New research shows how taking B12 supplements in excess can change your skin's bacteria levels, causing inflammation and acne

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Fact: Your body needs B12-a vitamin found in meat and shellfish that keeps your blood cells and nervous system healthy and helps your body produce DNA. But what effect does it have on the outside of your body?

Turns out, in some people, B12 can worsen acne. That's because this water-soluble vitamin can change your skin's bacteria, leading to acne-forming inflammation, according to a new study published in Science Translational Medicine.

In the study, researchers from UCLA took skin samples from people with and without acne, and found that Propionibacterium acnes (a type of bacteria that's present in people with clear skin and with acne) expressed itself differently in each group. In people with acne, P. acnes was less active in producing B12. The researchers then looked at people with clear skin who received a supplemental dose of B12 and found that the P. acnes on their skin became less active in producing B12, in a way that resembled the P. acnes of acne patients.

"When people have a high level of B12 in their body, the bacteria can sense it and it stops producing as much B12," says Huiying Li, Ph.D., lead author of the study. But once P. acnes shuts down the B12 biosynthesis pathway, the bacteria starts producing more porphyrins, naturally-occurring compounds that can lead to inflammation and acne. Put simply, "If there is excessive vitamin B12, the bacteria stop producing their own vitamins, and this causes inflammation," explains Jessica Wu, M.D., Los Angeles dermatologist, author of Feed Your Face.

The findings are part of a growing body of research showing how the microbiome-the community of bacteria and other microorganisms that live in us (and on us)-influences our health. "Bacteria definitely plays a role in acne and not in the way we used to think," says Dr. Wu. "Previously, we thought that pimples and whiteheads were caused by infection, and treatments were designed to kill the acne-causing bacteria." The current way of thinking about acne, on the other hand, is that it's the result of an imbalance between good and bad bacteria. Finding ways to keep the microbiome healthy and balanced is a new goal of acne treatment. (Try one of these Alternative Adult Acne Treatments.)

So what does this mean for you and vitamin B12? The short answer is to try to get nutrients from food rather than supplements, urges Dr. Hu. "That's what your body's designed to absorb and use." (Find out The Truth about Vitamin B12 Injections.) If you eat meat and fish, you probably get enough B12 from your diet. If you're vegetarian or vegan, get your levels checked. It's possible that you're getting enough B12 from other foods (plain yogurt, nutritional yeast, or fortified foods, such as nut milks and cereal), or your doctor may recommend using a supplement.