Why vitamin B12 problems can sneak up on you
Vitamin B12 (cobalamin) is essential for healthy nerves, red blood cell production, and DNA synthesis. Because those jobs touch many different systems, a deficiency doesn’t always announce itself with one unmistakable symptom. Some people do feel the classic “tired all the time” fatigue early on, but others notice subtler changes first—like tingling in the feet, feeling unsteady in the dark, or unusual blood test results that don’t quite fit the expected pattern.
From a fitness perspective, these signs matter because they can affect coordination, training quality, recovery, and safety. And from a medical perspective, recognizing the pattern early is important because nerve-related symptoms can become harder to reverse the longer they persist.
What B12 does in the body (and why deficiency looks so varied)
B12 helps power two key enzyme reactions. One supports the conversion of homocysteine to methionine, which is tied to methylation processes important for nerve function and many cellular tasks. The other supports the conversion of methylmalonyl-CoA to succinyl-CoA, a step that affects fatty acid and energy metabolism and is closely linked to myelin integrity. Myelin is the protective coating around nerves that helps signals travel efficiently.
When B12 is low, problems can show up in:
Nerves: altered sensation, numbness, tingling, pain, or balance changes.
Blood: anemia or changes in red blood cell size and shape.
Brain and mood: slower thinking, memory issues, irritability, or low mood (not everyone experiences these, and they can overlap with many other conditions).
That mix is why B12 deficiency can be missed—especially if a person doesn’t look pale, isn’t obviously anemic, or assumes tingling is from “tight shoes,” “a pinched nerve,” or training soreness.
The sensation changes that often show up before obvious fatigue
One of the most recognizable non-fatigue presentations of B12 deficiency is a change in sensation, often starting in the feet. People may describe:
Tingling or “pins and needles” in the toes or soles.
Numb patches or reduced ability to feel light touch.
Burning or electric sensations that come and go.
Reduced vibration sense (for example, noticing less sensation from a vibrating tool or massager).
These symptoms relate to peripheral neuropathy and can be symmetrical (both feet) and gradual. In active people, it’s easy to attribute them to training load, footwear, cycling position, or a prior back issue. But when tingling is persistent, progressive, or paired with other clues (like unusual lab results), it deserves a proper medical evaluation.
Fitness note: If your feet feel “asleep” more often, or you’re losing sensitivity, it can subtly affect running mechanics and balance—raising the risk of ankle sprains, falls, or overuse injuries.
Balance and coordination: when the nervous system is the clue
B12 deficiency can affect the spinal cord and the sensory pathways that help you know where your body is in space (proprioception). That can lead to:
Unsteadiness or clumsiness that feels out of proportion to your conditioning.
Worse balance in low light (for example, feeling unsure when walking at night or in a dim room).
Trouble with heel-to-toe walking or feeling like your gait has changed.
More reliance on vision to stay steady (watching your feet more than usual on stairs or uneven ground).
This balance pattern can be especially concerning because it may indicate involvement beyond the peripheral nerves. Not every balance issue is B12-related—inner ear conditions, medication effects, neurological disorders, and low blood pressure can all play roles—but B12 deficiency is one of the treatable causes clinicians keep in mind.
Fitness note: Balance is trainable, but if it’s suddenly declining—or declining alongside tingling or numbness—more single-leg drills aren’t the solution by themselves. You need to address the underlying cause.
Blood count changes: the lab pattern that can point to B12
B12 deficiency is famous for causing megaloblastic anemia, where red blood cells become larger than normal. On a complete blood count (CBC), this may appear as:
Elevated MCV (mean corpuscular volume), meaning larger average red blood cell size.
Low hemoglobin/hematocrit (anemia) in some cases.
High RDW (red cell distribution width), reflecting variable red cell size.
Low white blood cells or platelets in more severe cases (this can happen but isn’t always present).
However, one reason B12 deficiency can be missed is that not everyone has the “textbook” CBC. Some people have neurological symptoms with little or no anemia. Others may have mixed deficiencies—such as iron deficiency plus B12 deficiency—making MCV look normal because one condition pushes cell size down while the other pushes it up. That’s one reason clinicians look at the whole picture rather than one number.
Fitness note: Athletes often watch hemoglobin and ferritin for performance. If fatigue doesn’t match iron status, or if MCV is creeping up, it’s worth asking whether B12 (and folate) have been assessed.
Who is at higher risk (even if they eat “pretty well”)
Vitamin B12 is naturally found in animal-derived foods (meat, fish, dairy, eggs) and in foods fortified with B12. Deficiency risk rises when intake is low, absorption is impaired, or needs change. Higher-risk groups include:
People eating vegan diets without consistent B12 supplementation or reliable fortified foods.
Vegetarians who have low intake of eggs/dairy or inconsistent fortified foods.
Older adults, who are more likely to have reduced stomach acid or absorption issues.
People with pernicious anemia, an autoimmune condition that impairs intrinsic factor needed for B12 absorption.
Those with gastrointestinal conditions that affect absorption (for example, certain inflammatory bowel diseases or conditions involving the stomach or small intestine).
People with a history of bariatric surgery or other GI surgeries that change absorption.
Long-term use of certain medications that can interfere with B12 absorption in some people (your clinician or pharmacist can review your specific meds and risk).
Even among omnivores, absorption can be the limiting factor. That’s why dietary intake alone doesn’t always predict status.
How clinicians evaluate B12 deficiency (and why “normal” isn’t always simple)
Testing often starts with a serum B12 level plus a CBC. Depending on results and symptoms, a clinician may also consider additional markers that can help clarify borderline cases:
Methylmalonic acid (MMA): Often rises when B12 is functionally low.
Homocysteine: Can be elevated in B12 deficiency (and also in folate deficiency and other situations).
Folate status may be checked because folate and B12 deficiencies can overlap in blood findings.
No test is perfect in isolation. Some people have serum B12 in a borderline range but still have symptoms and elevated MMA; others have low serum B12 without major symptoms. This is why doctors interpret lab results alongside the clinical picture—symptoms, risk factors, neurologic exam, and other labs.
What treatment looks like (and why follow-up matters)
Treatment depends on the cause and severity. In some cases, increasing B12 intake through supplements and/or fortified foods is enough. In others—especially when absorption is impaired—clinicians may recommend higher-dose oral B12 or B12 injections. The exact plan and dose should be guided by a healthcare professional, particularly if neurological symptoms are present.
Follow-up matters because:
Symptoms take time to change. Blood counts may improve before nerve symptoms do.
Not all tingling is B12-related. If symptoms persist, your clinician may look for other causes (thyroid disease, diabetes, medication effects, spine issues, and more).
The underlying cause might require long-term management. For example, if someone has pernicious anemia or malabsorption, they may need ongoing supplementation rather than a short course.
Important caution: If you suspect B12 deficiency, avoid self-treating in a way that delays evaluation—especially if you have balance problems, numbness, weakness, or changes in walking. Those can signal significant nerve involvement and deserve prompt assessment.
How to support B12 status through food (and when food may not be enough)
Dietary sources of B12 include meat, fish, shellfish, eggs, and dairy. For people who don’t eat animal products, B12-fortified foods (some plant milks, cereals, and nutritional yeast products) can help—if they’re eaten consistently in adequate amounts. Because B12 isn’t reliably present in most unfortified plant foods, supplementation is commonly recommended for those following vegan diets.
Still, if the issue is absorption rather than intake, simply eating more B12-rich foods may not correct the problem. That’s why persistent symptoms or concerning lab patterns should be evaluated medically instead of handled by diet tweaks alone.
Training clues that can overlap with B12 deficiency
Fitness changes don’t diagnose anything, but they can provide hints that something is off. People with low B12 sometimes notice:
More frequent “missteps” on trails, stairs, or uneven surfaces.
Grip or coordination issues with fine motor tasks (opening jars, buttoning, handling small equipment).
Unusual muscle weakness or a sense that the nervous system isn’t firing smoothly.
Reduced tolerance for high-intensity work if anemia develops (because oxygen delivery can be affected).
Of course, overtraining, poor sleep, low calorie intake, iron deficiency, infections, and many other factors can cause similar issues. The key is persistence and pattern: sensory changes plus balance changes plus a CBC abnormality is a combination clinicians take seriously.
When to seek medical care promptly
Talk with a healthcare professional if you have ongoing or worsening:
Numbness, tingling, or burning sensations in hands or feet.
Balance problems, frequent falls, or noticeable gait changes.
Unexplained anemia or rising MCV on blood work.
Cognitive or mood changes that are new, persistent, or concerning—especially with other neurological symptoms.
Seek urgent care if you develop sudden weakness, significant trouble walking, severe confusion, chest pain, or shortness of breath—those symptoms can have many causes and need immediate evaluation.
Putting it all together
Vitamin B12 deficiency doesn’t always start with obvious fatigue. For many people, the first clues are neurological—subtle tingling, reduced sensation, or balance changes—while others are tipped off by blood count shifts such as a rising MCV or unexplained anemia. Because the cause can be dietary, absorption-related, or autoimmune, and because symptoms overlap with common training and lifestyle issues, the most helpful approach is to connect the dots: symptoms, risk factors, and labs.
If you recognize yourself in this pattern, it’s worth having a straightforward conversation with your clinician. B12 deficiency is one of the more treatable explanations for nerve and blood changes—and the sooner it’s identified, the better the odds of full recovery.