What shapes gut flora, and what about "30 plants a week"?

You may have heard that you should eat 30 different plants a week to have a healthy gut flora. Or that your genes decide which bacteria you have.

But what actually shapes the gut flora in an adult? Here is what the large studies show, and where the number 30 comes from.

Genes matter less than you might think

An Israeli study examined both the genome and the gut flora of 1,046 healthy people from different ethnic backgrounds. The gut flora was not significantly linked to their genetic ancestry, and genes played only a small role [1].

On the other hand, the gut flora of people who lived together was similar, even when they were not related. And over 20% of the differences between people were linked to diet, medication and body measurements [1].

In short: your environment matters more than your genes.

Medication and stool matter most

A Belgian study of 1,106 people, which was repeated in a Dutch study of 1,135, looked for everything that was linked to the gut flora. The researchers found 69 factors, and 92% of them also held up in the Dutch study [2].

Two things stood out:

Stool consistency, meaning whether it is hard or soft, had the largest effect on the gut flora.
Medication explained the most of the difference between people overall [2].

How you were born, by caesarean section or vaginally, could not be seen in the gut flora of the adults [2].

Much is still unexplained. In the Israeli study, just over a fifth of the differences between people were linked to diet, medication and body measurements [1]. And these are associations, not proof of cause. Read about what a single course of antibiotics does to the gut flora in the article on antibiotics and gut flora.

Diet changes the flora in a few days, and it reverts just as quickly

In an American trial, ten healthy volunteers ate only animal foods such as meat, eggs and cheese for five days, and in another period only plant foods [3].

The animal-based diet changed the gut flora as early as the day after the food reached the gut. Bacteria that tolerate bile increased, and bacteria that break down plant fibres decreased. But two days after the animal-based diet ended, their gut flora was back to its usual self [3].

This is an important point: the gut flora responds to what you eat now. A week with lots of greens changes it, but the change only lasts as long as the diet does.

Where does "30 plants a week" come from?

The number comes from the American Gut Project, a large citizen science project in which over 10,000 people, mostly from the USA, the UK and Australia, posted their own stool sample and filled in a questionnaire [4].

One of the questions was: "How many different plants do you eat in a typical week?" One of the answer options was "more than 30".

The researchers found that the number of different plants was more closely linked to the gut flora than whether people called themselves vegetarian, vegan or omnivore [4]. The detailed comparison included only the extremes: 41 people who answered over 30, against 44 who answered under 10. The group with over 30 plants had a more varied gut flora and fewer genes for resistance to certain antibiotics [4].

It is an interesting finding, but notice what it does not show:

30 is not a threshold the researchers found. It is an answer category in a questionnaire. The study says nothing about what happens between 10 and 30.
It is an association, not proof. People who eat many different plants probably also do many other things differently.
Nobody measured health. The study shows a more varied gut flora, not that the participants were healthier.
The diet was self-reported, and the participants signed up themselves.

"30 plants a week" is therefore a good rule of thumb for eating a varied diet. But it is not a research finding, and you have not failed if you end up at 22.

Sweeteners and additives

Two smaller randomised trials have examined individual substances found in many processed foods:

Sweeteners. 120 healthy adults were given saccharin, sucralose, aspartame or stevia for two weeks at doses below the acceptable daily limit. All four sweeteners changed the gut flora, and saccharin and sucralose worsened blood sugar control. The effect varied from person to person, and what it means for health in the longer term is not clear [5].
The emulsifier CMC (carboxymethylcellulose, E466). 16 healthy adults ate the same diet with or without CMC for 11 days. With CMC, the diversity of the gut flora fell, fewer short-chain fatty acids were formed, and the participants had slightly more stomach pain after meals [6].

Both trials are small and short. They test individual substances, not "ultra-processed food" as a whole. Whether this leads to disease in the longer term is a hypothesis, not something the trials have shown.

Exercise: works as long as you keep it up

32 people who did not exercise did cardio training three times a week for six weeks and then stopped for six weeks. The gut flora changed during the training, and in those of normal weight the amount of short-chain fatty acids in their stool rose. In the participants with severe obesity, it did not. When the training stopped, the changes largely disappeared again [7].

The trial had no control group and is small. But it points the same way as the diet trial: the gut flora follows what you do now.

Age: more personal over the years

A study of over 9,000 people from three groups found that the gut flora becomes more and more personal from mid-life onwards. In healthy people over about 80, that development continued, while it stalled in those in poorer health. Older people who still had a gut flora dominated by the bacterium Bacteroides lived shorter lives during the four years the researchers followed them [8].

This is an association. It does not show that the gut flora extends life, and it could just as well be health that shapes the gut flora.

What you can do

Eat many different plants. Vegetables, fruit, pulses, wholegrains, nuts, seeds and herbs all count. Not because 30 is a magic number, but because variety is what the studies point to. Read more about prebiotics and fibre types.
Think habits, not quick fixes. The gut flora quickly reverts when the diet or exercise stops.
Take the medication you need. Medication affects the gut flora, but that is no reason to drop treatment. Talk to your doctor if you are unsure.
If you want to try fermented food, see the article on fermentation at home.
Skip the test. If you want to measure your gut flora, first read the article on gut flora tests.

The bottom line

Environment matters more than genes. Diet, medication and who you live with are linked to the gut flora.
Diet changes the gut flora in a few days, but it reverts when the diet changes again.
"30 plants a week" is an answer option in a questionnaire, not a threshold from research. Variety is what counts.
Certain sweeteners and emulsifiers changed the gut flora in small trials. What this means for health is unclear.

Sources

Rothschild D et al. Environment dominates over host genetics in shaping human gut microbiota. Nature. 2018. PMID 29489753
Falony G et al. Population-level analysis of gut microbiome variation. Science. 2016. PMID 27126039
David LA et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014. PMID 24336217
McDonald D et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems. 2018. PMID 29795809
Suez J et al. Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell. 2022. PMID 35987213
Chassaing B et al. Randomized Controlled-Feeding Study of Dietary Emulsifier Carboxymethylcellulose Reveals Detrimental Impacts on the Gut Microbiota and Metabolome. Gastroenterology. 2022. PMID 34774538
Allen JM et al. Exercise Alters Gut Microbiota Composition and Function in Lean and Obese Humans. Med Sci Sports Exerc. 2018. PMID 29166320
Wilmanski T et al. Gut microbiome pattern reflects healthy ageing and predicts survival in humans. Nat Metab. 2021. PMID 33619379

This article is general information and not medical advice.