Dietary sources and requirements

Cobalamin is synthesized solely by micro-organisms. Ruminants obtain cobalamin from the foregut but the only source for humans is food of animal origin. The highest amounts are found in liver and kidney (up to 100 |g/100 g) but it is also present in shellfish, organ and muscle meats, fish, chicken and dairy products - eggs, cheese and milk - which contain small amounts (6 |g/L). Vegetables, fruits and all other foods of non-animal origin are free from cobalamin unless they are contaminated by bacteria. Cooking does not usually destroy cobalamin.

A normal Western diet contains between 5 and 30 |g of cobalamin daily. Adult daily losses (mainly in the urine and faeces) are between 1 and 3 |g (about 0.1% of body stores) and, as the body does not have the ability to degrade cobalamin, daily requirements are also about 1-3 |g. Body stores are of the order of 2-3 mg and are sufficient for 3-4 years if supplies are completely cut off.

Figure 5.6 (a) Intrinsic factor; (b) intrinsic factor-cobalamin complex. Intrinsic factor has been estimated to have a molecular radius of 3.6 nm, vitamin B12 0.8 nm and the complex 3.2 nm.

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