Vitamin B12...

Ellen Says: Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as fatigue, depression, and poor memory may be experienced. However, these symptoms by themselves are too nonspecific to diagnose deficiency of the vitamin.

Vitamin B12 deficiency can also cause symptoms of mania and psychosis.
Vitamin B12 deficiency has the following pathomorphology and symptoms:
Pathomorphology: A spongiform state of neural tissue along with edema of fibers and deficiency of tissue.

The myelin decays, along with axial fiber. In later phases, fibric sclerosis of nervous tissues occurs. Those changes apply to dorsal parts of the spinal cord and to pyramidal tracts in lateral cords. The pathophysiologic state of the spinal cord is called subacute combined degeneration of spinal cord.

In the brain itself, changes are less severe: They occur as small sources of nervous fibers decay and accumulation of astrocytes, usually subcortically located, and also round hemorrhages with a torus of glial cells. Pathological changes can be noticed as well in the posterior roots of the cord and, to lesser extent, in peripheral nerves.

Clinical symptoms: The main syndrome of vitamin B12 deficiency is Biermer's disease (pernicious anemia). It is characterized by a triad of symptoms:
Anemia with bone marrow promegaloblastosis (megaloblastic anemia)
Gastrointestinal symptoms
Neurological symptoms

Each of those symptoms can occur either alone or along with others. The neurological complex, defined as myelosis funicularis, consists of the following symptoms:
Impaired perception of deep touch, pressure and vibration, abolishment of sense of touch, very annoying and persistent paresthesias
Ataxia of dorsal cord type

Decrease or abolishment of deep muscle-tendon reflexes
Pathological reflexes — Babinski, Rossolimo and others, also severe paresis
During the course of disease, mental disorders can occur. These include irritability, focus/concentration problems, depressive state with suicidal tendencies, and paraphrenia complex.
These symptoms may not reverse after correction of hematological abnormalities, and the chance of complete reversal decreases with the length of time the neurological symptoms have been present.

Vitamin B12 is found in foods that come from animals, including fish and shellfish, meat (especially liver), poultry, eggs, milk, and milk products.  One half chicken breast provides some 0.3 µg (micrograms) per serving or 6.0% of one's daily value (DV); 85 grams (3 oz) of beef, 2.4 µg, or 40% of one's DV; one slice of liver 47.9 µg or 780% of DV; and 85 grams (3 oz) of molluscs 84.1 µg, or 1,400% of DV.
Eggs are often mentioned as a good B12 source, but they also contain a factor (avidin) that blocks absorption.

 Certain insects such as termites contain B12 produced by their gut bacteria, in a way analogous to ruminant animals.