Your GI tract houses trillions (yes, with a T) of microbes: bacteria, viruses, fungi, and more. The swarm of microscopic critters living in your intestines makes up the gut microbiome and influences everything from your digestion to your immune system. And it’s even tied to how much fat is in your liver.
“The gut-liver axis is important throughout the continuum of liver disease,” says Rohit Loomba, MD, chief of the gastroenterology and hepatology division at the University of California San Diego School of Medicine. “It’s clinically very evident – it’s right in our face, and we have to act upon it.”
When your microbial inhabitants are in the right balance – not too many of a given species, nor too few – it helps you stay healthy. But when one or a few species start to dominate, it can mean trouble.
For example, studies show that people with too much fat in their liver tend to have higher or lower levels of certain bacterial species living in their gut.
We’re talking here about fatty liver that’s not caused by alcohol overuse. Doctors call it “metabolic dysfunction-associated steatotic liver disease” (MASLD). Or you might hear its old name, nonalcoholic fatty liver disease (NAFLD). It’s the most common liver condition, affecting about a third of adults worldwide – and it’s rising.
This is key: Your gut and liver must be able to work as a team for your long-term health. Your behavior – what you eat, how active you are – determines whether or not they can work well together.
Read on to find out why, and what you can do – today -- to help them do their jobs.
Eavesdropping on Your Gut-Liver Conversations
Your gut and liver are next-door neighbors, constantly chatting with one another. Something that throws one organ off-kilter – a gut microbiome imbalance, for example – can wreak havoc on the other.
The portal vein, a 3-inch-long expressway linking the two organs, brings blood and nutrients from your gut and stomach to your liver. The liver then removes anything harmful before it reaches the rest of your body.
Your liver also makes bile, a sticky yellow-green fluid that helps break down fat in food. It seeps through a network of ducts from the liver into the small intestine during meals, or to the gallbladder for storage when you’re not eating. Gut microbes digest the acids in bile and turn them into lots of secondary bile acids. These secondary acids have critical responsibilities including activating important hormones and helping the small intestine absorb vitamins, Loomba explains.
That’s how it’s supposed to work. But Loomba notes that if the gut microbiome is out of whack, so is the population of secondary bile acids. This can thwart the balance of receptors in the small intestine, changing the way you absorb fat and nutrients. It can also alter the microbiome even more. The result: a vicious cycle of imbalance between the two organs. “There’s a complex interplay happening,” Loomba says.
On top of all that, when certain gut bacteria spiral out of control, they can send a flood of inflammatory molecules through the portal vein. This leads to liver inflammation and can cause tissue damage.
The Way to a Healthy Gut and Liver Is Through Your Stomach
MASLD often goes hand-in-hand with other metabolic conditions. Up to two-thirds of people with type 2 diabetes have it. So do about 75% of people who are overweight and more than 90% of people with severe obesity, according to the National Institute of Diabetes and Digestive and Kidney Diseases.
What ties all that together? Food. Everything you eat affects the balance of microbes in your gut, and in turn, your liver health.
“Whatever food we consume, it is also food for the bacteria in our gut,” Loomba says. “We are shaping our microbiome continuously with what we eat for breakfast, lunch, and dinner.”
It doesn’t take long to tip the gut barrier into unhealthy territory. “If we consume a ‘Supersize Me’ fast-food diet, even for a month, it will make the gut leaky during that time,” Loomba says. That means our intestines “leak”inflammation-causing toxins through the digestive wall into the body – not a good thing.
It may sound like fatty food is a big deal here, and it’s true that it causes lots of problems, including contributing to fatty liver. But refined sugars, like the ones that sweeten processed foods, candy, and soft drinks, are also a key culprit. “Sugars are really bad for the liver,” Loomba says.
Fructose, in particular, is an issue. A 2009 study showed that the livers of people who drank fructose-sweetened drinks for 10 weeks, their livers produced more fat. In an animal study from 2008, the same was true for mice that downed lots of fructose over two months.
The blood of those sugar-laden mice also had higher levels of endotoxins, or LPS (lipopolysaccharides), toxic molecules found on the outside of certain bacteria. Studies show that a high-fructose diet makes it easier for gut bacteria, and their endotoxins, to leak out from the intestines and into the blood. The leakage happened because the protective barrier of cells, mucus, and defensive proteins lining the small intestine was damaged. When endotoxins escape and reach the liver, they cause inflammation, fat buildup, and can even injure or kill liver cells.
Leaky gut can also be caused by too much alcohol, as well as by eating lots of fat – particularly saturated fat – over a long period of time. Researchers detailed those risk factors in the journal Hepatology Communications in 2023.
Some studies suggest that the intestines of people with MASH – short for metabolic dysfunction-associated steatohepatitis, which is a worse form of MASLD – tend to be leakier. This may directly affect the progress of fatty liver disease, including MASH.
“If this process continues, it leads to chronic fibrosis or [liver] scarring over the years,” Loomba says. “If it continues for decades unabated, it can lead to cirrhosis and future development of liver cancer.”
Though fructose is naturally found in fruit, you’d have to eat a mountain of fruit to get anywhere near the levels that were tested in those studies. Fruit also gives you nutrients including fiber, but you won’t miss out if you limit fructose in drinks or other processed foods.
Cutting down on refined sugars and alcohol can make a big difference. Obesity and type 2 diabetes can also change your microbial makeup and make the gut leaky, Loomba says.
But a poor diet can worsen MASLD even if you’re not obese. That’s what happened in a small 2014 study where healthy people were assigned to eat a typical Western diet for a month. Their blood levels of endotoxins soared by 71%. On the other hand, people on a “prudent-style” diet, with far less saturated fat and a lot more fiber, had a 38% decrease in endotoxin activity.
Although the study was too small to know if that’s a typical result and too short to see if any health problems came up, there’s no benefit to having more endotoxins drifting through your blood. Endotoxins in the blood cause a strong immune response and can injure your liver cells. Really severe endotoxemia, as it’s called, can lead to sepsis, bleeding, and can land you in a hospital bed in the ICU, Loomba says.
Fats to Favor, Avoid
The first thing doctors will tell you if you have fatty liver disease is to watch your weight and eat a healthier diet.
Part of the liver’s job is to store extra fat that the body can’t use right away. Getting too much fat and calories in your diet is a direct driver of fatty liver.
It probably comes as no surprise that a familiar dietary foe – saturated fats like those found in butter, red meat, and fast food – are the worst offenders, Loomba says. Meanwhile, “good” fats like those found in salmon, sunflower seeds, walnuts, and plant-based oils may help protect us from liver fat accumulation and inflammation. These are polyunsaturated fatty acids, or PUFAs.
Fiber, a nutritional superstar, also does some heavy lifting here. Gut microbes digest it into short chain fatty acids, or SCFAs. Research is still in early stages, but studies show that SCFAs are likely a key piece of the fatty liver puzzle. They do many things in the body, including reducing inflammation, preventing fat buildup, and maintaining your gut barrier.
Boosting microbes that make SCFAs could be one approach for improving liver disease, says Ashwani Singal, MD, a transplant hepatologist and medical professor at the University of Louisville School of Medicine.
Leverage Probiotics and Prebiotics
You can also boost SCFAs by eating more prebiotics: foods that contain sources of nutrition, including fiber, for your “good” gut bacteria in order to help them thrive (see list). Some people can’t tolerate prebiotics, because they can worsen symptoms of some inflammatory conditions. Consult your doctor before making any big changes to your diet.
There are no formal recommendations yet, but probiotics, which contain living cultures of “good” bacteria, are also a hot topic in potentially improving MASLD. Animal studies show that probiotics can help restore balance to gut microbes, strengthen the intestinal barrier, and reduce liver fat buildup.
Prebiotic and probiotic supplements are widely available, but their quality varies. So if you’re considering one, talk with your doctor about recommended brands.
Probiotics or prebiotics haven’t been shown to tame MASLD alone, Loomba says, but when used wisely, they can give your gut microbes a boost in the right direction.
Repairing the Gut-Liver Axis
If you carry extra weight, especially around your middle, and your diet isn’t especially healthy, the chances your gut-liver axis is wobbly are very high – but it’s possible to return it to firm footing.
For example, if your leaky gut is still in the early stages, just a few months of a better diet – one with more fiber and less sugar and saturated fat – can allow the gut’s protective barrier to heal itself, Loomba says. “This process is reversible – it’s not one-way,” he says. Fatty liver can also be unwound through lifestyle changes. Physical activity can be especially effective against fatty liver and also may improve gut health.
Of course, if you quit those changes, the problems will return.
“The majority of people are just waxing and waning” between periods of eating healthy or not so much, Singal says.
And when fatty liver starts to creep into its later stages, the damage can become permanent. So prevention is the best bet.
For instance, with the OK from your health professionals, add foods one by one from the prebiotic and probiotic list until they’re a regular part of your diet – and gradually subtract other foods that are high in fat and sugar but low in nutrition. You can do this over days, weeks, or months. “Having a balanced diet is critical,” Loomba says.
The key to making lasting changes is to be patient and build new habits one at a time. Make small choices that foster a healthy microbiome – your liver will thank you for it.
