The Other Vitamin B12 Controversy: Must We Eat Meat?
Our second most popular post on TalkingNutrition is one from September 12, 2013 based on some highlights from talks at a conference on controversies in measuring vitamin B12 status. The reason for its popularity is that it comes up at the top of searches for vitamin B12 controversies. We get about 10 visitors a day to this post. However, the post is written for nutrition science experts, and I wonder if it contains the information that the average person is looking for on vitamin B12 controversies. There are namely several other controversies regarding vitamin B12, and they mostly relate to whether our requirement for vitamin B12 means that we have to eat meat.
Vitamin B12 is needed for various important processes in the body, including red blood cell formation and normal neurological function (see a factsheet from the Office of Dietary Supplements with a thorough overview on vitamin B12). Rather than merely being a source of cobalt as some websites suggest, the various cobalamins with vitamin B12 activity participate in a number of important biochemical reactions. Vitamin B12 is absolutely essential: deficiency results in permanent neurological damage, and before it was discovered that pernicious anemia could be cured by feeding people large quantities of liver, it was fatal.
Animal products contain the only “naturally occurring” source of vitamin B12 in the diet, particularly liver and filter-feeding seafood such as mussels, but meat, fish, eggs and dairy products all contain some vitamin B12. However, the ultimate source of vitamin B12 in foods is bacteria and archaea*. Industrial production processes use most commonly Pseudomonas denitricans and Propionibacterium shermanii to make vitamin B12. Fungi and yeasts do not appear to synthesize vitamin B12, although it may be present in some edible fungi due to vitamin B12 absorption from the fungi’s substrate (see Bito, and an excellent, exhaustive discussion on whether plant foods and soil contamination of food can meet vitamin B12 requirements from Norris). Bacteria in the intestinal tract of ruminants such as cows and sheep produce vitamin B12 when sufficient cobalt is provided in the diet. For example, around 3% of dietary cobalt in cows’ diets is converted into vitamin B12. Animals that are more omnivorous such as pigs or chickens obtain sufficient vitamin B12 from small quantities of insects or other animal products that they consume, or from ingesting soil that contains vitamin B12. Farmed animals generally are given vitamin B12 as part of their diet.
The biological need for vitamin B12 is reflected in our genes. The process for actively absorbing vitamin B12 is complex and relies on a number of carrier proteins such as haptocorrin and intrinsic factor to release it from the food matrix and allow it to be absorbed as a complex via specific receptors on cells in the ileum. Genetic mutations can affect the function of these proteins required for vitamin B12 absorption and metabolism, leading to various inborn deficiency disorders (Watkins and Rosenblatt).
Animal foods are the only sources of vitamin B12 in the human diet, outside of supplements. The absorption of vitamin B12 is determined by a number of our genes, therefore must have been there over the evolution of humanity. Does this mean that people have to eat animal foods? Does the need for vitamin B12 mean that a 100% plant-based diet is unhealthy? No! The presence or absence of animal-based foods does not affect whether a diet is healthy. The healthiness of a diet is determined by whether it nourishes us, i.e. its actual nutrient content. Advances in nutrition science and industrial production of small, complex molecules have allowed us to produce vitamin B12 without the need for eating animals. Eat meat or avoid it, it’s your choice.
Even so, vegans are at greater risk of vitamin B12 deficiency than ovo-lactovegetarians or omnivores (see review from Pawlak and co-workers), and this is logical as they do not obtain vitamin B12 from their food. Vegans should be aware of the lack of a source of vitamin B12 in their diet and either undergo regular testing for deficiency, or take a vitamin B12 supplement.
Bito, T.; Teng, F.; Ohishi, N.; Takenaka, S.; Miyamoto, E.; Sakuno, E.; Terashima, K.; Yabuta, Y.; Watanabe, F. Characterization of vitamin b12 compounds in the fruiting bodies of shiitake mushroom (lentinula edodes) and bed logs after fruiting of the mushroom. Mycoscience 2014, 55, 462-468, http://dx.doi.org/10.1016/j.myc.2014.01.008.
Martens, J.H., Barg, H., Warren, M., Jahn, D. Microbial production of vitamin B12. Applied Microbiology and Biotechnology. Volume 58, Issue 3, 2002, Pages 275-285. DOI: 10.1007/s00253-001-0902-7 DOI: 10.1007/s00253-001-0902-7 http://www.ncbi.nlm.nih.gov/pubmed/11935176
Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. How prevalent is vitamin B(12) deficiency among vegetarians? Nutr Rev. 2013 Feb;71(2):110-7. doi: 10.1111/nure.12001. Epub 2013 Jan 2. http://www.ncbi.nlm.nih.gov/pubmed/23356638
Watkins D, Rosenblatt DS. Inborn errors of cobalamin absorption and metabolism. Am J Med Genet C Semin Med Genet. 2011 Feb 15;157C(1):33-44. doi: 10.1002/ajmg.c.30288. Epub 2011 Feb 10. http://www.ncbi.nlm.nih.gov/pubmed/21312325
*Vitamin B12-producing genera include: Aerobacter, Agrobacterium, Alcaligenes, Azotobacter, Bacillus, Clostridium, Corynebacterium, Flavobacterium, Micromonospora, Mycobacterium, Norcardia, Propionibacterium, Protaminobacter, Proteus, Pseudomonas, Rhizobium, Salmonella, Serratia, Streptomyces, Streptococcus and Xanthomonas