http://www.amazon.com/dp/0073316083/?tag=tff-amazonparser-20
Chapter 25.
There's really no way to easily digest this for someone who wants a TL;DR on how digestion works to assimilate micros and macros into the cells of the body.
In short, there are virtually no non-animal sources of vitamin B12 that can actually be absorbed into the body reliably. All of the identified vitamins (e.g. A, D, E, K, B complexes, etc.) are crucial to functions within the human body, and B12 is no exception. The damage caused by vitamin B12 deficiency is very serious and irreparable in most cases. In a nutshell, this damage includes degeneration of the nervous
system. More info can be found here:
https://en.wikipedia.org/wiki/Vitamin_B12#Deficiency
(As an aside, Vitamin B12 is one of the more complex vitamins because it is so important that the body keeps a robust reserve of it. That's why many vegans will "feel great" when first going vegan - because they are cleaning the terrible processed food out of their diet and have not yet hit the symptoms of B12 deficiency. Then, as the reserve depletes they start to feel like shit
all the time. See
@Kelly C's post for an example of this.)
So this leaves vegans with a conundrum. They NEED B12 to live, but animals are the source of 99.99999999% of B12. So a vegan needs to be discerning and make sure they are taking regular vitamin B12 supplements just to
avoid terrifying irreparable damage to the nervous system.
To me, that hardly sounds like an ideal, natural and logical diet.
If that doesn't scare you away from veganism (which it should), then you may say, "Why can't I just eat those yeasts and only those yeasts and select few non-animal sources of B12?"
That's a great question! I am glad you asked.
Yes, there are yeasts and some select other non-animal sources, but they are very very few. This presents two problems:
- These sources of Vitamin B12 are generally not used by the body very well.
- They are so few and far between, you will be reliant on one or two far-from-acceptable species that produce vitamin B12....for the rest of your life. To reiterate, there is irreparable damage to your nervous system on the line.
So let's tackle the first point....
Not all vitamins are made equal. The source of the vitamin is just as important as the vitamin itself. For example, isolated ascorbic acid (the source of Vitamin C in Vitamin C tablets) is not absorbed or utilized in the body nearly as well as eating a pepper or orange with the same amount of vitamin C. It's almost as if you didn't eat a large portion of the vitamin C at all. To understand this, you need to understand the biochemistry of vitamins and how they are utilized in the body - something most people (even doctors and nutritionists) never truly delve into and understand. In short, when something is readily absorbed and used by the body it is said to be "bioavailable" or have a "high bioavailability".
Similar to ascorbic acid, non-animal B12 sources are not very bioavailable for several reasons that would take a whole chapter in a textbook to explain.
So, if you are willing to deal with that and risk being on a single source of B12 that has low bioavailability (
again, bear in mind this experiment may leave you with irreparable nervous system damage), you have to also consider that a lack of variety in diet is typically a very bad thing. There are myriad reasons why having a high variety in diet yields a healthier organism, and these are very well documented across several species including humans. So to reconcile that with the fact that you need to live with very few sources of crappy vitamin B12 just makes no sense.
For any vegans, consider these points. Then take this info, and do some research on your own (because I can be wrong, after all). If, after that, veganism sounds good to you, that's on you. But I highly recommend you don't cut corners. Read the A&P textbook. Understand the biological processes.
If you say "Oh I don't have time for all that" I would ask you to consider that this information will not affect just your life, but the lives of future generations in your family - both due to epigenetic AND that your understanding will shape your children's understanding, as well.
The impact of having a high level understanding of your body is tremendous and transformative. I wouldn't take it lightly.