Vitamin B12 or cobalamin is a nutrient that is necessary for, among other things, the production of red blood cells and the proper functioning of the nervous system. It does not occur naturally in plant foods.
Therefore, anyone on a vegan diet must take vitamin B12 supplements.
Vitamin B12 deficiency
Vitamin B12 deficiency can have neurological consequences such as tingling in the fingers (paresthesia), itchy skin, memory loss, coordination disorders, muscle weakness, (extreme) fatigue and depression. A deficiency of vitamin B12 can also lead to a form of anemia.
The consequences of a long-term B12 deficiency can be very serious and result in permanent damage to health.
Daily recommended amount
There is an ongoing debate among doctors and dietitians about the optimal daily amount of vitamin B12 for omnivores. For example, the European Food Safety Authority (EFSA) recommends a daily adequate intake (AI) of 4.0mcg of vitamin B12. The Dutch Health Council and the Nutrition Center stick to a lower recommended daily allowance of 2.8mcg.
The recommended doses for vegans are much higher than those numbers. This is because the body absorbs vitamin B12 from supplements you take once a day less efficiently than B12 from foods you eat throughout the day.
Supplement advice
Vegan adults between 11 and 65 years old should take a supplement of either 50-100mcg/day or 2000mcg/week or 2x1000mcg/week vitamin B12 in the form of cyanocobalamin.
50mcg is enough, but many supplements contain 100mcg, which is also fine. The body will excrete excess vitamin B12.
There are several types of B12 that are active for humans, namely cyanocobalamin, methylcobalamin, adenosylcobalamin/dibencozide and hydroxocobalamin. Cyanocobalamin is the best researched form and is therefore most commonly recommended for oral supplementation (pills).
Many supplements contain methylcobalamin. The current supplementation advice for this form is 1000-1500mcg/day. So much more than for the cyano-B12.
Vitamin B12 in supplements are made by bacteria and therefore always vegan.
Why do vegans need so much vitamin B12?
The body has two different mechanisms for absorbing vitamin B12:
1. through receptors in the gut.
Your body can just absorb about 1.5mcg per eating moment via the receptors in the gut, after which they are saturated for a few hours.
Omnivores eat something with B12 at every meal. They absorb 1.5mcg at every meal and easily achieve their daily amounts of 2.8mcg – 4.0mcg per day.
But vegans, who take a supplement once a day, get all the B12 at once and the receptors in the gut just absorb the 1.5mcg at the time that the supplement is taken. Obviously, this is less than the advised amount. But luckily for us vegans, there is another B12 absorption mechanism, namely:
2. through so-called ‘passive diffusion’ through the membranes of the digestive tract.
Some of the remaining vitamin B12 is absorbed through passive diffusion. But this is very inefficient:
only 1% of the B12 in a supplement is absorbed like that.
Therefore, high-dose supplements are advised.
Let’s do the math
From a daily supplement of 250mcg you absorb approximately 2.5mcg (=1%) through passive diffusion. Plus 1.5mcg through the receptors. So in total 4mcg, which is sufficient.
From a weekly supplement of 2000mcg you absorb approximately 20mcg (=1%) through passive diffusion. Plus 1.5mcg through the receptors. So in total 21.5mcg. Divided over 7 days, this is about 3mcg of vitamin B12 per day, which is sufficient.
The risk of adverse effects from too much vitamin B12 is very small.
There are virtually no known undesirable effects in people who take (too) much vitamin B12 for a long time.
Body storage of B12
Vitamin B12 is the only water-soluble vitamin that can be stored by the body. People who eat animal products usually (but not always!) have a supply of vitamin B12 in the liver that the body can use for 3 to 5 years before a deficiency develops. A deficiency therefore develops gradually, but if you eat vegan for a long time, it is important to supplement vitamin B12 from the beginning to maintain the body’s reserves.
It is strongly advised not to wait to take vitamin B12 supplements, because the damage to health can be irreversible.
Measuring blood values
It is important to have your blood values measured regularly as a vegan and especially if you take no or insufficient B12 supplements. It is also recommended to do a blood test when symptoms appear that may indicate a B12 deficiency. There may be an absorption disorder where the body is not able to absorp B12 in the supplements properly.
References
Allen, L.H., 2010, Bioavailability of B12. Int J Vitam Nutr Res. 2010 Oct;80(4-5):330-5
Voedingscentrum – Vitamine B12
Deshmuk et al. 2010, Effect of physiological doses of oral vitamin B12 on plasma homocysteine: a randomized, placebo-controlled, double-blind trial in India. Eur J Clin Nutr. 2010 May;64(5):495-502.
Del Bo et al. 2019, Effect of two different sublingual dosages of vitamin B 12 on cobalamin nutritional status in vegans and vegetarians with a marginal deficiency: A randomized controlled trial. Clin Nutr. 2019 Apr;38(2):575-583.
