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25). Humans obtain vitamin B12, also known as cobalamin, pri…
25). Humans obtain vitamin B12, also known as cobalamin, primarily from animal-based foods because it is produced by bacteria and accumulates in animal tissues. Common dietary sources include meat, fish, eggs, and dairy products. People who do not consume animal products can get B12 from fortified foods, such as cereals and plant-based milks, or from supplements. Since the human body cannot produce B12 on its own, it must be obtained through diet to support essential functions like red blood cell formation, nerve function, and DNA synthesis. The stereochemistry of vitamin B12, known as cobalamin, is critical for its biological activity and absorption in humans because only the naturally occurring “active” forms fit properly into transport proteins and cellular enzymes. Vitamin B12 has a complex three-dimensional structure with multiple chiral centers, and even small changes in its configuration can make a form biologically inactive or poorly utilized. In the body, absorption depends on highly specific binding proteins in the stomach and small intestine, such as intrinsic factor, which recognize only correctly structured B12 molecules. Because of this strict specificity, not all forms of B12 are absorbed efficiently, and some synthetic or analog compounds may have low or no biological activity. This highlights a broader principle in nutrition: not all vitamins have high or uniform bioabsorption, since uptake often depends on molecular shape, transport mechanisms, and metabolic conversion efficiency. Which of the following conclusions best summarizes the information provided about vitamin B12?
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