Vitamin B12 is found in
Vitamin B12 is required for the formation of RBCs Neurological functions and DNA synthesis. Vitamin B12 functions as a cofactor for methionine synthase and L-methylmalonyl-CoA mutase.
Methionine synthase catalyzes the conversion of homocysteine to methionine. Methionine is required for the formation of S-adenosylmethionine, a universal methyl donor for almost 100 different substrates, including DNA, RNA, hormones, proteins, and lipids.
L-methylmalonyl-CoA mutase converts L-methylmalonyl-CoA to succinyl-CoA in the degradation of propionate, an essential biochemical reaction in fat and protein metabolism. Succinyl-CoA is also required for hemoglobin synthesis.
Natural occurring Vitamin B12 is bound to proteins so it needs to be hydrolyzed by gastric acid before getting bound intrinsic factor. Synthetic vitamin B12 is free so no need to acted upon by gastric acid.
Pernicious anemia is an autoimmune disease that affects the gastric mucosa and results in gastric atrophy. This leads to the destruction of parietal cells, achlorhydria, and failure to produce intrinsic factor.
Serum vitamin B12 less than 170–250 pg/mL (120–180 picomol/L) or elevated level of homocysteine above 13 micromol/L, or Elevated methylmalonic acid levels (values >0.4 micromol/L) is considered vitamin B12 deficiency, Elevated is homocysteine is not specific to Vitamin B12 deficiency.
|0–6 months||0.4 mcg||0.4 mcg|
|7–12 months||0.5 mcg||0.5 mcg|
|1–3 years||0.9 mcg||0.9 mcg|
|4–8 years||1.2 mcg||1.2 mcg|
|9–13 years||1.8 mcg||1.8 mcg|
|14+ years||2.4 mcg||2.4 mcg||2.6 mcg||2.8|
Sources of Vitamin B12
Animal products are sources of vitamin B12. plant sources do not contain vitamin B12.
Table 2: Selected Food Sources of Vitamin B12 
|Clams, cooked, 3 ounces||84.1||1,402|
|Liver, beef, cooked, 3 ounces||70.7||1,178|
|Nutritional yeasts, fortified with 100% of the DV for vitamin B12, 1 serving||6.0||100|
|Trout, rainbow, wild, cooked, 3 ounces||5.4||90|
|Salmon, sockeye, cooked, 3 ounces||4.8||80|
|Trout, rainbow, farmed, cooked, 3 ounces||3.5||58|
|Tuna fish, light, canned in water, 3 ounces||2.5||42|
|Cheeseburger, double patty and bun, 1 sandwich||2.1||35|
|Haddock, cooked, 3 ounces||1.8||30|
|Breakfast cereals, fortified with 25% of the DV for vitamin B12, 1 serving||1.5||25|
|Beef, top sirloin, broiled, 3 ounces||1.4||23|
|Milk, low-fat, 1 cup||1.2||18|
|Yogurt, fruit, low-fat, 8 ounces||1.1||18|
|Cheese, Swiss, 1 ounce||0.9||15|
|Beef taco, 1 soft taco||0.9||15|
|Ham, cured, roasted, 3 ounces||0.6||10|
|Egg, whole, hard boiled, 1 large||0.6||1|