As you are reading these sections please keep in mind that the science of measurement and definitions of normal vs. optimal levels for these substances is still evolving. Plus there is incomplete standardization of these tests in the US - meaning the reference normal range may be different from one lab to the other. But these few paragraphs will give you a good background with which to interpret new discoveries and opinions as they emerge!
One of my favorites (such a complex and hotly debated vitamin). There are only a few essentials you need to know - but they are important to understand fully. The active forms have a variety of ___-cobalamin names, but you will mostly hear about B12 as "cyanocobalamin". Now we know why we call it B12 for short.
B12 is only made by bacteria and fungi, which explains why we have a hard time finding it in our modern antiseptic foods. Ruminants like cows have b12 because bacteria in their gut make it for them (and thus eating or drinking cows provides b12). Our intestinal bacteria also make B12, BUT these little guys live away from the place in our intestines where we are able to absorb B12...this is why we must eat B12.
Take B12 seriously - B12 deficiency is not rare. A prolonged deficiency of B12 has serious yet reversible effects (such as anemia - low red blood cell levels) and even more serious but irreversible effects (such as subacute degeneration of the dorsal columns - permanent damage to nerves that enable you to feel the floor and stay balanced, which is something I have a hard time doing even with normal dorsal columns).
Low B12 levels are also associated with depression and other neurological conditions.
Here's the good news - B12 is cheap, it has little if any side-effects, you need very little of it (around 2-3 micrograms/day) and you (well your liver) can hoard stores of b12 for one or more years (you'll hear ranges from 1-7 years, but NOTE: you will eventually run out if you don't eat any b12). Your liver, at least, understands how important this vitamin is!
Now here's the thing - B12 deficiency is more common than we'd like in both non-vegans and vegans. Why? Because most b12 deficiency is not from eating too little, but rather from being unable to absorb it (for reasons I wont get into here)...This said, vegans do consistently have lower levels of B12 than non-vegans.
So here's what you should do:
1/ make sure you have a reliable daily source of b12. These include a multivitamin, B12 supplement or fortified soy or rice milk, fortified cereals and "cliff" bars (or similar).
Yes you might be able to get B12 from a few dietary sources (e.g. Red Star Brand nutritional yeast, marmite). And there are some folks who claim other sources have b12 (traditionally made tempeh, your gut bacteria, blue-green algae, nori, etc.) but it is highly unlikely that these sources provide adequate amounts of B12 for all people. And frankly it is foolish to bet our nerves and blood on it. To complicate matters some of these other "sources" have "inactive" B12 analogues which can compete for absorption in your gut with active B12 thereby worsening your real B12 levels - another reason not to count on these less reliable sources.
I realize some people think there is something "unnatural" and bad about taking supplements, but it is the price we must pay for eating less dirt, feces and insects than our primate ancestors.
2/ being vegan gets you a free b12 level lab test. Actually it may get you as many as you want! This is because doctors are classically conditioned to associate the word vegan with the word b12 deficiency. It's part of the fun training we get. So get your b12 level checked and find out where you stand.
The B12 blood test your doctor will do is not a perfect test (like most tests we use), so the result may need some interpretation. For instance, if it comes back in the upper half of normal (often >400 pg/mL) - rock on, it's normal. Recheck once in about 2 years. And keep eating a reliable daily source as per above. If it comes back "normal" but it's at the very low end (say below 300/350) then you should request a more specific test (e.g. methylmalonic acid and homocysteine) to make sure all is kosher. Talk to your doctor about this. Assuming the confirmation test is normal, you still need to be extra diligent about taking in b12 at least once per day and consider a repeat lab in 3-6 months . If a confirmation test or your initial test comes back "deficient", then you have some important work to do with your doc. (Note: an alternative strategy for a low-normal B12 is aggressive supplementation for a few months and then recheck with the standard test. Also note that labs may use different units such as pmol/L).
Look for updates on the topic of what the optimal b12 range is. If indeed it is optimal to be above 300-350 pg/ml, than that would mean a huge proportion of people around the world have deficient B12 levels. Yes this may seem extreme, but it is not an impossibility in our modern sanitized world - which has eliminated our 'natural' exposure to B12.
B12 Summary - find a daily reliable source, get your B12 level checked - it's simple.
For the clinically/medically minded see this article in the AAFP for a good B12 review.
For a nice informal review of B12 studies relating to vegan diets see this article from the 35th World Vegetarian Congress
As always, this blog does not provide individualized medical advice - for that you should find a good doctor.
If any of the above is unclear, please feel free to post related questions.