We have bacteria to thank for many things, (though of course some "bad-guy bacteria" cause health problems), but one of the most important is that they make vitamin B12, which we humans need to survive.Vitamin B12 is one of those complex, all-important vitamins that cause serious problems when we miss out on them; even slight deficiencies in B12 can lead to mania, depression, fatigue and anemia. Serious lack of the compound could mean permanent damage to the brain and nervous system.
Animals ingest B12 when they are grazing; the bacteria are in the soil and on the animals' food, and when we consume them or their products, we get that B12 from their bodies. Yes, vitamin B12 is microorganisms, not a specific compound.
If you eat meat and seafood regularly, you could still be deficient in B12 (and this is especially true if you are over 50), but if you are vegetarian and don't eat eggs or dairy much, and definitely if you are vegan, you are going to have to be aware to ensure you get enough B12. In any case, you can't get too much B12 since the body flushes it out, so err on the side of getting plenty.
But first off, how much does anyone need? The NIH's chart below shows that your age and reproductive status are important considerations:
A simple and reliable way to get B12 is with a supplement, but many of us (myself included) try to get the majority of our nutrition from our food. But the top foods that are highest in B12 include bivalves, octopus, crustaceans, lean beef, liver and lamb; not good news for veggies.
If you are a vegetarian who eats eggs and dairy, B12 can be easily found in those foods; you will just need to keep rough track that you are getting enough from cheese and eggs; you'd need about 3 oz. of cheese, which is about half your daily requirement, plus 2-3 eggs to break even (that's a super-hearty omlette!). Throw in some fortified tofu though, and it's easy to get your daily minimum.
Many foods are fortified with B12 (some at levels many times the daily requirement), especially breakfast cereals, energy or snack bars, alternative milks (like soy, almond and rice), soy products, and faux meats often contain it as an added nutrient, so check your labels. But if you don't eat many of these foods (since they are processed, I keep away from most of them), you might need to make sure you are getting it elsewhere.
For vegans, the only significant way to get B12 outside a supplement is by consuming nutritional yeast. According to the Vegetarian Resource Group, "Red Star T-6635+, has been tested and shown to contain active vitamin B12. This brand of yeast is often labeled as Vegetarian Support Formula with or without T-6635+ in parentheses following this name. It is a reliable source of vitamin B12. Nutritional yeast, Saccharomyces cerevisiae, is a food yeast, grown on a molasses solution, which comes as yellow flakes or powder. It has a cheesy taste. Nutritional yeast is different from brewer’s yeast or torula yeast. Those sensitive to other yeasts can often use it."
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