Vitamin B12 Deficiency and its Neurological Consequencesby Viatcheslav Wlassoff, PhD | July 30, 2014
B complex vitamins are important for various functions of the human body. Be it energy production, body defense mechanisms, or red blood cell formation, the B group vitamins play pivotal roles and carry on the functions by working in tandem or all alone. Vitamin B12, in particular, has a great impact on neurological health.
When it comes to neurological concerns, the vitamins B1 (thiamine), B2 (riboflavin), B3 (niacin), biotin, B6 (pyridoxine), folic acid and B12 (cobalamin) are especially important. These B group vitamins have specific effects on brain cell functions and their deficiency leads to different neurological and psychological problems. Vitamin B12 deficiency is not very common in young adults but older people are more prone to this condition.
Vitamin B12, or cobalamin, is an essential vitamin for the proper functioning and development of the brain and the nerve cells. It plays an important role in the maintenance of the sheaths that cover and protect the nerves of the central and the peripheral nervous system, ensuring proper and faster nerve-impulse transmission.
A fatty substance called myelin is essential for the formation of these sheaths. Vitamin B12 plays a significant role in the synthesis and maintenance of myelin. The neurological problems caused by vitamin B12 deficiency later in life are due to the damage caused to the myelin sheath.
The actual mechanism by which myelin damage occurs remains unclear. However, two possible mechanisms are proposed:
1. Vitamin B12 assists an important step of the one-carbon cycle
A proper functioning of the central nervous system is dependent on the one-carbon metabolism that takes place in the brain. The one-carbon cycle actually includes a series of biochemical reactions that occur in every cell of the human body. The cycle is crucial for the synthesis of DNA, RNA, neurotransmitters, membrane phospholipids and myelin. Among these, the neurotransmitters (serotonin, dopamine, acetylcholine and nor-epinephrine) are essential for the maintenance of the cognitive functions of the brain and the myelin is required for neuronal protection and communication as described above.
Vitamin B12 is a vital participant of the cycle because it acts as a co-factor in a very important step of the one-carbon pathway: the synthesis of methionine. The reasons for this step being so crucial are:
- Methionine is an amino acid that donates the ‘methyl groups’ required for the methylation reactions (biochemical reactions that are essential for the synthesis of the above mentioned substances) to occur. Without methionine, myelin and neurotransmitters that are needed for neurological development, maintenance and functions, cannot be produced.
- Methionine is synthesized from homocysteine, an amino acid that is related to many neurodegenerative diseases. A high level of this amino acid can lead to brain damage and cognitive disturbances. The synthesis of methionine thus prevents the accumulation of this harmful amino acid in the brain.
Other B complex vitamins like folate, vitamin B6 and vitamin B2 are also essential for the one-carbon pathway.
2. Vitamin B12 acts as a coenzyme in another important reaction that is needed for myelin synthesis and stabilization
Another biochemical reaction, the conversion of methylmelonyl CoA into succinyl CoA, also requires the co-enzyme cobalamin. If this reaction does not occur, methylmalonyl CoA gets converted to methylmalonic acid (MMA), which is a myelin destabilizer. The excess MMA leads to the synthesis of abnormal fatty acids instead of myelin. These abnormal fatty acids are incorporated into the neuronal lipids leading to the formation of a fragile myelin sheath. Subsequently, abnormal myelination or demyelination occurs. The results are severe central and peripheral nervous system dysfunctions.
Vitamin B12 deficiency causes various neuropsychiatric problems ranging from neuropathy to dementia in the elderly
Vitamin B12 deficiency is found to cause neurological and psychiatric problems in adults between 40-90 years of age. It rarely affects people younger than this. The neurological manifestations include myelopathy (disease of the spinal cord), neuropathy (disease of the nerves), sensory disturbances, gait abnormalities and weakness while the psychiatric problems range from cognitive and behavioral disturbances to dementia.
a) Subacute Combined Degeneration (SCD)
This is a progressive degenerative condition affecting the spinal cord caused by vitamin B12 deficiency. The degeneration may also involve the brain, the nerves of the eyes and the peripheral nerves. Initially the myelin sheath is damaged, followed by the degeneration of the axons (nerves). The initial symptoms of this condition are usually tingling sensation and numbness felt on both sides and clumsy movements. Other symptoms include weakness, visual problems, cognitive disturbances, abnormal reflexes and bladder and erectile dysfunctions. Early treatment of this condition with vitamin B12 supplementation gives good results. A delay in treatment means an incomplete recovery of lost functions.
b) Peripheral neuropathy
Peripheral neuropathy is also a usual presentation of vitamin B12 deficiency. In this condition the transmission of nerve signals between the spinal cord and different parts of the body is disrupted. The problem may be due to a direct damage caused to the nerves or demyelination leading to axonal damage. The symptoms are tingling sensation, pain and numbness. Depending on the type of the nerve affected, there can be loss of sensation or muscle mass and activity. This condition also needs early treatment to limit the damage and obtain effective results.
c) Psychiatric problems related to vitamin B12 deficiency
The psychiatric manifestations of vitamin B12 deficiency include delusions, hallucinations, cognitive changes (like memory decline), depression and dementia. These problems are described by various mechanisms like the deranged production of neurotransmitters and the elevated levels of homocysteine and methylmalonic acid (MMA). When there is no other obvious cause of a psychiatric disorder, screening for and supplementation with vitamin B12 need to be considered.
d) Vitamin B12 deficiency in brain shrinkage and neurodegenerative disorders
Vitamin B12 deficiency and the resulting increase in homocysteine levels can lead to brain atrophy (shrinkage). Brain atrophy usually occurs with normal aging. But this process is accelerated in the presence of neurodegenerative diseases.
Various studies have shown that cobalamin deficiency is more common in patients with Alzheimer’s disease, Parkinson’s disease and various conditions causing dementia. But a causal relationship between vitamin B12 deficiency and these diseases remains inconclusive. However, most of the studies that relate hyperhomocysteinemia with Alzheimer’s disease and other causes of dementia report positive results. This means that elevated levels of homocysteine in the blood increase the risk of these diseases.
There also exists controversy in the reports of studies that evaluate an association between vitamin B12 deficiency and Multiple Sclerosis.
e) Vitamin B12 and vascular complications
Increased level of homocysteine in blood is also a recognized risk factor for stroke and other vascular complications. Thus there is a possible relationship between vitamin B12 deficiency and vascular diseases.
Vitamin B12 and folate deficiencies in the fetal and early life cause poor brain development and cognitive functions
Folate and vitamin B12 play very important roles in brain development, especially during the fetal and first 2 years of life. Furthermore, these vitamins are essential till puberty as myelination that starts during fetal life continues till puberty. As mentioned above, vitamin B12 deficiency results in restricted myelination and depending on the area of the nervous system affected, the child can present with varied cognitive and intellectual problems. Pregnant women having folate and B12 deficiency need supplements to prevent the neurological complications in the children.
Vitamin B12 thus proves to be a very important micro-nutrient for brain health in the children and the elderly. There are various conditions that may lead to the deficiency of this vitamin. But if the cause is due to a dietary deficiency, a proper and timely supplementation of this ‘nervous vitamin’ can prevent the damage to the nervous system. The vitamin is abundant in animal foods and even vegan diets are being fortified with this vitamin.
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