For example, Dr. Michael Greger (of NutritionFacts) recommends:

For adults under age 65, the easiest way to get B12 is to take at least one 2,500 mcg supplement each week or a daily dose of 250 mcg.

From John A. McDougall's speech I heard (though I haven't checked the info, it might be just a personal anecdote with no science behind it):

According to anthropology and archaeologists' findings the ancient human diet was mostly plant based with very occasional meat consumption (like once a month or even more rarely in very warm regions).

This statement means our bodies should be adapted to live without B12 for a month or longer, but still I read that doctors recommend we supplement every day/week at max.

My question is, why should we supplement so often? Isn't a pill per month sufficient?

  • Please add a link or citation to support the stated "fact" that ancient humans ate meat once a month or less. Otherwise the claim of factuality is unsubstantiated.
    – Nic
    Nov 15, 2018 at 17:31
  • 1
    @Nic got it from McDoughall's speech, not sure if it is backed up with scientific evidence, gonna change the question so that it says it may be just an invalid claim. Thanks for the point.
    – Maxgmer
    Nov 16, 2018 at 12:34

4 Answers 4


The UK Vegan Society provide an info page on B12 where they state (emphasis added):

To get the full benefit of a vegan diet, vegans should do one of the following:

  • Eat fortified foods two or three times a day to get at least three micrograms (mcg or µg) of B12 a day
  • OR Take one B12 supplement daily providing at least 10 micrograms
  • OR Take a weekly B12 supplement providing at least 2000 micrograms.

If relying on fortified foods, check the labels carefully to make sure you are getting enough B12. For example, if a fortified plant milk contains 1 microgram of B12 per serving then consuming three servings a day will provide adequate vitamin B12. Others may find the use of B12 supplements more convenient and economical.

The less frequently you obtain B12 the more B12 you need to take, as B12 is best absorbed in small amounts. The recommendations above take full account of this. There is no harm in exceeding the recommended amounts or combining more than one option.

Thus, the reason for high frequency supplementation is that absorption is better when doses are small, but to get enough B12 from small doses you need to take many such small doses. A huge monthly dose might be sufficient to prevent you getting ill, but it certainly wouldn't be optimal.

I've read that in the past people used to get B12 from non-animal sources, because of farming methods or hygiene practices. The bottom line is, we don't know how people used to get their B12; we only know what works in the present.

  • thanks, for the info, +1)
    – Maxgmer
    Nov 15, 2018 at 9:54

It is basically because we no longer drink untreated water and we no longer eat fruits and vegetables without washing them. B12 is produced by a bacteria in the soil. It is hard to absorb 1 month worth of B12 in a single dosis

  • thanks, +1) might be a valid point as there actually is the bacteria according to what I ve found.
    – Maxgmer
    Nov 15, 2018 at 9:55

B12 is a stored vitamin. You can take high doses on a less regular basis and doctors do prescribe injections of vitamin B12 for patients with severe deficiency. Most vegans aren’t severely deficient, they’re at a high risk of deficiency without dietary supplements. Bioavailability and absorption are the main reasons for this recommendation. It’s better to take lower doses often as you won’t absorb as much of a higher dose (which is why these are injections).

As for the anthropology, it is more complex than that. Early hominins may have been on a largely plant-based diet but they weren’t strictly vegan, they were scavengers and hunters. They ate basically anything necessary to survive. It is difficult to generalise about early human diets due to the diversity between populations, even with more accurate genomic data recently. Plants were often their main source of carbohydrates and proteins but a small amount of other foods if enough for micronutrients. They occasionally also ate fish and eggs. They’re also known to have eaten bone marrow (tools have been found to extract it) of animals killed other predators even before hunting. These are also sources rich in vitamin B12. As mentioned by others, the source of B12 in animals is the bacteria that they eat. They were also exposed to more bacteria than we are now living in clean sanitised environments including our food and drinking water.

It’s also inaccurate to claim that early hominins were still “healthy”, despite being on a largely plant-based diet. By modern standards, they were not. Life expectancy is a poor indicator here since infant mortality and disease were high, so those surviving into adulthood still lived over the average life expectancy. They still did not live nearly as long as we do today. We cannot cite a plant-based diet as the only cause of this. There are many other differences to our environment and lifestyle. This was a time when an infection could fester and be life threatening. A broken bone or losing your teeth could be a death sentence. We simply cannot compare our health to this time period. These were by no means the best conditions to be healthy. Your doctors recommendations are based on medical and biochemical studies on what is best for your health and well-being, not how your ancestors managed to survive despite suffering many now preventable illnesses.

Improvements in nutrition have been shown to have caused our average height to increase meaningfully, even in the last few centuries. A better understanding of our needs for vitamins is a contributing factor for this (such as the reduction in scurvy and rickets). A deficiency in vitamins in childhood is much more of a concern than adulthood as it can affect your growth and development irreversibly. This continues to occur in the developing world and certainly did in early hominins. There are known time periods of population bottlenecks and low genetic diversity that are evidence that human populations came close to extinction several times (a few 100,000 years ago). Starvation in harsh conditions has been suggested to have caused this (and the may have lead to the migrations that followed).

  • This is a good answer, but would be greatly improved with references or citations.
    – Nic
    Nov 15, 2018 at 15:51

Dr. John A. McDougall himself recommends:

If you follow the McDougall Diet for more than 3 years, or if you are pregnant or nursing, then take a minimum of 5 micrograms of supplemental vitamin B12 each day.

As for human adaptation to availability of B12, you need to consider sources other than meat eating. For example...

Feces of cows, chickens, sheep and people contain large amounts of active B12. Until recently most people lived in close contact with their farm animals, and all people consumed B12 left as residues by bacteria living on their un-sanitized vegetable foods.

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