30 plants a week: science or slogan?

The idea of eating 30 different plants a week has become one of the most repeated pieces of gut-health advice.

It is an appealing number because it is simple, memorable and practical.

But the science behind it is more nuanced than the slogan suggests.

Where did the “30 plants” idea come from?

The number is most commonly traced to the American Gut Project, a large citizen-science microbiome study published in mSystems in 2018.

The project analysed microbiome samples and lifestyle data from more than 10,000 participants, primarily from the United States, the United Kingdom and Australia.

One of the study's observations was that people reporting consumption of more than 30 different plant types per week had different gut microbiome profiles from those reporting 10 or fewer.

The researchers also found differences in stool metabolites between the two groups and lower abundance of several antibiotic-resistance genes among those eating the broader range of plants.

Importantly, this was an observational association. Participants were not randomly assigned to eat 10 plants or 30 plants.

So the study does not establish that 30 is an optimal threshold, nor that moving from 29 to 30 plants produces a specific biological effect.

What it does suggest is that dietary diversity may explain aspects of the gut microbiome that broad labels such as “vegan” or “omnivore” do not capture particularly well.

Why might plant diversity matter?

Different plants contain different combinations of dietary fibre, resistant starch, oligosaccharides, polyphenols, micronutrients and other compounds.

Gut microorganisms also differ in the substrates they can use.

Some organisms preferentially ferment resistant starch. Others can utilise particular oligosaccharides, pectins, gums or fibres from cereal cell walls.

Microbes can also interact through cross-feeding, where metabolites produced by one organism become substrates for another.

A varied diet therefore provides a wider chemical environment than repeatedly consuming the same small group of plant foods.

That biological logic is plausible, but it is still different from proving that a particular weekly plant count is necessary for health.

Is there experimental evidence for plant diversity?

Until recently, much of the evidence linking plant diversity with the gut microbiome was observational.

A 2026 randomised controlled trial published in the British Journal of Nutrition provides a useful newer piece of evidence.

The BIOME trial randomised 399 adults aged 35 to 65 years to one of three groups for six weeks:

• a whole-food plant blend containing more than 30 ingredients;
• a single-strain probiotic containing Lacticaseibacillus rhamnosus GG; or
• an energy-matched control food.

The plant blend contained fruits and vegetables, mushrooms, herbs, nuts, seeds, spices and whole grains, and participants consumed 30g per day while otherwise maintaining their usual diet.

Of the 349 participants included in the modified intention-to-treat analysis, the diverse plant group showed changes in the relative abundance of 57 microbial species, compared with 14 species in the control group and 4 species in the probiotic group.

The overall microbiome composition also changed significantly within the plant-blend group over the six-week intervention.

However, the trial did not show that every conventional measure of microbial diversity increased, and the researchers did not demonstrate that simply counting to 30 plants is itself the causal mechanism.

The intervention was a specific commercial blend containing more than 30 whole-food ingredients, fibre, polyphenols and micronutrients.

It is therefore better interpreted as evidence that a deliberately diverse plant intervention can meaningfully alter gut microbial composition, rather than proof of a universal “30 plants” prescription.

What happened to digestive symptoms?

The BIOME trial also measured gastrointestinal symptoms.

Participants consuming the diverse plant blend reported reductions in constipation, indigestion, heartburn, flatulence and overall gastrointestinal symptoms compared with the control group.

Approximately 69% of participants in the plant-blend group experienced a reduction in total gastrointestinal symptoms, compared with approximately 56% of the control group.

The changes were modest and the participants were generally healthy, but the findings are notable because they challenge the assumption that adding more diverse fibre-rich foods must necessarily worsen gas or digestive discomfort.

Does “30 plants” mean 30 vegetables?

No.

The American Gut work used the broader concept of plant diversity, not simply vegetables.

Depending on how a plant-counting approach is constructed, examples may include:

• vegetables;
• fruit;
• whole grains;
• legumes;
• nuts;
• seeds;
• herbs;
• spices;
• mushrooms; and
• sea vegetables.

This is one reason a plant-diversity target can be easier than it first sounds.

A bowl containing oats, chia, walnuts, raspberries and kiwi already contains five different plant foods. A Japanese-style dinner with rice, edamame, mushrooms, wakame, daikon, spinach, cucumber, spring onion and sesame can add many more.

Does every plant count equally?

Probably not.

A teaspoon of cinnamon and a cup of lentils are clearly not nutritionally equivalent simply because both can be counted as one plant.

The original research does not justify treating plant counting as a precise scoring system.

Quantity still matters. Fibre intake still matters. Overall diet quality still matters.

The value of counting plants is therefore less about achieving a perfect score and more about exposing dietary repetition.

If the same five or six plant foods appear every week, a plant count makes that visible.

Can you have a diverse diet and still eat too little fibre?

Yes.

Plant diversity and total fibre intake overlap, but they are not the same measurement.

A person could eat small amounts of many different plants and still fall short of the Australian Adequate Intake for dietary fibre.

Conversely, a person could achieve a high fibre intake from a narrow range of foods.

The more useful aim is therefore to consider both:

• quantity: are you eating enough fibre overall?
• diversity: is that fibre coming from a reasonably broad range of sources?

Read: How much fibre do you need each day? →

What about fermented foods?

Fermented foods add another dimension to the discussion.

In a 2021 Stanford randomised trial published in Cell, participants assigned to a diet high in fermented foods showed an increase in gut microbial diversity and reductions in several inflammatory markers over the intervention period.

The high-fibre group in the same trial showed different microbiome and immune effects, but did not show the same increase in microbial diversity over the study period.

This is useful because it reinforces a broader point: dietary fibre is important, but the gut ecosystem responds to more than fibre quantity alone.

Fermented foods, diverse plant compounds and different fibre structures can all influence the microbial environment in different ways.

So should you aim for 30 plants a week?

It is reasonable to use 30 plants as a practical prompt, but not as a biological rule.

The number is best understood as a behavioural tool that encourages people to broaden their diet.

Aiming for a wider range of whole grains, legumes, vegetables, fruits, nuts, seeds, herbs and spices is consistent with the broader evidence on healthy dietary patterns.

But there is no evidence that 30 is a magic threshold, and there is no reason to assume that 28 plants is meaningfully inferior to 30.

The more defensible principle is simpler:

eat enough fibre, and obtain it from a varied range of plant foods.

Where does a fibre supplement fit?

A supplement cannot reproduce the nutritional complexity of a varied diet.

Whole foods provide combinations of fibres, polyphenols, micronutrients and other plant compounds that isolated ingredients do not fully replicate.

But dietary diversity and dietary adequacy are both difficult to achieve consistently.

A multi-source fibre blend can therefore serve a narrower role: contributing additional fibre from several different sources on days when food intake falls short.

That is the principle behind MacroBlue. It is designed to complement dietary diversity, not replace it.

Read: Why seven fibres? →

Sources

• McDonald D, Hyde E, Debelius JW, et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems. 2018.
• Creedon AC, Bernard HM, Amati F, et al. Does a diverse whole-food plant-based dietary intervention improve gut microbiome composition, gut symptoms, energy and hunger in healthy adults? A randomised controlled trial. British Journal of Nutrition. 2026.
• Wastyk HC, Fragiadakis GK, Perelman D, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021.