We are now also seeing this pattern more frequently in the years following major global viral events, which have left behind lingering immune and inflammatory shifts in many people. These post viral changes can disrupt absorption, alter gut microbiota, and stress the cellular transport system for nutrients like B12. Many patients who never felt the same after that period show subtle B12 transport issues that standard labs overlook.
Here is where things get even more confusing.
Sometimes, serum B12 is elevated because certain bacteria in the small intestine are overproducing B12, which can happen with small intestinal bacterial overgrowth (SIBO). This may make your labs look impressive, but much of that B12 is inactive or poorly utilized. Similarly, oral supplements and B12 injections can artificially raise serum B12 levels without fixing the underlying cellular issue.
Stopping treatment because the serum looks “high” can have serious consequences, including worsening neurological symptoms, fatigue, and cognitive decline. True clinical guidance should be based on functional testing and the patient’s symptoms, not just a single number on a lab sheet.
A comprehensive approach to B12 looks beyond the surface. This includes urinary methylmalonic acid (uMMA) and holotranscobalamin to measure active transport, an assessment of intrinsic factor and ileal absorption, and evaluation for SIBO, environmental exposures, and post viral immune shifts. When a true transport block or absorption problem is identified, hydroxocobalamin injections can bypass the broken system, deliver B12 directly into the bloodstream, and finally allow the cells to receive what they have been missing.
In simple terms:
High blood B12 does not always mean your body is using it. Sometimes your cells are starving because the transport system is blocked by toxins, genetics, gut bacteria, or post viral changes in absorption. Proper evaluation and treatment can be life changing, and the decision should always be based on how you feel and how your cells are functioning, not just what the lab says. True B12 therapy is not just about raising a number on a lab test. It’s about ensuring your body can transport, utilize, and integrate it safely. Monitoring potassium levels and providing the right cofactors can make therapy more effective and help prevent unnecessary complications.
If you are experiencing fatigue, neurological changes, brain fog, or symptoms that seem “mystery level” despite normal labs, it might be time to explore cellular B12 transport and functional testing.
