Vitamin B12 (critical)

Evidence quality

Evidence on vitamin B12 biochemistry and deficiency prevention is high. Microbiological research establishes that biosynthesis occurs in a limited set of bacteria and archaea, while clinical and nutritional evidence supports fortified foods or supplements as reliable sources for people consuming no animal products (Fang et al., 2017; NIH ODS, 2025).

Biological origin

Vitamin B12, or cobalamin, is synthesized by microorganisms rather than by plants or animals. Animals obtain and retain B12 through microbial synthesis within a digestive ecosystem or through dietary intake, fortification, or supplementation. Industrial B12 used in supplements and food or feed fortification is likewise produced through microbial fermentation (Fang et al., 2017; EFSA FEEDAP Panel, 2018).

The presence of B12 in animal-derived foods therefore reflects accumulation and food-chain transfer, not animal biosynthesis. In ruminants, dietary cobalt is required for rumen microorganisms to synthesize B12. A 2025 review reports that cobalt supply, diet composition, forage-to-concentrate ratio, and rumen microbiota affect this synthesis and notes a current recommended cobalt concentration of 0.2 milligrams per kilogram of dietary dry matter for dairy cattle (Siregar et al., 2025).

Human dietary sources

The NIH Office of Dietary Supplements states that B12 is naturally present in foods of animal origin and is not naturally present in ordinary plant foods, while fortified breakfast cereals and nutritional yeasts provide readily available, bioavailable B12. It identifies vegetarians and vegans as groups at increased risk of deficiency and states that fortified foods and supplements can substantially reduce that risk (NIH ODS, 2025).

Some algae, fermented foods, and other products have been reported to contain cobalamin or related compounds. A review of plant-associated sources proposed dried purple laver as a potential source but also documented that many products marketed or discussed as plant B12 sources require chemical and biological verification (Watanabe et al., 2014). Because content, bioavailability, serving size, and inactive analogues vary, these products should not replace a source with a declared B12 amount when adequacy is the objective (NIH ODS, 2025; Watanabe et al., 2014).

Deficiency evidence

A 2024 systematic review and meta-analysis using multiple biomarkers found that vegan adults had lower serum B12 and higher homocysteine than omnivores, with trends toward higher methylmalonic acid and lower holotranscobalamin. Subgroup analysis indicated that unsupplemented vegans had less favorable biomarker profiles, while B12 supplement use reduced the difference (Niklewicz et al., 2024). Serum B12 alone does not capture every aspect of functional status, which is why studies may also use methylmalonic acid, homocysteine, or holotranscobalamin (Niklewicz et al., 2024).

Animal-feed supplementation

EFSA assessed cyanocobalamin produced by microbial fermentation for use as a nutritional feed additive for all animal species and regarded B12 additives produced by the assessed organisms as effective in meeting animals' requirements. This confirms that direct B12 feed supplementation is an established animal-nutrition pathway (EFSA FEEDAP Panel, 2018). Ruminant systems can instead or additionally provide cobalt to support ruminal microbial synthesis, and the amount of B12 in milk varies with diet, supplementation, microbial conditions, and genetics (Siregar et al., 2025).

These sources establish that feed B12 and cobalt supplementation exist and are technically recognized; they do not establish the proportion of livestock globally receiving either intervention. Animal foods can contain B12 without supplementation when microbial synthesis and dietary conditions are sufficient (EFSA FEEDAP Panel, 2018; Siregar et al., 2025).

Reliability of incidental exposure

The inherited stub links modern B12 sourcing partly to reduced incidental exposure from soil and water. The retrieved sources do not quantify how much historical human B12 intake came from such contamination or show that unwashed produce can reliably meet requirements. Official guidance instead identifies fortified foods and supplements as dependable sources for vegan diets (NIH ODS, 2025).