**3. The absorption and distribution of vitamin B12**

**1. Introduction**

demyelination may emerge [5].

out the importance of identifying and treating it.

**2. The metabolism of vitamin B12**

**Figure 1.** The mechanism of vitamin B12.

Vitamin B12 is only synthesized by microorganisms in nature and thus, is obtained by human beings through their diet [1]. Since the most important source of vitamin B12 is animal proteins,

104 Epidemiology of Communicable and Non-Communicable Diseases - Attributes of Lifestyle and Nature on Humankind

Vitamin B12 deficiency may be caused by a lower dietary intake (impaired absorption or decreased intake), an autoimmune issue related to intrinsic factors or gastrointestinal system diseases resulting in vitamin B12 malabsorption [2]. The most important symptoms and findings of severe vitamin B12 deficiency are anemia and neurological problems. Vitamin B12 deficiency is one of the most common causes of macrocytic anemia [3, 4]. If it is not treated, anemia symptoms and neurological disturbances resulting in spinal cord and cerebral cortex

Epidemiology concerns health and disorders, etiological agents, the symptoms of disorders, diagnoses and the benefits of clinical care, and its discontinuation. Determining risk factors and at‐risk groups as well as educating them about vitamin B12 deficiency, proper diet, and replacement would prevent any irreversible complications of this type of deficiency. The goal of this study is to review epidemiological studies related to vitamin B12 deficiency and to point

The major metabolic pathway of vitamin B12 formation is shown in **Figure 1**.

Vitamin B12 is essential for DNA synthesis in cells. It has two different forms in cells.

vegetarians may lack sufficient quantities of this vitamin in their diets.

The absorption of vitamin B12 is a multiple staged process. Vitamin B12 intake through dietary sources initially combines with binding proteins (R‐protein) in the saliva. Then it reaches the intestine where pancreatic protease is extracted and it combines with intrinsic factors which contain glycoprotein. Vitamin B12 is absorbed efficiently when it combines with such intrinsic factors. In fact, very little uncombined free vitamin B12 is absorbed. The vitamin B12 and intrinsic factor binds with a specific receptor on the mucosa cells of the terminal ileum and is extracted to the circulation system from the intestine wall. Vitamin B12 is bound with transcobalamin proteins in circulation. The most important transcobalamin protein is transcobalamin II that is the main transporter protein in distributing vitamin B12 to the tissues and liver [5].

Tissues rich in vitamin B12 include parenchymal tissues (above 100 mcg/100 g), fish, muscular organs, dairy products, and egg yolks (1‒10 mcg/100 g) [5]. In the West, daily vitamin B12 intake by nonvegetarians is approximately 5‒7 mcg/day, which is sufficient for normal homeostasis of body functions [6]. However, vegetarians are at risk of vitamin B12 deficiency because they only consume 0.25‒0.5 mcg/day vitamin B12 from their diet [6]. Vitamin B12 is stored well in tissues; for adults, vitamin B12 levels are 2‒5 mg and this is mostly located in the liver (ap‐ proximately 1 mg). Daily loss of vitamin B12 level is 0.1%. When someone no longer obtains vitamin B12 through their diet, depletion of the stored vitamin may take as long as 3‒4 years [6].
