All eight B vitamins are synthesized by human intestinal bacteria and can be absorbed by the host.
"These eight chemicals called B, all of them are made by bacteria and we absorb them." (said at 1:01:00)
While human intestinal bacteria possess biosynthetic pathways capable of producing all eight B vitamins (B1, B2, B3, B5, B6, B7, B9, and B12), they cannot all be meaningfully absorbed by the human host. Most bacterial synthesis occurs in the colon. Although the colon possesses transporters capable of absorbing several B vitamins (such as biotin, thiamin, folate, and riboflavin), it cannot absorb bacteria-synthesized vitamin B12. Physiological absorption of vitamin B12 requires gastric intrinsic factor and takes place exclusively in the terminal ileum, upstream of where most colonic bacterial B12 synthesis occurs; hence, colonic B12 is excreted in feces and unavailable to humans without coprophagy, requiring dietary intake.
- partial: Role of the gut microbiota in human nutrition and metabolism. (Journal of gastroenterology and hepatology 2013)
"The gut microbiota produce a variety of nutrients including short-chain fatty acids, B vitamins, and vitamin K." (abstract, results, passage verified)
pubmedfull study (doi) - contradicts: Vitamin B12 synthesis by human small intestinal bacteria. (Nature 1980)
"In man, physiological amounts of vitamin B12 (cyanocobalamin) are absorbed by the intrinsic factor mediated mechanism exclusively in the ileum. Human faeces contain appreciable quantities of vitamin B12 or vitamin B12-like material presumably produced by bacteria in the colon, but this is unavailable to the non-coprophagic individual." (abstract, results, passage verified)
pubmedfull study (doi)