The new study solved the mystery by showing that the vitamin alters the metabolic activity of the skin bug responsible for acne, Propionibacterium acnes. The study found that, in the presence of vitamin B12, the skin bacteria that are commonly linked to acne start pumping out inflammatory molecules known to promote pimples. The researchers looked at the bacteria's gene expression, hoping to figure out why Propionibacterium acnes, which is the most common skin microbe, causes pimples in some people but not in others. Vitamin B12 alters how the genes of facial bacteria behave — a change that in turn promotes inflammation, which has been linked to pimples in the past. In humans, vitamin B12 plays roles in metabolism, red blood cell formation and the maintenance of the central nervous system. But "it has been reported several times that people who take B12 develop acne," says Huiying Li, a molecular pharmacologist at the University of California-Los Angeles and a co-author of the study. Prior studies citing an association between these two elements confirmed the possibility of a link — research dating back to the 1950s has suggested that vitamin B12 supplements could contribute to some people's acne. Now, Li and her team think they may have figured out how B12 acts on P. acnes, thanks to experiments her team performed on bacteria taken from a small number of people. [4 Common Skin Woes, and How to Fix Them] 
 
 A week after receiving the vitamin B12 injection, one of the 10 participants broke out in pimples. Together, the new findings suggest that when too much B12 is present, the bacteria have changes in their gene expression that suppress further synthesis of the vitamin. They discovered that excess B12 quenched the bacteria's own production of the vitamin, which had the effect of boosting levels of an inflammatory compound, porphyrin, released by the bugs. The study proves the link between B12 and skin bacteria - but that doesn't mean people should stop taking B12, the researchers stress. Most of the studies that have found an increase in acne with vitamin B12 involved large doses of the vitamin, given by injections, she pointed out. Researchers led by Dr Huiyang Li, from the University of California, Los Angeles, write in the journal Science Translational Medicine: "Our findings suggest a new bacterial pathogenesis pathway in acne and provide one molecular explanation for the long-standing clinical observation that vitamin B12 supplementation leads to acne development in a subset of individuals. A deeper understanding of microbial-disease pathways could lead to more targeted treatments, the researchers wrote in their study, published today (June 24) in the journal Science Translational Medicine.
– Nature giveth, and nature taketh away. The same vitamin credited with helping the brain and nervous system—B12—is now accused of contributing to acne, reports the Independent. A new UCLA study suggests a link, though researchers emphasized that more research needs to be done to confirm what's actually happening. Still, the findings "could raise questions about whether people with acne should take vitamin B12 supplements," observes Live Science. It seems that the vitamin changes the way skin bacteria behaves, which promotes inflammation, which in turn leads to pimples. Some people are more vulnerable than others. "This study does present some compelling evidence suggesting that supplementation with B12 can cause or exacerbate acne in a subset of individuals," says a co-author of the study. She emphasizes that the research shows correlation, not causation. While previous studies have linked the vitamin to acne flareups, this is the first to provide details on how it interacts on a molecular level with bacteria, especially Propionibacterium acnes. One upside of the research is that it might eventually lead to better acne medication, reports the Verge. (Kids may be getting too many vitamins in cereal.)