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As a mostly plant eating person, there are a few nutrients I'm supposed to pay special attention to, one of them being vitamin B12. In general, the B vitamins help convert food (carbohydrates) into fuel (glucose), which can then be "burned" for energy. Together, the B vitamins also help the body metabolize fats and proteins and are necessary for healthy skin, hair, eyes and liver. They are water soluble, so this means they are not stored in our body. Even though I hated chemistry, I find the science behind food pretty interesting. I don't know if the power of a nutrient truly amazes anybody else but just look at the structure of this thing! Amazing, huh?
Absorption of B12 ideally happens in the ileum where receptors, called cubilins, are present. Our intestines have a thin "slimy" coat, called glycocalyx (like the layer on a fish), and underneath that layer are the finger-like microvilli, both increase nutrient absorption.
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Once the cobalamin-IF is absorbed into the intestinal cell (enterocyte), it can now part ways with the IF and hitch a ride with transcobalamin II to the blood. From the time of ingestion to the time cobalamin enters the blood, it can take three to four hours, with peak levels present four to eight hours after ingestion. Alright, so now that the B12 has been absorbed what does our body use it for?
There are two main enzymatic reactions in the body that require vitamin B12: the conversion of homocysteine to methionine (an essential amino acid) and the conversion of methylmalonyl CoA to succinyl CoA (important for the TCA cycle). I'm not going to go into details of the mechanisms here. B12 is extremely important in healthy nerve cells and it aids in the production of RNA and DNA. Coupled with B9, it helps regulate the production of red blood cells and helps iron (another problem vitamin for vegetarians/vegans) function better in the body. Together with B6 and B9, B12 helps control levels of homocysteine, which have been associated with heart disease. If that wasn't enough, when paired with folate, they work together to produce S-adenosylmethionine (SAM-e), involved in immune function and mood. As you can see, this single vitamin works with so many of the other compounds in our body to keep us healthy.
The recommended amount of vitamin B12 for adults is 2.4 micrograms/day for males and females with the amounts increasing to 2.6 when pregnant and 2.8 during lactation. Low levels can cause a range of symptoms such as fatigue, shortness of breath, diarrhea, nervousness, numbness, or tingling sensations in the fingers and toes. Additionally, neurological problems occur in 75-90% of deficient people, characterized by loss of concentration, memory loss, disorientation, swelling of myelinated fibers and possibly dementia. Deficiency can lead to megaloblastic macryocytic anemia characterized by large immature red blood cells in the bone marrow. Eating a well balanced diet helps prevent deficiency. Just 3 oz of salmon provides 302% of the RDA of B12 and 3 oz of a sirloin contains 62% of the RDA. For more on the top 10 foods highest in B12, check out this article at HealthAliciousNess.
Other Sources used:
My textbook from micro/macronutrient metabolism, The fifth edition of Advanced Nutrition and Human Metabolism