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Vitamin B12 deficiency, also known as hypocobalaminemia, refers to low blood levels of vitamin B12. A wide variety of signs and symptoms may occur including a decreased ability to think and changes in personality such as depression, irritability, and psychosis. Abnormal sensations, changes in reflexes, and poor muscle function can also occur as may inflammation of the tongue, decreased taste, low red blood cells, reduced heart function, and decreased fertility.〔 In young children symptoms include poor growth, poor development, and difficulties with movement.〔(【引用サイトリンク】url=http://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/#h5 )〕 Without early treatment some of the changes may be permanent. Common causes include poor absorption from the stomach or intestines, decreased intake, and increased requirements. Decreased absorption may be due to pernicious anemia, surgical removal of the stomach, chronic inflammation of the pancreas, intestinal parasites, certain medications, and some genetic disorders. Decreased intake may occur in those who eat a vegan diet or have malnutrition. Increased requirements occur in HIV/AIDS and in those with rapid red blood cell breakdown.〔 Diagnosis is typically based on vitamin B12 blood levels below 120–180 picomol/L (170–250 pg/mL) in adults. Elevated methylmalonic acid levels (values >0.4 micromol/L) may also indicated deficiency. A type of low red blood cells known as megaloblastic anemia is often but not always present.〔 Supplementation is recommended to prevent deficiency in vegetarians who are pregnant.〔 Once identified it is easily treated with supplementation by mouth or injection. There are no concerns from excess vitamin B12 among those who are otherwise healthy.〔 Some cases may also be helped by treating the underlying cause. Other cases may require ongoing supplementation as the underlying cause is not curable. Vitamin B12 deficiency is common.〔 It is estimated to occur in about 6% of those under the age of 60 and 20% of those over the age of 60. Rates may be as high as 80% in parts of Africa and Asia. ==Signs and symptoms== Vitamin B12 deficiency can lead to vitamin B12 deficiency anemia and neurologic dysfunction. A mild deficiency may not cause any discernible symptoms, but as the deficiency becomes more significant symptoms of anemia may result, such as weakness, fatigue, light-headedness, rapid heartbeat, rapid breathing and pale color to the skin. It may also cause easy bruising or bleeding, including bleeding gums. GI side effects including sore tongue, stomach upset, weight loss, and diarrhea or constipation. If the deficiency is not corrected, nerve cell damage can result. If this happens, vitamin B12 deficiency may result in tingling or numbness to the fingers and toes, difficulty walking, mood changes, depression, memory loss, disorientation and, in severe cases, dementia. The main syndrome of vitamin B12 deficiency is pernicious anemia. It is characterized by a triad of symptoms: # Anemia with bone marrow promegaloblastosis (megaloblastic anemia). This is due to the inhibition of DNA synthesis (specifically purines and thymidine) # Gastrointestinal symptoms: alteration in bowel motility, such as mild diarrhea or constipation, and loss of bladder or bowel control. These are thought to be due to defective DNA synthesis inhibiting replication in a site with a high turnover of cells. This may also be due to the autoimmune attack on the parietal cells of the stomach in pernicious anemia. There is an association with GAVE syndrome (commonly called watermelon stomach) and pernicious anemia. # Neurological symptoms: Sensory or motor deficiencies (absent reflexes, diminished vibration or soft touch sensation), subacute combined degeneration of spinal cord, seizures, or even symptoms of dementia 〔Reversible dementia in young persons due to cobalamin deficiency. Kumar S, Narasimha A, Holla B, Viswanath B, Narayanaswamy JC, Math SB, Chandrashekar CR. The Journal of Neuropsychiatry and Clinical Neurosciences 2013;25:E62-E63. doi:10.1176/appi.neuropsych.12040083 http://journals.psychiatryonline.org/article.aspx?articleid=1660553〕 and or other psychiatric symptoms may be present. The presence of peripheral sensory-motor symptoms or subacute combined degeneration of spinal cord strongly suggests the presence of a B12 deficiency instead of folate deficiency. Methylmalonic acid, if not properly handled by B12, remains in the myelin sheath, causing fragility. Dementia and depression have been associated with this deficiency as well, possibly from the under-production of methionine because of the inability to convert homocysteine into this product. Methionine is a necessary cofactor in the production of several neurotransmitters. 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, loss of sense of touch, very annoying and persistent paresthesias # Ataxia of dorsal chord type # Decrease or loss of deep muscle-tendon reflexes # Pathological reflexes — Babinski, Rossolimo and others, also severe paresis Vitamin B12 deficiency can cause severe and irreversible damage, especially to the brain and nervous system. These symptoms of neuronal damage 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 deficiency symptoms also include shortness of breath and increased pallor. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vitamin B12 deficiency」の詳細全文を読む スポンサード リンク
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