Vitamin B12 is an essential nutrient in human nutrition. It is synthesized by bacteria and archaea, in the rumen. Therefore, dairy and beef products are considered the main source of vitamin B12 in the human diet (Martens et al., 2002). Vitamin B12 acts as a co-factor for several enzymes involved in carbohydrate, amino acid, and fatty acid metabolic pathways in humans and other mammals. Vitamin B12 deficiency can cause pernicious anemia, cognitive disabilities, neuropathy, and sustained spinal cord degeneration (Hoffbrand, 2015). According to USDA (2012), a 250-ml glass of milk provides 46% of the daily requirements of vitamin B12 (2.4 µg for humans above 13 years old). However, vitamin B12 levels in the milk of individual cows have been shown to vary significantly (Duplessis et al., 2019). The main known factors that influence the microbial synthesis of B12 in the rumen are genetics and dietary composition (the effects of season or stage of lactation are likely confounded with dietary composition). Apparent ruminal synthesis of vitamin B12 was shown to be positively associated with dietary cobalt, neutral-detergent fibre (NDF) and sugars, and negatively associated with dietary non-fibre carbohydrate (Schwab et al., 2006). According to Beaudet et al. (2016), apparent ruminal synthesis of vitamin B12 was 3-fold greater for cows receiving a high-forage diet compared with a high-starch diet. It is important to note that the metabolism of vitamin B12 is highly interconnected with the status of other B vitamins, such as Biotin. However, in this short review, I will focus on vitamin B12 for simplicity.
Similar to mammals, most bacteria require vitamin B12 for critical metabolic functions but, at the same time, lack the ability to synthesize their own vitamin B12. It is unknown which groups of ruminal bacteria are able to synthesize B12, however, based on a human study (Degnan et al., 2014), only 25% of the commensal microbiota in the intestine can synthesize vitamin B12, and the rest are considered vitamin B12 utilizers.
A recent collaborative study lead by McGill University in QC, Canada (Franco-Lopez et al., 2020) examined the correlations among the levels of vitamin B12 in the rumen, milk, and feces. The concentrations of B12 in the rumen and plasma were positively correlated with the yield of vitamin B12 in the milk (concentration of B12 X milk yield). However, these correlations, although significant, were considered weak (r < 35), likely due to variation among different individual animals and herds.
Further, the study investigated the bacterial communities in the rumen, milk, and feces and the correlations between these communities and the abundance of vitamin B12 in the bovine rumen, milk and feces. It is important to note here that correlation studies cannot imply causation (cause and effect); thus, we cannot determine if a particular microbial group was driven by B12 or was the driver. The genus Prevotella was more abundant in animals with high vitamin B12 concentration. It is interesting that Prevotella does not produce vitamin B12 but rather takes the opportunity of the presence of vitamin B12 in the rumen to grow and proliferate. Provetella metabolizes sugars, amino acids, and small peptides for growth and is considered a major propionate producer in the rumen (Strobel, 1992). Our research (AlZahal et al., 2017) showed that Prevotella dominated during high-forage feeding (42% phylum Bacteroidetes), and its abundance was increased to 65% when cows were switched to high-grain. This increase in dominance was associated with an increase in ruminal propionate concentration. It is important to note that Provetella is a very diverse genus with a large number of newly discovered species, the functions of which are widely unknown.
On the other hand, Franco-Lopez et al. (2020) showed that the phylum Bacteroidetes, the family Succinivibrionaceae, and the genera Ruminiclostridium, Butyrivibrio, and Succinimonas were more abundant with animals with low vitamin B12 concentration. The relationship between these bacterial groups, which are considered as important degraders in the ruminal community, and vitamin B12 abundance is not clear. The authors in this study aimed to explore the connection between bovine microbial communities and vitamin B12 abundance in the rumen and milk and to highlight gaps in our knowledge regarding vitamin B12 metabolism. It is, indeed, a complex topic given that vitamin B12 is critical to the growth and function of the gastrointestinal microbiome, the productivity and health of the ruminant animal, and the health of consumers.
Although the supply of B12 vitamin (and other B vitamins) from dietary sources and microbial synthesis in the rumen can generally be sufficient to avoid deficiency, Girard and Matte (2006) and others suggested that these supplies were “insufficient for optimizing metabolic efficiency, production, composition and the nutritional quality of milk in high-producing dairy cows.” Supplementation of B vitamins to high-yielding cows and during the transition period showed a tangible potential to improve cows’ health and healthfulness of ruminant products. Nonetheless, more integrative research is needed to understand conditions under which B vitamins apparent synthesis in the rumen becomes limiting.
References
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