New research finds high-fat diets may change the makeup of people’s gut bacteria.

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New research finds Western diets can affect the gut microbiome. Getty Images

A high-fat diet changes the bacterial communities in the gut and increases biomarkers of inflammation.

The typical “Westernized” diet of processed and fast foods — high in fat and added sugars — has been linked to many health problems, including obesity, type 2 diabetes, and heart disease.

New research shows that some of the health effects of these foods depend on how they interact with your gut microbiome. This is the community of bacteria and other microorganisms that live in the intestines.

One recent study published this month in the Gut medical journal found that a high-fat diet is linked to “unfavorable” changes in the communities of bacteria in the gut, with possible negative effects on health.

This is relevant for developing countries moving from a traditional high-carb, low-fat diet to a more Westernized, high-fat diet and to countries like the United States that are already there, write the authors of the study.

The study also raises questions about the long-term effects of higher-fat diets like paleo and keto on the gut microbiome.

In the new study, researchers assigned 217 healthy 18- to 35-year-olds to one of three diets for six months:

  • lower fat with 20 percent of calories from fat
  • moderate fat with 30 percent of calories from fat
  • higher fat with 40 percent of calories from fat

The diets for all three groups included the same amount of protein and dietary fiber.

In comparison, the ketogenic diet is 60 to 75 percent fat, while the paleo diet is about 40 percent fat.

At the start and end of the study, researchers collected fecal samples in order to analyze the participants’ gut microbiome. They also measured inflammatory biomarkers in the blood at both times.

After six months, the gut microbiome of people on a high-fat diet had shifted in ways that could have a negative impact on their health.

One group of bacteria decreased in people eating a high-fat diet and increased in those on a low-fat diet.

This group includes beneficial bacteria that produce short-chain fatty acids. These molecules help regulate inflammation in the body and protect the cells that line the intestines.

Another group of bacteria increased in the high-fat group. This group includes bacteria that show up in higher amounts in people with type 2 diabetes, compared to those with normal blood sugar levels.

People on a high-fat diet also had an increase in certain molecules linked to inflammation.

Tiffany Weir, PhD, an associate professor of food science and human nutrition at Colorado State University, who wasn’t involved in the study, says the findings aren’t surprising.

When you reduce the amount of carbohydrates in the diet, “the organisms that are best able to use those for energy will suffer, and as a consequence, fewer metabolites like the short-chain fatty acids will be produced,” she said.

The study has some limitations. One is that researchers included only young, healthy adults. A high-fat diet may affect the microbiomes of other people differently.

Also, all three groups lost weight during the study, with people on the low-fat diet losing the most. The weight loss could have positively affected the microbiome or reduced inflammation levels.

Weir says it will be interesting to see if the benefits of the faster weight loss while on a high-fat, low-carb diet outweigh the possible negative changes to the gut microbiome.

Previous studies have found a link between the gut microbiome and diet, including with high-fat diets like keto.

Some research shows that keto has benefits for conditions like epilepsy or type 2 diabetes. But this diet may also reduce the diversity of the gut microbiome, at least over the short term.

Higher gut microbiome diversity — greater numbers and types of bacteria — is thought to have positive effects on health.

So, the keto diet may not be beneficial over the long run, unless for a specific health condition.

“The ketogenic diet was really designed for specific clinical applications,” Weir said. “So, to strictly follow a self-prescribed ketogenic diet without medical reasons or oversight seems like it may backfire by introducing new issues rooted in the gut.”

Paleo is another popular higher-fat diet. Little direct research, though, has been done looking at its effects on the gut microbiome.

But studies of the Hadza tribe, modern-day hunter-gatherers, show that eating a natural “paleo” diet is linked to higher gut diversity, compared to people living in Western countries.

The Hadza also don’t experience obesity, type 2 diabetes, or other metabolic diseases of the developed world.

This suggests that eating a paleo diet may have beneficial effects on the gut microbiome.

Other lifestyle and diet factors, though, are likely involved in keeping the Hadza healthy.

“The paleo diet, if followed properly, is neither high fat nor low carbohydrate,” Weir said. “True paleo diets should include lots of vegetables and fruits that are high in the fibers needed to maintain a healthy microbiota.”

Other research supports the need for this kind of variety in the diet. A recent study in lupus-prone mice showed that resistant starch — which is fermented in the intestines — has beneficial effects on the gut microbiome.

Resistant starch works by promoting the growth of certain bacteria that produce short-chain fatty acids, which in turn suppresses “bad” bacteria.

The author of that study, Dr. Martin Kriegel, an assistant professor of immunobiology and rheumatology at Yale School of Medicine, cautioned that diet can affect people’s microbiome differently, depending on genetic and other factors.

“The microbiome is certainly about personalized medicine,” Kriegel said. “We normally view a disease as a single entity, but driven by different components of the microbiome or different genetics, there are really subgroups.”