How Long Does It Take to Become Vitamin B12-Deficient and What Are the Symptoms?

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For those eating plant-based diets, encouragement of vitamin B12 supplementation cannot be overemphasized.

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Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

In these next five videos, I’m going to be covering everything vitamin B12. How fast B12 deficiency can set in, the earliest signs to watch out for, why standard blood tests can miss it, and just how dangerous it can be left untreated. Most importantly, the best way to protect yourself.

For those eating plant-based diets, encouragement of vitamin B12 supplementation cannot be overemphasized. What happens if you don’t get enough B12? Well, the vitamin is necessary for the synthesis of DNA, so a deficiency in B12 can cause pretty much anything. Brain problems, psychiatric problems, sensory problems, constitutional problems, muscle problems, and nerve problems. It can also cause clotting disorders and bleeding disorders. It can make people catatonic, hallucinate, psychotic, and even suicidal. One patient got electroshock therapy before they realized (oops!) he was just B12 deficient.

Even more tragic are life-threatening manifestations of B12 deficiency in infants when their moms aren’t supplementing: coma, respiratory failure, severe brain damage. Vitamin B12 deficiency in pregnancy is a diagnosis not to miss.

I’ve done B12 videos like this over and over again over the years. Can’t we just stick to the consensus clinical guidelines on preventing, diagnosing, and treating B12 deficiency? No, we can’t, because there aren’t any. There are lots of recommendations out there, but little robust evidence to back them up; they’re based more on assumptions than solid clinical evidence. That’s why you’ll see papers like this: “How I treat cobalamin (vitamin B12) deficiency.” Why should we care what some random doctor does? Because there aren’t a lot of clearcut answers everyone agrees upon.

For example, it is currently unknown how long it takes for vitamin B12 deficiency to occur in someone going completely plant-based without supplementing with B12 or B12-fortified foods. This textbook suggested it might take as long as 10 or 20 years, but that was based on a study of baboons and a single case report. On the flip side are those suggesting we can see evidence of a functional B12 deficiency within four weeks, but if you look at that source, that’s not right either. Three years is a common time frame we hear, but that’s just from this 70-year-old autopsy study where a back-of-the-envelope calculation said that if the average liver weighs like three pounds (1.35 kg), and B12 is stored in the liver, and there’s a certain amount of B12 per gram, then the average liver would contain enough for three years––but that’s all just in theory.

If we look at people undergoing a type of bariatric surgery that messes with B12 absorption, we start seeing problems within just a few months in terms of impaired metabolism, a rise in methylmalonic acid and homocysteine, two compounds that are normally cleared in part by B12. This challenges the concept that liver stores of B12 are sufficient for long-term maintenance of B12 status. Here’s an interesting recent case: a vegan who had been taking B12 decided to stop it for undisclosed reasons. This allowed for a natural experiment.

Though his blood levels of B12 stayed within normal limits, his homocysteine went through the roof. We like to see it below 12 µmol/L, but it jumped to 18 in a matter of months, though there is a complicating factor. His folate plunged at the same time, and folate is also necessary to detoxify homocysteine. Evidently, when he stopped supplementing with B12, he also stopped eating as many fruits and vegetables (folate is concentrated in greens) because it was wintertime and his access was limited, so that may have contributed, too. The bottom line, the researchers concluded, is that this reiterates the need for continuous B12 supplementation in persons following an unfortified plant-based diet. Some medical professionals even suggest it might be a good preventative measure for all vegans to annually check their B12 status. The number one question primary care doctors are advised to ask symptomatic patients on a plant-based diet is: “What Is Your Preferred Source of Vitamin B12?”

Please consider volunteering to help out on the site.

Motion graphics by Avo Media

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

In these next five videos, I’m going to be covering everything vitamin B12. How fast B12 deficiency can set in, the earliest signs to watch out for, why standard blood tests can miss it, and just how dangerous it can be left untreated. Most importantly, the best way to protect yourself.

For those eating plant-based diets, encouragement of vitamin B12 supplementation cannot be overemphasized. What happens if you don’t get enough B12? Well, the vitamin is necessary for the synthesis of DNA, so a deficiency in B12 can cause pretty much anything. Brain problems, psychiatric problems, sensory problems, constitutional problems, muscle problems, and nerve problems. It can also cause clotting disorders and bleeding disorders. It can make people catatonic, hallucinate, psychotic, and even suicidal. One patient got electroshock therapy before they realized (oops!) he was just B12 deficient.

Even more tragic are life-threatening manifestations of B12 deficiency in infants when their moms aren’t supplementing: coma, respiratory failure, severe brain damage. Vitamin B12 deficiency in pregnancy is a diagnosis not to miss.

I’ve done B12 videos like this over and over again over the years. Can’t we just stick to the consensus clinical guidelines on preventing, diagnosing, and treating B12 deficiency? No, we can’t, because there aren’t any. There are lots of recommendations out there, but little robust evidence to back them up; they’re based more on assumptions than solid clinical evidence. That’s why you’ll see papers like this: “How I treat cobalamin (vitamin B12) deficiency.” Why should we care what some random doctor does? Because there aren’t a lot of clearcut answers everyone agrees upon.

For example, it is currently unknown how long it takes for vitamin B12 deficiency to occur in someone going completely plant-based without supplementing with B12 or B12-fortified foods. This textbook suggested it might take as long as 10 or 20 years, but that was based on a study of baboons and a single case report. On the flip side are those suggesting we can see evidence of a functional B12 deficiency within four weeks, but if you look at that source, that’s not right either. Three years is a common time frame we hear, but that’s just from this 70-year-old autopsy study where a back-of-the-envelope calculation said that if the average liver weighs like three pounds (1.35 kg), and B12 is stored in the liver, and there’s a certain amount of B12 per gram, then the average liver would contain enough for three years––but that’s all just in theory.

If we look at people undergoing a type of bariatric surgery that messes with B12 absorption, we start seeing problems within just a few months in terms of impaired metabolism, a rise in methylmalonic acid and homocysteine, two compounds that are normally cleared in part by B12. This challenges the concept that liver stores of B12 are sufficient for long-term maintenance of B12 status. Here’s an interesting recent case: a vegan who had been taking B12 decided to stop it for undisclosed reasons. This allowed for a natural experiment.

Though his blood levels of B12 stayed within normal limits, his homocysteine went through the roof. We like to see it below 12 µmol/L, but it jumped to 18 in a matter of months, though there is a complicating factor. His folate plunged at the same time, and folate is also necessary to detoxify homocysteine. Evidently, when he stopped supplementing with B12, he also stopped eating as many fruits and vegetables (folate is concentrated in greens) because it was wintertime and his access was limited, so that may have contributed, too. The bottom line, the researchers concluded, is that this reiterates the need for continuous B12 supplementation in persons following an unfortified plant-based diet. Some medical professionals even suggest it might be a good preventative measure for all vegans to annually check their B12 status. The number one question primary care doctors are advised to ask symptomatic patients on a plant-based diet is: “What Is Your Preferred Source of Vitamin B12?”

Please consider volunteering to help out on the site.

Motion graphics by Avo Media

Doctor's Note

This is the first video in a five-part series on vitamin B12. Stay tuned for:

For more on vitamin B12, check out the topic page.

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