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What can extremely low B12 cause?

Vitamin B12, also known as cobalamin, is an essential vitamin that plays a critical role in several bodily functions. B12 is crucial for DNA synthesis, red blood cell production, neurological function, and maintaining healthy nerve cells. When the body does not get enough B12, it can lead to a condition called vitamin B12 deficiency. An extremely low B12 level is defined as less than 150 pg/mL. Prolonged and severe deficiency of this essential vitamin can lead to serious complications. In this article, we will explore what can happen when B12 levels get extremely low in the body.

Symptoms of Extremely Low B12

Some of the common symptoms of extremely deficient B12 levels include:

  • Fatigue and weakness
  • Pale skin
  • Shortness of breath and rapid heartbeat
  • Glossitis – inflamed and swollen tongue
  • Mouth ulcers
  • pins and needles sensation or numbness in hands and feet
  • Impaired balance and coordination
  • Memory loss and confusion
  • Depression and irritability
  • Vision problems like blurred or double vision

These symptoms tend to creep in slowly and worsen over time as B12 reserves continue depleting. Many neurological symptoms may not manifest until deficiency has set in severely.

Causes of Extreme B12 Deficiency

There are several reasons why someone may end up with critically low B12 levels.

Inadequate Dietary Intake

Since B12 is naturally present only in foods of animal origin, vegetarians and vegans are at a higher risk of developing deficiency if they do not supplement properly. Plant foods do not contain B12 unless fortified. People who follow macrobiotic diets or limit intake of animal foods and byproducts like eggs and dairy are also vulnerable. Children brought up on vegan diets with no supplementation or B12 monitoring can end up extremely deficient.


Even if dietary intake is adequate, certain gastrointestinal disorders can hamper absorption of B12 from food. These include:

  • Atrophic gastritis – chronic inflammation of the stomach lining
  • Pernicious anemia – an autoimmune disorder that destroys stomach cells that produce intrinsic factor needed for B12 absorption
  • Celiac disease – gluten damages intestinal villi needed for absorption
  • Crohn’s disease
  • Ulcerative colitis
  • Gastrectomy or gastric bypass surgery
  • Pancreatic insufficiency
  • Medications – PPIs, metformin, antibiotics

Inadequate Intrinsic Factor

Intrinsic factor is a protein made in the stomach that binds with B12 and allows its absorption in the small intestine. People who lack this protein cannot absorb vitamin B12 effectively. Causes include:

  • Autoimmune atrophic gastritis
  • Stomach or small intestine surgery
  • Pernicious anemia

Other Causes

Some other factors that can lead to extremely low B12 include:

  • Excess alcohol consumption
  • Pregnancy and lactation – fetal and infant needs deplete mother’s reserves
  • Aging – decreased stomach acid impairs absorption
  • HIV infection
  • Kidney dialysis
  • Rare inborn errors that affect vitamin B12 metabolism

Prolonged deficiency usually occurs due to either dietary inadequacy, impaired absorption or intrinsic factor deficiency.

Complications of Extremely Low B12 Levels

If extremely deficient B12 levels are left untreated over months and years, it can lead to severe and sometimes irreversible complications.

Megaloblastic Anemia

One of the most common consequences of critically low B12 levels is megaloblastic or macrocytic anemia. B12 is needed along with folate for red blood cell maturation. In deficiency, immature and enlarged red blood cells called megaloblasts accumulate in the bone marrow, while fewer mature red blood cells are released into circulation. This results in anemia with symptoms like weakness, fatigue, pale skin, palpitations and shortness of breath.

Neurological Problems

Low B12 can wreck havoc on the nervous system as it is needed to synthesize myelin sheath around nerves. If untreated for years, it can lead to irreversible nerve damage. Neurological issues include:

  • Numbness and tingling in hands and feet
  • Muscle weakness and spasms
  • Loss of reflexes
  • Impaired sense of vibration, position sense and touch
  • Difficulty walking
  • Dementia
  • Psychosis
  • Depression
  • Memory loss
  • Vision loss

Glossitis and Mouth Ulcers

The tongue can become inflamed, beefy-red in color with a burning sensation known as glossitis. Severe deficiency also leads to painful mouth ulcers.

Nerve Damage

Prolonged and untreated deficiency can lead to irreversible nerve damage including spine cord degeneration. This can cause symptoms like difficulty walking, loss of sensation in the limbs, paralysis, loss of bladder or bowel control.

Pernicious Anemia

This is an autoimmune condition often triggered by vitamin B12 deficiency, where the immune system attacks stomach cells that produce intrinsic factor. This impairs B12 absorption leading to worsening, severe and life-threatening deficiency.

Birth Defects

When a pregnant woman has extremely deficient B12 levels, it puts the fetus at risk of serious neurological and developmental disorders. Neural tube defects are one of the consequences.

Cardiovascular Disease

Studies show that severely low B12 levels are linked with a higher risk of congestive heart failure, ischemic heart disease and stroke. Deficiency causes high homocysteine levels which damage blood vessels.

Increased Infection Risk

B12 supports immune function. Very low levels are associated with recurrent infections like pneumonia as it reduces white blood cell production and function.


Deficiency affects both male and female fertility and reproductive function according to research. It likely impairs egg and sperm production.

Diagnosing Extreme B12 Deficiency

If a person exhibits multiple symptoms of vitamin B12 deficiency, the doctor will check blood levels of B12 along with other markers and tests to confirm diagnosis. These include:

  • Serum B12 – Levels less than 150 pg/mL indicate deficiency. Below 100 pg/mL is considered extremely deficient.
  • Methylmalonic acid – This compound is elevated when B12 levels are very low.
  • Homocysteine – High levels indicate deficiency.
  • Complete blood count – To check for macrocytic anemia.
  • Peripheral blood smear – Abnormally large red blood cells are seen.
  • Neurological exam – To assess nerve damage.
  • Schilling test – To confirm poor intestinal absorption of B12.
  • Intrinsic factor antibodies test – Presence indicates pernicious anemia.
  • Endoscopy and biopsy – To diagnose gastritis, celiac or Crohn’s disease.

Detecting and treating severely low B12 levels early is important to avoid complications.

Treatment for Extremely Low B12

Treating underlying conditions causing deficiency is the first step. Treatment for reversing extremely low levels involves B12 supplementation, usually by injection first to rapidly boost B12 when deficiency is severe, along with diet changes.

B12 Injections

Intramuscular injection of 1000 mcg cyanocobalamin daily for a week, then weekly shots for a month are given to replenish body’s B12 stores, especially when deficiency is due to malabsorption. This enables body to absorb B12 even without intrinsic factor. High dose oral supplements are also sometimes used.

Oral Vitamin B12

For milder cases, oral vitamin B12 supplementation at very high doses (1000-2000 mcg) may be recommended to normalize levels along with addressing underlying causes. Sublingual tablets that dissolve under the tongue are better absorbed.

Diet Modifications

If deficiency is due to low dietary intake in vegans, consuming more vitamin B12 fortified foods, dairy, eggs and seafood can help. Avoid alcohol and nitrous oxide use. Gastric bypass patients may need lifelong B12 supplements and injections.

Targeting Underlying Conditions

Pernicious anemia is treated with injections. Celiac disease requires gluten-free diet. Crohn’s disease treatment will help improve B12 absorption. Proton pump inhibitors may need to be stopped under medical guidance.

Preventing B12 Deficiency

To avoid extremely low B12 levels, the following preventive measures can be implemented:

  • Include animal foods like dairy, eggs, fish and lean meat in the diet. Vegans must take B12 supplements or eat fortified foods.
  • Treat conditions that impair absorption like celiac, Crohn’s disease properly.
  • If on medications that reduce absorption, monitor vitamin B12 levels periodically.
  • Get regular health screens done to catch deficiency early.
  • Pregnant and breastfeeding women should ensure adequate B12 intake.
  • Older adults should get levels tested and supplement if low.

Getting sufficient B12 of 2.4 mcg daily from food and supplements can prevent development of extremely low levels and associated complications.


Vitamin B12 deficiency is fairly common, especially in high risk groups. Prolonged and critically low levels under 150 pg/mL can lead to symptoms like fatigue, anemia, memory loss, neuropathy and mouth ulcers. If left untreated, severely inadequate B12 can cause permanent neurological damage, disability, infertility, birth defects, heart disease and other life-threatening complications. However, this can be prevented through proper dietary intake, supplementation when required and by addressing conditions that impair absorption. Detecting and treating deficiency in the early stages is crucial.