Vitamin B12 is an essential vitamin that plays a crucial role in many bodily functions. It helps make DNA and red blood cells, keeps the nervous system healthy, and is involved in metabolizing food. Meat is one of the main sources of B12 in most people’s diets. But where exactly does the B12 in meat come from?
The B12 found in meat and other animal products is not directly produced by the animals themselves. B12 is actually made by certain bacteria that reside in animals’ digestive systems. Ruminant animals like cows and sheep have specialized stomachs containing bacteria that synthesize B12. The B12 gets absorbed into their tissues and milk. In turn, when we eat meat and dairy from these animals, we consume the B12.
So the B12 found in animal foods like beef, lamb, and milk originates from the bacteria within the animal, not the animal cells themselves. This article will go into more detail on the sources of B12 found in different types of meat and animal products.
Where Does B12 in Beef and Lamb Come From?
Cattle, sheep, and other ruminant animals have a specialized digestive system that allows them to break down and absorb nutrients from fibrous plant material. Their stomach has four compartments, the largest being the rumen. The rumen contains a diverse microbiome of bacteria, protozoa, and fungi that ferment and digest the cellulose and other compounds from grasses, hay, and grains.
Several species of bacteria in the rumen produce cobalamin, which is another term for vitamin B12. The main bacteria responsible are Prevotella ruminicola and Succinivibrio dextrinosolvens. The B12 is absorbed into the animal’s bloodstream and tissues. About 2-5 mg of B12 is produced by rumen bacteria per day in cattle.
So when we eat beef and lamb, we are consuming B12 that was synthesized by the bacteria in the cow’s rumen. The muscle tissues become enriched with B12 levels much higher than the bacteria themselves. A 100g serving of beef contains about 2.1 mcg of B12 on average.
B12 in Chicken and Pork
Chickens, pigs, and other monogastric animals only have one stomach compartment and do not ruminate their food. But they still acquire B12 and other nutrients from the microbiome in their digestive tracts.
The small intestines of chickens contain a diverse community of bacteria, including species from the Lactobacillus, Clostridium, and Bacteroides genera. Studies have found strains like Lactobacillus reuteri and Enterococcus faecalis that can produce vitamin B12 in the chicken gut. The B12 gets transferred into the meat tissues as the chickens absorb it. Chicken liver is particularly high in B12, containing over 60 mcg per 100g.
Pigs also acquire B12 from intestinal bacteria, though different species. Porcine gut microbiota includes Bifidobacterium, Lactobacillus, Prevotella, and Succinivibrio species that are capable of B12 synthesis. About half of the fecal bacteria isolated from pigs in one study showed B12 production abilities. The B12 gets incorporated into the pig’s muscles and organs, providing a source when we eat pork. Ham and bacon contain around 1 mcg of B12 per 100g serving.
B12 in Fish and Shellfish
The B12 in fish and shellfish has a more direct microbial origin. Aquatic environments contain various microorganisms like algae, bacteria, and archaea that produce B12.
Fish get B12 by feeding on algae, plankton, worms, and other organisms that have absorbed the vitamin from their environment. The B12 accumulates in the fish tissue as they ingest it through their diets. Cold water oily fish are great sources of B12, providing 5-10 mcg per half fillet. This includes fish like salmon, tuna, mackerel, and sardines.
Shellfish are unique in that they obtain B12 both through their diets and from symbiotic bacteria that live within their bodies. Clams, oysters, and mussels contain endosymbiotic B12 producing bacteria from genera like Clostridium, Bacillus, Lactococcus, and Pseudomonas. The shellfish provide shelter and nutrients for the bacteria, while the bacteria generate a surplus of B12 that the shellfish use. Just 100g of cooked clams, mussels, or oysters contains up to 100 mcg of B12.
B12 in Eggs and Dairy
The B12 in eggs and dairy products can also be traced back to the intestinal bacteria of the animals they come from.
The microbiome of chickens and other egg-laying poultry synthesize B12 that gets transferred to their eggs. Hens fed probiotic supplements with Bifidobacterium and Lactobacillus showed increased levels of B12 in egg yolks. Just one large egg yolk contains about 0.6 mcg of B12.
For dairy products, cows and other ruminants pass the B12 produced by rumen bacteria into their milk supply. Milk is a major source of B12, especially because bacteria continue to generate it during storage in the cow’s udder. One cup of whole milk provides about 1.2 mcg of the vitamin. Hard cheeses and yogurt also provide B12 concentrated from the milk.
Necessity of B12 for Animals
Since B12 is produced by microbes rather than the animals themselves, is it necessary as a nutrient for the animals? Or is it more of a byproduct that then benefits us when we consume animal foods?
B12 is actually an essential vitamin for the animals as well. They require it for important physiological functions like energy metabolism, nerve function, and red blood cell formation just like humans. Without adequate B12, animals can suffer from fatigue, failure to thrive, and neurological disorders.
Farm animals normally get all the B12 they need from synthesis by their gut microbiomes. But animals raised in very hygienic environments with heavy antibiotic use may end up with depleted B12. In those cases, B12 supplementation is sometimes given to prevent deficiency.
So in summary, the B12 produced by symbiotic bacteria is significant both for the health of the animals, and for providing this essential micronutrient to those who eat animal products.
B12 in Processed and Fortified Foods
Beyond natural animal products, B12 is also added to many processed and fortified foods through food technology.
B12 used in supplements and fortified foods is commonly produced through microbial fermentation. Certain bacterial species like Propionibacterium freudenreichii and Pseudomonas denitrificans are used to biosynthesize large quantities of B12 for commercial uses.
Vitamin B12 is added to breakfast cereals, nutritional yeasts, plant-based milks, and vegetarian meat substitutes in order to provide this nutrient that is otherwise lacking in plant foods. Food manufacturers must use a form of B12 that is bioavailable and biologically active for humans, such as cyanocobalamin or methylcobalamin.
The fortification levels aim to provide at least 2-6 mcg per serving, an amount found naturally in many animal products. This makes foods that are fortified with B12 good vegetarian and vegan sources.
Do Humans Make B12? Can It Be Obtained Without Animal Foods?
Unlike ruminants and some other animals, humans cannot synthesize vitamin B12 ourselves. Nor do our bodies harbor the types of symbiotic bacteria that produce large amounts of B12 internally.
Though there are bacteria naturally present in the human digestive system, they reside in the large intestine while B12 absorption occurs in the small intestine. The types and populations of bacteria also differ from those in animal guts.
While trace amounts of B12 may be produced in the human colon, this is not significant enough to meet our needs. For example, analysis of human feces shows it contains B12, but only about 1.4% of it is bioavailable and not bound to bile salts. Eating feces is also a very dangerous practice that can spread gastrointestinal infections.
For vegans and strict vegetarians not consuming animal foods, getting an adequate intake of B12 requires consumption of properly fortified foods or supplements. Common vegan sources include nutritional yeast, fortified plant milks and cereals, and B12 supplements.Those avoiding animal foods need to take special care to get 2-6 mcg daily from these sources.
How B12 is Absorbed in the Body
Though B12 is made by microbes, it must follow a complex pathway to get from foods into our bloodstream where it can be used by our cells:
– Digestion – When protein foods containing B12 are eaten, stomach acid frees B12 from the food.
– Binding – B12 attaches to a protein called haptocorrin in the stomach.
– Absorption – B12 travels to the small intestine. Pancreatic enzymes break down the B12-haptocorrin complex and B12 binds to intrinsic factor.
– Transport – The B12-intrinsic factor complex is absorbed into cells of the small intestine via receptors. Then it enters the bloodstream.
– Delivery – B12 is carried on blood proteins, mainly transcobalamin II, to cells of the body for use in essential processes.
– Storage – Excess B12 gets stored in the liver until needed. The liver can store 3-5 years worth of B12.
Symptoms and Risks of B12 Deficiency
Since humans are unable to make B12 ourselves, deficiency can occur if intake is insufficient over time. Groups at higher risk include strict vegans and vegetarians, older adults, those with GI disorders affecting absorption, and long-term users of acid-blocking medications.
If deficient, the body’s B12 stores in the liver eventually become depleted. Symptoms of deficiency include:
– Fatigue and weakness
– Neurological issues like numbness and memory problems
– Megaloblastic anemia
– Poor balance
– Depression
– Vision loss
– Cognitive decline
Deficiency can potentially cause permanent nerve damage if left untreated. So people who fall into the high risk groups should have their B12 levels checked by a doctor and supplement if needed.
Conclusion
To summarize, the B12 present in animal source foods like meat, dairy, eggs, and seafood originates from the microorganisms these animals host in their digestive systems and bodies. Bacteria are the producers while animals are the package and delivery method.
As humans cannot synthesize it ourselves, we must obtain B12 from adequately eating these animal foods, fortified products, or properly supplemented sources. Throughout evolution, our diet has adapted to utilize B12 from foods thanks to the unique symbiosis between animals and microbes.