Vitamin B12 may be an underlying cause of acne, new research has shown.
The essential nutrient, found in meat, fish, eggs and dairy products, alters the activity skin bacteria leading to unsightly spots and pimples in susceptible individuals, scientists found.
They believe the discovery could lead to new treatments for the distressing skin disorder, and shed light on why some people who take vitamin B12 supplements come out in spots.
While it was already known that taking the vitamin pills can result in acne flare-ups, no-one had previously been able to explain the phenomenon.
The new study solved the mystery by showing that the vitamin alters the metabolic activity of the skin bug responsible for acne, Propionibacterium acnes.
This causes the bacteria to secrete an inflammatory compound that in turn triggers pimples.
But the effect is not the same for everyone - some people are more vulnerable than others due to the complex interaction of bacteria and host.
The US team led by Dr Huiyang Li, from the University of California, Los Angeles, wrote 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.
"Our study .. provided evidence that .. interactions between the host and the skin microbiota play essential roles in disease development."
The research involved testing 10 volunteers who were all given vitamin B12 supplements. Within a week, just one of them broke out in spots.
To investigate what had happened the researchers collected bacterial samples from the participants' noses using deep-cleansing pore strips.
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. Porphyrin is known to drive the inflammatory skin response linked to acne.
The researchers added: "Future follow-up studies may potentially lead to the development of new therapeutics for this medically important disease."
Vitamin B12 plays key roles in the normal functioning of the brain and nervous system, and red blood cell formation. Deficiency of the vitamin can lead to fatigue and lack of energy.
People on strict vegan diets which exclude all animal products may be at risk from having too little vitamin B12.
Although naturally occurring in animal products, the vitamin is actually synthesised by bacteria.