If your New Year's resolution is to adopt a healthful diet, you shouldn't expect to see immediate effects; your gut microbes might not let you.

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A Western diet may alter gut microbes in a way that weakens the response to a plant-based diet.

In a new study, researchers found that the diversity of gut microbiota - the population of microbes that reside in the digestive tracts - is altered by a typical Western diet.

Furthermore, the study reveals that a gut microbiome that has been conditioned by a Western diet may weaken the effects of a healthful, calorie-restricted, and plant-based diet.

Senior study author Jeffrey Gordon, director of the Center for Genome Sciences and Systems Biology at Washington University in St. Louis (WUSTL), and colleagues publish their findings in the journal Cell Host & Microbe.

A Western diet - sometimes referred to as the "American standard diet" - is typically defined as one that is low in fruits, vegetables, seafood, poultry, and whole grains, but high in red meats, carbohydrates, saturated fats, sugar, and processed foods.

Studies have shown that the Western diet is a key contributor to obesity, and it can also raise the risk of heart disease, cancer, and various other diseases.

In order to reduce the risk of such conditions, a plant-based diet - that is, high in vegetables, fruits, whole grains, and legumes, but low in animal products - is considered one of the best options.

According to the new research, however, individuals wanting to switch from a Western diet to a plant-based diet are unlikely to reap the benefits straight away.

For their study, Gordon and colleagues collected fecal samples from individuals who followed either an unrestricted Western diet or a plant-based, calorie-restricted diet.

On analyzing the fecal samples, the researchers identified a greater diversity of microbes in the samples from people who followed the plant-based diet.

Western diet-conditioned microbes weaken response to plant-based diet

Next, the team introduced the human gut microbe communities conditioned by each diet to germ-free mice - mice that are absent of all microbes. The rodents were then fed either the diet that their human donors followed or the alternative diet.

The team found that both groups of mice responded to the new diets. However, mice with a gut microbiota that had been conditioned by the Western diet demonstrated a weaker response to the plant-based diet.

The researchers then placed two different groups of mice together in a cage. Both groups of mice had a microbiota derived from humans. One group had a microbiota conditioned by a Western diet, whereas the other group had a microbiota conditioned by a plant-based diet.

The team found that the gut microbes conditioned by a plant-based diet soon crept into the Western diet-conditioned microbiota; these plant-conditioned microbes significantly increased the Western diet-conditioned microbiota's response to a plant-based diet.

"We need to think of our gut microbial communities not as isolated islands but as parts of an archipelago where bacteria can move from island to island. We call this archipelago a metacommunity," notes first author Nicholas Griffin, an instructor at WUSTL.

"Many of these bacteria that migrated into the American diet-conditioned microbiota were initially absent in many people consuming this non-restricted diet," he adds.

The researchers are hopeful that their results will help to identify ways to improve people's responses to healthful diets, but they caution that further studies are warranted to pinpoint the mechanisms involved in microbial exchange.

"We have an increasing appreciation for how nutritional value and the effects of diets are impacted by a consumer's microbiota. We hope that microbes identified using approaches such as those described in this study may one day be used as next-generation probiotics.

Our microbes provide another way of underscoring how we humans are connected [...] to one another as members of a larger community."

Jeffrey Gordon

Read how gut microbes switch host genes on and off in response to diet.