A study in 2018 found that, when scientists altered the gut microbes of mice, the mice went on to develop severe allergic skin reactions that they hadn't experienced before — and that when the gut microbes were translated to healthy mice, those mice started to get allergies, too. So what's the connection?

If you've started to develop skin allergies after experiencing some kind of gut change — like taking antibiotics or shifting your diet — it's possible your gut microbes may be responsible. The reason? Your immune system.

An overview of the gut-skin axis published in Frontiers In Microbiology in 2018 noted that scientists have observed the typical western diet can change the gut microbiome, and that influences the immune system — poorly. The chain, they explained, might go something like this: the microbial community in the gut changes, it stops producing anti-inflammatory metabolites that help regulate the immune system, which means inflammation suges and the immune system starts to over-react to threats. That opens the door to allergies, on the skin and elsewhere. "Reduced local and systemic immune tolerance resulting from an altered gut microbiome may help explain the observed rise of both autoimmune and atopic disease observed in the western world," the scientists argued. It's not a proven argument yet, but it's gaining a lot of traction.

Acne might also be a way in which gut health shows on your skin. The scientists in the Frontiers Microbiology overview explain that people with acne vulgaris can show symptoms that also show up in people with 'intestinal dysbiosis', the technical term for a disordered gut. This is made more complex by the fact that acne can be treated with antibiotics, which are known to wipe out colonies of bacteria in the gut (though they do recover over time).

The scientists noted there are a few theories for the links between acne and the gut; an uptick in 'inflammatory signals' from a damaged gut might cause acne flare-ups, or low levels of stomach acid might cause serious microbial imbalances over time. It's a developing field of research, so we're still in the dark about how precisely it works.

Studies have shown that the administration of probiotics — substances with a lot of healthy bacteria, like probiotic yogurts — can help with acne in some people. Lactobacillus is apparently a particularly helpful bacteria for helping with acne.

Kim Kardashian may be the most famous psoriasis patient in the world, but many other people have this chronic autoimmune condition too. It involves huge over-production of skin cells that then form scabs and flakes on the skin — and for some of us, the key to psoriasis might be in the gut.

The overview in Frontiers In Microbiology pointed out that psoriasis patients often have dysbiosis too. They have low levels of very helpful bacteria, including Lactobacillus, and high levels of unhelpful ones like salmonella, and one study showed that people with psoriasis in particular had low levels of two specific gut species compared to people with other chronic health disorders. Scientists are not sure how that might affect psoriasis and whether it's a consequence, a cause, or something else, but the scientist suggest that this imbalance might cause issues for the immune system and translate to chronic skin problems.

The National Psoriasis Foundation says that the microbiome is an ongoing research area for shedding light on the condition. Dr. Martin Blaser of the Human Microbiome Program explained that "the microbiome in psoriasis is a new frontier." Bacterial diversity appears to be a big issue — the microbiomes of people with psoriasis tend to be less varied than people without — and it's suggested that might trigger immune system issues, but science still need to do a lot more work to understand the link.

Rosacea is chronic skin redness, and scientists are unclear on how or why it forms; it seems to be partly genetic, but there's a lot of mystery. The gut, however, might be a missing piece of the puzzle. A study in the World Journal of Dermatology in 2017 noted that rosacea has been "linked to Helicobacter pylori (H. pylori) infection," an issue that can seriously disarrange the gut microbiome. They also noted that, according to a small study, people with rosacea show a greater tendency for bacteria overgrowth in their small intestines — which, when treated, tends to reduce their symptoms. Probiotics in combination with medication have been found to help some people with rosacea issues.

A lot of the science around the gut-skin axis has focused on serious skin conditions, so it's not yet clear what the relationship between gut health and milder skin issues might be. Microbiologist Professor Justin Sonnenburg told the New York Times in 2018 that it's not yet very clear how the gut and skin relate to one another, and that there's still a lack of big studies to tell us how the mechanics of the relationship work. So beware anybody making grand claims about the skin-clearing powers of a probiotic or fermented supplement. However, if you have serious skin issues, it's worth considering talking to your doctor about your gut health to see if it may help.