This information is provided for reference use only and is not intended as medical advice. Consult a doctor.

Vitamin B12 is required for healthy function. It participates in chemical reactions in the body relating to DNA, blood cells, and hormones. It is the wing-man for essential enzymes involved in fat and protein metabolism.

Bacteria produce B12. Plants are not a source of B12 unless they are contaminated with soil, feces, insects, or are B12 fortified. However, soil and groundwater (containing B12-producing bacteria) are unreliable sources and I am not aware of them being considered effective. “No unfortified plant food contains any significant amount of active vitamin B-12” – American Dietetic Association’s position paper on vegetarian diets. Seaweed has been previously noted to contain B12, but this is thought to be an analogue that is not biologically active.

They go on to say that vegetarians can absorb sufficient B12 from regular dairy consumption. Vegans must consume fortified foods like milk alternatives, breakfast cereals, and meat alternatives, or else take a supplement.

The Vegan Society recommends 3mcg of B12 obtained through 2-3 servings of fortified foods throughout the day, a daily B12 supplement of at least 10mcg, or a weekly B12 supplement providing at least 2000mcg.

Vegan Health by Jack Norris, RD and The Vegan RD by Ginny Messina, MPH, RD both recommend 25-100mcg daily dose, or 1000mcg biweekly, for ages 14-64.

Absorption by Intrinsic Factor

Note that both ug and mcg indicate micrograms and are equivalent.

The RDA for adults is 2.4mcg. Pregnant women require 2.6mcg and breastfeeding women require 2.8mcg. Please visit the Mayo Clinic’s site on B12 dosing for children’s guidelines and other additional information. Supplementation of B12 is recommended for vegans and lacto-ovo vegetarians that are pregnant and/or breastfeeding to ensure healthy development of the child.

Absorption of B12 is achieved by intrinsic factor, but influenced by other biological factors in the gut as well. Absorption is maximal when fasted. Intrinsic factor limits our ability to absorb B12 in one sitting. The National Academy of Medicine’s report (formerly Institute of Medicine) on the Dietary Reference Intake of B12 considers that we absorb “50% at 1ug dose, 20% at 5ug dose, and just over 5% at 25ug dose.” So, about 1ug at a time, but total absorption slowly creeps up as dosage increases. A second dose 4-6 hours later will provide similar absorption. The daily recommendation assumes a 50% absorption at low dietary dosages.

Intrinsic factor is what grabs B12 and then attaches to the lining of our small intestine where it can be absorbed. Generally, 1.5 to 2.5mcg causes saturation. All of our B12 hands are filled, and we cannot grab any more until a few hours pass and those hands are emptied.

Some people absorb much greater amounts at a time, up to 7mcg, but don’t count on it.

After this point, absorption is limited to passive diffusion.

Dietary consumption of B12 may be maximized by consuming B12 throughout the day rather than all at once. Wait 4 hours between B12-containing meals.

Absorption by Diffusion (Supplements)

On average, absorption by diffusion is 1.2% (Berlin et al, 1968) but varies considerably.

The dosages of B12 supplements are absurdly high because after the initial approximately 2mcg absorbed by intrinsic factor you will only absorb B12 by diffusion.

At 500mcg dosage, 2.8-13.4mcg is absorbed when fasted, with only 1.8-7.5mcg absorbed with a meal.

There is wide variation in total absorption.

1000mcg is typically the dose used for malabsorption to guarantee sufficient intake even for those on the lowest end of the range.

Considering the math, a 500mcg dose breaks down like this:

2mcg absorbed by intrinsic factor if present.

498mcg remaining in small intestine.

1.2% of 998mcg is 5.9mcg absorbed by diffusion on average.

Total of 7.9mcg.

And a 1000mcg dose breaks down like this:

2mcg absorbed by intrinsic factor if present.

998mcg remaining in small intestine.

1.2% of 998mcg is 11.9mcg absorbed by diffusion on average.

Total of 13.9mcg.

Deficiency

This study (Carmel R, 2008) cited by the NIH on their Health Professional Fact Sheet finds that, due to the tight regulation of B12 (cobalamin specifically), deficiency is unlikely unless intrinsic factor fails. However, in dietary insufficiency, the deficiency takes longer to develop because of the continual reabsorption of excreted B12 and erratic intake. The majority of clinical B12 deficiencies are due to malabsorption, with failing intrinsic factor to blame. One study (Savage et al, 1994) found 94% of clinical deficiencies had malabsorption related to intrinsic factor, and only 1% were vegetarians, in a sample of 406 patients. In short, it’s likely to be genetic or a problem with your gut.

Deficiency in vegans and those eating macrobiotic diets could definitely become a concern in the long-term as stores deplete, but the National Academy of Medicine could not make a conclusion on recommendations because dietary information in these populations is not always sufficient to conclude that observed B12 levels were due to diet and not due to other factors such as differences in supplementation practices among different groups. Recommendations for vegans provided by vegan registered dieticians was noted at the top of the page.

The NIH considers breakfast cereal one of the few dietary sources of B12 for vegans on their B12 Health Professional Fact Sheet. Note that some milk alternatives and meat alternatives are B12 fortified but check the label to make sure.

Treatment

This 2003 paper published in American Family Physician states that oral supplementation of B12 is just as effective as muscular injections. Oral supplementation reverses deficiency when intrinsic factor is absent and also when the stomach has been removed. The 2011 update states that oral and sublingual administration are equivalent.

The treatment consists of 1,000 to 2,000 mcg of oral B12 per day for two weeks, with 1,000 mcg per day for life as a maintenance dose.

For those without pernicious anemia or malabsorption disorders, 250mcg/day is an option according to this clinical practice summary.

Adverse Effects

B12 is water-soluble and excess in the blood is removed through urine. However, greatest loss of B12 is that which was never absorbed and is excreted with feces.

Dosages of 500mcg to 5000mcg have been associated with acne, primarily with hydroxocobalamin rather than the cyanocobalamin often used in supplements.

There is no Tolerable Upper Intake Level of B12. Up to 5000mcg administered by injection has not produced adverse effects. The low absorption rate of high oral dosages contributes to its safety as a supplement.

According to the Dietary Reference Intake of B12 :

In healthy people, there are no associated adverse effects from food or supplementation.

The 95th percentile of intake is 18-37mcg for adults, varying by age and gender.

Supplementation that is 2 orders of magnitude higher than this does not appear to present any risk.

2 orders of magnitude higher would be 1800-3700mcg.

However, much of the data supporting its safety was not specifically trying to assess adverse effects.

1000mcg/day has been most commonly studied to be more than sufficient at replenishing B12 stores and maintaining levels regardless of the cause of B12 deficiency and levels in excess of this may be absolutely unnecessary.