Globally, meat intake is slowly rising. However, in some regions, plant-based, vegetarian, and vegan diets are also on the rise.
Although people switch from meat for a number of reasons, two important ones are the impacts of animal products on human health and the health of the planet.
It’s now well established that a high intake of red and processed meat is linked to poorer health. On the other hand, eating a diverse range of plants is associated with better health.
At ZOE, we know that a thriving gut microbiome is vital for good health. So, it’s important to understand how shifting to a plant-based diet influences our microbes.
However, because studying links between diet and the microbiome is challenging, there are few large-scale studies on how these different dietary patterns influence our gut bacteria.
So, we decided to investigate how following a vegan or vegetarian diet might impact your gut bacteria.
Our results were recently published in the high-impact scientific journal, Nature Microbiology.
What did we do?
We analyzed poop samples from 21,561 participants, which included:
All participants were based in the United Kingdom, United States, or Italy and provided us with detailed information about their diet.
What did we find?
First, we analyzed the quality of participants’ diets using the Healthful Plant-Based Diet Index (hPDI). Based on this index, we found that on average, vegans had the healthiest diets, followed by vegetarians, and then omnivores.
Next, we looked at their gut microbiome alpha diversity, which is one indicator of gut microbiome health. When alpha diversity is measured via species richness, it looks at how many different types of bacteria an individual has in their gut microbiome.
We found that species richness was lower in vegetarians and vegans than in omnivores. This might seem counterintuitive, but experts are moving away from using alpha diversity alone as a measure of microbiome health because it doesn’t take into account the type of microbes present.
For instance, someone may have a high number of “bad” bacteria species. So, in this case, the gut microbiome would be diverse, but not healthy.
Dietary pattern linked to gut microbiome signature
We next found that the gut microbiome composition of vegans, vegetarians, and omnivores were significantly different from each other.
The greatest difference was between non-meat eaters and meat eaters. We saw that each dietary pattern had its own microbial “signature.”
Omnivore
The omnivore signature featured more bacteria related to meat consumption and digestion, like Alistipes putredinis, which is involved in fermenting protein.
They also had higher numbers of bacteria linked to inflammation, like Ruminococcus torques and Bilophila wadsworthia, which are associated with inflammatory bowel disease (IBD).
These species may be linked to an increased risk of colon cancer and poorer cardiometabolic health.
What food groups are responsible? Our scientists found that most of the bacteria that were more common in omnivores than vegans/vegetarians were correlated with eating white or red meat.
Vegans
The vegan signature featured more species of bacteria that specialize in fermenting fiber. This makes sense as plants are rich in fiber, whereas meat contains none.
Vegans also had larger numbers of bacteria that produce a short-chain fatty acid (SCFA) called butyrate. Some “good” gut bacteria produce SCFAs as they ferment fiber in your gut.
These compounds are one of the reasons why a plant-based diet helps support your health. SCFAs nourish your gut lining, reduce inflammation, and help maintain healthy blood sugar levels.
What food groups are responsible? Most of the bacteria species that were elevated in vegans/vegetarians correlated with vegetable and fruit consumption.
Vegetarian versus vegan
The largest difference between vegetarian and vegan microbiomes was the presence of Streptococcus thermophilus in vegetarians. This bacterium occurs in fermented milk products and is used to make yogurt.
What food groups are responsible? Most of the bacteria that were present in greater amounts in vegetarians than vegans correlated with dairy consumption.
Diet quality
As mentioned earlier, diet quality (as estimated using the hPDI index) varied between groups. However, individuals with high hPDI scores following all three of the dietary patterns shared some microbiome features.
In other words, the microbiome of someone following a healthy vegan diet had similarities with an omnivore following a healthy diet.
Diet, microbiome, and health markers
The researchers next looked for links between dietary patterns and bacteria that are correlated with cardiometabolic health markers like body mass index (BMI), blood pressure, and blood fats.
In a previous ZOE study, we ranked bacteria according to how well they correlate with positive or negative health markers.
Using this ranking, the study found that omnivores had fewer species of “good” bacteria and more species of “bad” bacteria than either vegetarians or vegans. Although the differences between omnivores and vegans were much greater than between omnivores and vegetarians.
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Here are some of the other key findings from this part of the study:
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Most of the ranked bacteria that appeared in the omnivore microbiome were linked to poorer cardiometabolic markers.
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Most of the ranked bacteria that appeared in the vegan/vegetarian microbiome signatures were linked to better cardiometabolic markers. But the vegan ranked gut bacteria were significantly better than the vegetarian ranked gut bacteria.
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The bacteria that were more prevalent in the vegan signature than the vegetarian signature were mostly linked to favorable markers.
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The bacteria more prevalent in the vegetarian signature compared with the vegan signature were a bit of both — some good, and some bad.
The authors of the study state:
“Overall, we found the gut microbiome signature of an omnivore diet to be more associated with unfavorable cardiometabolic health outcomes, whilst vegan diet signatures were linked with favorable cardiometabolic health. The signatures of vegetarian gut microbiomes were linked to intermediate outcomes.”
So, according to this study, the gut microbiomes of vegans are healthier than those of vegetarians. And vegetarians’ gut microbiomes are healthier than the omnivore microbiome.
Microbes from food
In the final part of the study, ZOE’s scientists and academic collaborators investigated another fascinating aspect of microbiome science. Some of the bacteria in your gut arrive after hitching a ride in your food.
For instance, above we mentioned S. thermophilus, which makes its way into our microbiome in dairy products.
To gauge how common this transfer from food to gut microbiome is, the researchers used a database of food microbiomes — a list of individual foods and the bacteria associated with them.
The researchers found that:
This difference was mostly due to bacteria present in dairy products, which were found in omnivores and vegetarians, but not in vegans.
Bacteria associated with fruit and veg were found in all three dietary patterns, but they were more common in vegans. For instance, Enterobacter hormaechei and Citrobacter freundi are found in soil and are more prevalent in vegans.
When our researchers looked at the abundance of these food-associated bacteria (rather than the number of different species), vegetarians had the largest population and vegans the smallest.
Again, this is probably due to the range of dairy-associated bacteria in vegetarian and omnivore microbiomes.
What does it all mean?
Alice Creedon, PhD., one of the study’s authors, summarized the results:
“We showed that dietary pattern — vegan, vegetarian, and omnivore — has a strong influence on the gut microbiome and specific gut microbes that are associated with better health.”
Vegans had the highest numbers of bacteria that produce SCFAs, for instance. We also found that omnivores who ate a high-quality diet — in other words, more plants — shared more vegan signature microbes.
“In vegetarians and omnivores,” explains Alice, “greater consumption of healthy plant-based foods could potentially lead to a microbiome composition linked to better health. Further studies are needed to investigate whether this is true — watch this space.”