Vitamin B12 is a nutrient made by bacteria that is essential in our bodies. A deficiency can lead to problems like anemia, neurological disorders, and damaged blood vessels. The bacteria that make B12 blanket the earth and also live in the digestive tracts of animals (and people, too!). Eating these animals is one way that people get this vitamin. The B12 that human gut bacteria makes is actually found too far down in our digestive tract to be readily absorbed, so we need to ingest it from an outside source.
B12 deficiency can be kind of tricky and go undiagnosed or misdiagnosed for a while. Sometimes it's mistaken for depression or a personality disorder. It's actually common in both omnivores and vegans, since one of the main barriers to getting enough is a problem with absorption. Recent studies suggest that only those taking supplements or eating B12 fortified foods are getting sufficient amounts of the nutrient. The amount of B12 found in supplements is so high that it is practically impossible to not absorb enough. Luckily, once it's absorbed, our bodies are very efficient at storing B12.
Back before the age of sanitation, people used to get B12 just by being closer to the earth. It was readily available by drinking unfiltered water from mountain streams and wells, and eating unwashed vegetables straight from the farm. The dirt left on these vegetables (and sometimes even small amounts of animal feces) contained the bacteria that produces the vitamin. In western society, we're surrounded by chlorinated water, hand sanitizer and antibiotics that kill off this bacteria. So getting the vitamin just from drinking water isn't an option anymore (and neither is cholera, so that's good). This is definitely a step in the right direction in terms of public health. But in our evolution away from feces-covered vegetables, we must also evolve towards the safest way to get vitamin B12, which is either supplementation or fortified foods.
If you choose to take a supplement, you can find this at most any pharmacy or supermarket, usually under the name cyanocobalamin (you can also find methylcobalamin, the active form, but it's a little more expensive). Our bodies need only about 4 micrograms per day, but since not all of what goes in is absorbed, a daily dose of B12 is about 250 mcg. There is also the option of taking a weekly supplement of 2500 mcg. In addition, there is a whole world of B12-fortified foods to choose from - everything from cashew milk, Red Star brand nutritional yeast and breakfast cereal to Clif and Luna bars. Look for foods with at least 25% of the daily value on the label. Eating three servings a day (separated by several hours) of these foods will provide a sufficient amount.
B12 deficiency can be a serious issue for those who are not aware of the need. Luckily, it is an easy problem to prevent. For individuals who choose to avoid all animal products for an extended period of time, it is definitely a good idea to take a supplement or eat fortified foods. That way, you can maintain a diet that is protective against chronic diseases such as heart disease, diabetes, digestive problems, obesity, and much more, but still avoid any chance of developing a B12 deficiency.
To learn more about vitamin B12, check out this article by Dr. McDougall, or this video from Dr. Gregger.
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