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Why is vitamin B12 different from other water-soluble vitamins in the body?

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Several translucent, oval-shaped vitamin E gel capsules are scattered on a white surface near a partially visible bottle. — vitamin Vitamin E in gel-cap form.

Vitamin B12 is unique among water-soluble vitamins primarily due to its complex chemical structure and its specific requirements for absorption. Unlike other water-soluble vitamins, vitamin B12 contains a metallic ion, cobalt, within its molecular structure.12

Furthermore, its absorption process is distinct. To be successfully absorbed by the small intestine, vitamin B12 must first bind with intrinsic factor, a glycoprotein secreted by the parietal cells in the stomach. This binding process protects the vitamin from digestion as it travels through the gastrointestinal tract. While other water-soluble vitamins are generally absorbed more directly, a lack of intrinsic factor can lead to inadequate absorption of vitamin B12, potentially resulting in pernicious anemia.31