What is the gut-skin axis?

Written by Dr. Kate Denniston 

If you’re interested in wellness or beauty, you might have heard about the gut–skin axis. Often when people talk about it, they suggest taking something for good skin. Usually it’s a probiotic — which doesn’t tend to work (more on that below).

The Interconnectedness of the Body

The whole body is far more interconnected than we tend to think. Our cells talk to each other across real distance, in highly sophisticated ways we are only beginning to understand. And when it comes to the gut–skin axis, that interconnectedness mostly comes down to immune signaling. That’s the link. About 70–80% of our immune system is located in the gut — and the gut microbiome influences the way our immune system behaves, including the way it responds in the skin. Since acne is an inflammatory condition, this matters a lot.

Here are three ways your gut affects your skin. Your inflammation levels, your metabolic health, and your hormones. All three come back to the same thing: which bacteria are thriving in there, and what you are feeding them.

1. Your gut lining helps control inflammation

A strong gut lining protects your skin from circulating inflammation. Your gut lining separates what’s inside your intestinal tract — undigested food particles and lipopolysaccharide (an inflammatory byproduct of unfavorable bacteria) — from entering your bloodstream. When these enter your bloodstream, they trigger inflammation throughout the body. And if you react to a lot of foods, that might be a sign of leaky gut.

In mice, lipopolysaccharide crossing a compromised barrier is enough on its own to drive body-wide inflammation — the work that coined the phrase “metabolic endotoxemia.”

And acne, eczema and psoriasis are all inflammatory conditions. Dermatology has classified all three as inflammatory, immune-mediated conditions for years. With acne this is more true than most people realize: when researchers biopsied follicles that looked completely normal in people with acne, the inflammation was already there — immune cells and IL-1 in the surrounding tissue before any comedone had formed. Inflammation isn’t the aftermath of a breakout. It’s the beginning of one.

2. Your gut bacteria shape your metabolic health

A good metabolism isn’t just your genes or the way you exercise. Your gut microbes do a lot of the work.

And the gut lining matters here too. Higher circulating endotoxin (from unfavorable gut bacteria) — that same lipopolysaccharide that crosses the gut barrier and triggers inflammation — predicts who goes on to develop diabetes. That’s 6,632 adults followed for nearly eleven years.

Beneficial bacteria like Akkermansia help regulate blood sugar and keep insulin in check. In a three-month randomized trial, people who took Akkermansia improved their insulin sensitivity by about 29% and lowered circulating insulin by about 34%. And insulin problems trigger acne — when young men with acne were put on a low-glycemic-load diet for twelve weeks, their lesion counts fell roughly twice as much as the control group’s, alongside a drop in free androgen index.

So when we talk about metabolic health —it’s easy to think about protein, but we also need to think about supporting the beneficial microbes that help our bodies maintain it. Akkermansia is the one with an actual human trial behind it.

The Christensenellaceae family has the most reproducible association with leanness of anything in this field — it’s strongly heritable, and consistently enriched in people at a normal weight — though nobody has ever given it to a person in a trial. Faecalibacterium prausnitzii and Roseburia are consistently lower in people with metabolic disease, which is a correlation. A lot of the research shows that polyphenols and other phytonutrients help these bacteria along.

3. Your gut bacteria regulate your hormones

When it comes to hormone balance, especially hormonal acne, it’s easy to think that taking a herb for hormones is the key. But what is actually more effective is supporting our body’s natural hormone balancing pathways. Our body was perfectly designed to balance hormones and eliminate excess ones. Bad bacteria, diet and stress can get in the way. All we have to do is remove what’s in the way of our body’s natural processes.

Good bacteria balance hormones. When we don’t have enough good bacteria, bad bacteria keep unwanted hormones recirculating through our bodies and our skin. These bacteria increase an intestinal enzyme called beta-glucuronidase — this enzyme interferes with our body’s natural hormone elimination pathways.

So what do you actually do with all this

The way to support good gut bacteria isn’t probiotics long term. Research increasingly shows that probiotics might not colonize the gut, and may only have a transitory benefit. In one study that actually scoped people rather than just testing their stool, the strains survived the journey — but the changes were transient. So the strains were detectable while people kept taking the capsules, and once they stopped, their gut went back to looking the way it did before. The bacteria were passing through rather than moving in and multiplying — so whatever benefit there was lasted about as long as the supply did. Whether they colonized at all was person-specific, region-specific and strain-specific. And it likely depended on what they ate — whether they had enough diverse fibers and phytonutrients in their diets to fuel the bacteria.

The way to sustain colonized beneficial bacteria long term is by supporting their growth through diverse fibers, and using phytonutrients to selectively shape the composition of the gut microbiome — nutrients that increase the good bacteria and simultaneously inhibit the bad ones.

Akkermansia and other beneficial bacteria grow when fed phytonutrients. We are finding out more and more that a lot of foods, good or bad, have effects on our bodies through the gut microbiome — and that even foods with “medicinal effects” like immune boosting activity may, at least in part, be acting through the microbiome and the microbiome’s effects on the rest of us.

Elderberry is the best example we have. In a 28-person trial, people taking a purified black elderberry polyphenol extract saw Akkermansia expand during supplementation — and in some of them it kept rising even after they stopped. People eating more plant fats — nuts, seeds, olive oil — saw the biggest increase.

The diversity in phytonutrients and fibers helps fuel and shape a healthy gut microbiome and a stronger gut lining. In the largest citizen-science microbiome dataset we have, people eating more than 30 plant types a week had more diverse microbiomes, more fiber-fermenting bacteria, and fewer antibiotic resistance genes than people eating fewer than ten.

What I like most about this research is that it shows us your body already knows how to regulate inflammation, handle hormones and keep a barrier intact. Most of what we can do is get out of the way, and feed our bodies and the organisms that support us.

We are early. A lot of what I’ve described here is mechanism rather than outcome. But the direction is clear enough to act on, and the actions are things that were good for you anyway.

If you want to support your gut–skin axis, the one main thing to do is eat more plants in your diet. Aim for diversity — a variety of different plants each week. Thirty is the ideal, but don’t be precious about hitting it exactly. Aim for different colored produce — at least eat all the different colors within a week (oranges, blues, greens, reds, etc). I’ve always liked Michael Pollan’s line from In Defense of Food about how simple healthy eating really is: “Eat food. Not too much. Mostly plants.”

-Kate