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296 SECTION II Diseases of Organ Systems

TABLE 12.6. Salient features pertaining to the metabolism
of vitamin B 12 and folate
Features B 12 Folate
Content in food Vegetables: poor source Vegetables: rich source
Meat: rich source Meat: moderate source
Milk: good source Milk: poor source
Effect of cooking 10–30% loss 60–90% loss
Adult: daily requirement 2–4 mcg 200 mcg
Adult: daily intake 5–30 mcg 100–500 mcg
Site of absorption Ileum Duodenum and jejunum
Body stores 2–5 mg 5–20 mg
Adequate for 3 years Adequate for 4 months

Vitamin B 12
1. Dietary sources of vitamin B 12 : B 12 is predominantly present in animal products (meat,
muscle, fish, eggs, cheese and milk); therefore, pure vegetarians may suffer from defi-
ciency. B 12 is synthesized by bacteria in nature. In humans, these bacteria are normal
inhabitants of large bowel but B 12 cannot be absorbed from this site and thus humans
are entirely dependent on dietary sources.
2. Absorption of B 12 : May be
(a) Active (Flowchart 12.4)
(i) Responsible for absorption of physiological amounts of vitamin B 12
(ii) Highly efficient but slow mechanism (takes 8–12 h from ingestion to attain-
ment of peak levels in blood)

Food with protein-bound vitamin B (in the form of adenosylcobalamin and methylcobalam)
12

Acid and pepsin in stomach
Proteolysis
from protein
Release of B 12

Combines with gastric ‘R’ protein
(R protein is derived from saliva and gastric juice and has a greater affinity for B 12 than intrinsic factor)

B –R protein complex cleaved by pancreatic enzymes in small intestine
12

Liberated B combines with intrinsic factor
12
(secreted by parietal cells in
fundus and body of stomach)

IF--B 12 binds to receptors on brush border of ileal mucosa in the
presence of calcium (pH >6)

IF is destroyed in the ileal cells and B 12 binds to transcobalamin (TC) II to be released into portal circulation

Distribution to tissues as holo-TC

Cellular uptake of B 12 via the megalin/TC II receptor complex
FLOWCHART 12.4. Absorption of vitamin B 12 by active mechanism.

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