VITAMIN B12: deficiency, symptom, diagnosis, and treatment

The human body needs vitamin B₁₂ to make red blood cells, nerves, and DNA, and carry out other functions. The average adult should get 2.4 micrograms of B₁₂ a day. Vitamin B₁₂ (cobalamin) is a water-soluble vitamin that is derived from animal products such as red meat, dairy, and eggs.

Intrinsic factor is a glycoprotein that is produced by parietal cells in the stomach and is necessary for the absorption of B₁₂ in the terminal ileum. Once absorbed, B₁₂ is used as a cofactor for enzymes that are involved in the synthesis of deoxyribonucleic acid (DNA), fatty acids, and myelin. As a result, B₁₂ deficiency can lead to hematologic and neurologic symptoms. B₁₂ is stored in excess in the liver; however, in cases in which B₁₂ cannot be absorbed for a prolonged period (e.g., dietary insufficiency, malabsorption, lack of intrinsic factor), hepatic stores are depleted, and deficiency occurs.

Aetiology of vitamin B₁₂ deficiency

Vitamin B₁₂ deficiency has 4 primary aetiologies:

1. Autoimmune: Anti-intrinsic factor antibodies in Pernicious anemia bind to and inhibit the effects of intrinsic factors, resulting in an inability of B₁₂ to be absorbed by the terminal ileum. 
2. Malabsorption: After gastric bypass surgery, the new alimentary pathway bypasses the parietal cells in the stomach. This results in reduced production of intrinsic factors and B₁₂ absorption. Any damage to the terminal ileum (with normal intrinsic factor production) will impair the absorption of B₁₂ for e.g. in surgical resection due to Crohn's disease. 
3. Dietary insufficiency: may occur in people who follow a strict vegan or vegetarian diet, depleting the excess store of B₁₂ in liver. 
4. Toxin exposure: Exposure to nitrous oxide can cause vitamin B₁₂ deficiency and neurologic symptoms. Metformin treatment can cause B₁₂ deficiency. (https://www.ncbi.nlm.nih.gov/books/NBK441923/) 

Alcohol abuse and transcobalamin II deficiency can also result in B₁₂ deficiency.

How does the body absorb Vitamin B₁₂ ?

• Hydrochloric acid in your stomach removes vitamin B₁₂ from the food it is in.
• Vitamin B₁₂ combines with something called intrinsic factor, a protein made by your stomach.

How common is B₁₂ deficiency?

Any person can develop vitamin B₁₂ deficiency at any age. People who are 60 years old or older are more likely to have vitamin B₁₂ deficiency compared to other age groups. 

Most people in the United States consume adequate amounts of vitamin B₁₂. Data from the 2017–March 2020 National Health and Nutrition Examination Survey (NHANES) show that mean daily intakes of vitamin B₁₂ from food and beverages were 5.84 mcg for men aged 20 and older and 3.69 mcg for women. For children and adolescents aged 2 to 19, mean daily intakes of vitamin B₁₂ ranged from 3.66 mcg to 4.52 mcg. (https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/) 

What are the symptoms of B₁₂ deficiency?

Vitamin B₁₂ deficiency can cause physical, neurological, and psychological symptoms. The symptoms of vitamin B₁₂ deficiency can develop slowly and can get worse over time. Some people may have no symptoms despite having a low level of vitamin B₁₂ in their bodies. People with vitamin B₁₂ deficiency can have neurological symptoms and/or damage without anemia (lack of red blood cells).

General physical symptoms of vitamin B₁₂ deficiency can include:

• Feeling tired or weak
• Nausea, vomiting or diarrhea
• Not feeling as hungry as usual
• Weight loss
• Having a sore mouth or tongue
• Having yellowish skin

Neurological symptoms of vitamin B₁₂ deficiency can include:

• Numbness or tingling in your hands and feet
• Vision problems
• Having a hard time remembering things or getting confused easily
• Having a difficult time walking or speaking like you usually do

Psychological symptoms of vitamin B₁₂ deficiency can include:

• Feeling depressed
• Feeling irritable
• Experiencing a change in the way you feel and behave

How is B₁₂ deficiency diagnosed?

The tests used to diagnose vitamin B₁₂ deficiency are a complete blood count (CBC) and a vitamin B₁₂ blood test level. A person is diagnosed with vitamin B₁₂ deficiency if the amount of vitamin B₁₂ in their blood is less than 150 per mL.

How is B₁₂ deficiency treated?

B₁₂ deficiency is often treated with cyanocobalamin, a human-made form of vitamin B₁₂. Depending on the cause of the deficiency, you may only need treatment until your vitamin B₁₂ levels are back to normal, or you may need vitamin B₁₂ therapy for the rest of your life. Options for vitamin B₁₂ treatment include:

• Vitamin B₁₂ oral medication.
• Vitamin B₁₂ intramuscular injections (a shot that goes into your muscle).
• Vitamin B₁₂ nasal gel.
• Vitamin B₁₂ nasal spray

(https://my.clevelandclinic.org/health/diseases/22831-vitamin-b12-deficiency)