Principles of Biochemistry Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Human Nutrition
Dietary vitamin B12 deficiency is extremely rare

Vitamin B₁₂ (Fig. 26-17) prevents The Development of pernicious anemia (Section 10.11). Neither plants nor animals can synthesize vitamin B₁₂; only specific Bacteria are capable of producing it. The bacteria of the human gastrointestinal tract can synthesize vitamin B₁₂ in quantities sufficient to meet daily human requirements. Large amounts of vitamin B₁₂ are also synthesized by bacteria residing in the rumen of ruminants and in the caecum of other herbivores, such as rabbits. Rabbits meet their requirements for vitamin B₁₂ and certain other Vitamins by periodically ingesting their own feces.

Pernicious anemia is a severe disorder characterized by a deficit of Hemoglobin and erythrocytes, along with serious impairments of the Central Nervous system. It develops not As a result of a dietary deficiency of vitamin B₁₂, but due to the inability of the intestine to absorb this vitamin owing to impaired secretion of a specific glycoprotein in The Stomach. This glycoprotein, known as the intrinsic factor, is essential for the absorption of vitamin B₁₂. Pernicious anemia is treated with injections of vitamin B₁₂ or oral administration of high doses of the vitamin, which can compensate for the impaired absorption. Because the human Liver can store vitamin B₁₂ in amounts sufficient to meet requirements for several years, true vitamin B₁₂ deficiency is extremely rare. Vegetarians meet their vitamin B₁₂ requirements either through intestinal bacteria or via bacteria ingested with plant foods.

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Fig. 26-16. Structure, function, and daily requirement of pantothenic acid.

Fig. 26-17. Stereochemical model of the backbone structure of vitamin B₁₂ (cobalamin; see also Section 10.11)