The 30 Plants a Week Finding

Two of the largest gut microbiome projects ever run — the American Gut Project and the British Gut Project — revealed something simple but powerful:

People who eat 30 or more different plant foods per week have the most diverse gut microbiomes and better overall health markers.

These studies analysed tens of thousands of stool samples from volunteers, comparing diet and lifestyle habits with the types of bacteria living in their intestines.

What they found

  • Diet diversity matters more than total fibre.
    Participants eating 30+ different plants each week had far more microbial diversity than those eating fewer than 10.
    In other words, variety beats volume — a small amount of many plants is better than a large amount of just one or two.

  • Plant foods feed different microbes.
    Each type of plant — whether a bean, berry, nut, herb, or grain — contains its own mix of fibres and polyphenols that specific microbes thrive on.
    The wider the menu, the stronger and more balanced your gut ecosystem becomes.

  • More “good” bacteria, fewer “bad” ones.
    Diverse diets were linked with higher levels of anti-inflammatory bacterial species and lower levels of pathogenic or opportunistic ones often associated with poor metabolic health.

Why it matters

A healthy, balanced microbiome supports:

  • Better digestion and smoother bowel movements
  • More consistent energy and metabolic balance, supported by diverse, fibre-rich foods
  • Reduced inflammation and stronger immune regulation
  • Improved long-term metabolic health, including links to insulin sensitivity in later studies

The gut plays a role in nearly every system of the body. Feeding it well means supporting your metabolism, your brain, and even your long-term weight control.

A diverse plant diet trains your gut — and your immune system — to stay calm, reducing the silent inflammation that drives weight gain and fatigue.

🧬 Mechanism in Simple Terms

Not all fibre is the same. Each plant contains its own mix of fibres, starches, and polyphenols, built from slightly different chemical structures.
Different gut microbes specialise in digesting different structures — some prefer the long, tough chains from grains or beans, others thrive on softer fruit fibres or leafy greens.

The more types of plants you eat, the more types of fibre you supply — and the more groups of microbes you can keep alive and active.

This helps explain why variety matters more than just “getting enough fibre.”
It’s diversity feeding diversity: diverse plant fibres → diverse microbes → broader metabolic benefits, from better digestion to reduced inflammation.
(Based on Armet et al., Cell Host & Microbe, 2022.)

🔎 What Each Study Shows

1. The American Gut Project (McDonald et al., mSystems 2018)

  • Analysed tens of thousands of stool samples from volunteers in the US and UK.
  • Found that participants who ate more than 30 different plant foods per week had greater gut microbial diversity than those eating fewer than 10.
  • This is observational evidence — showing a strong link between plant variety and microbiome diversity, but not direct proof of cause and effect.

2. Armet et al. (2022) – Rethinking Healthy Eating in Light of the Gut Microbiome (Cell Host & Microbe)

  • A review explaining why plant variety matters.
  • Describes how different fibre structures from different plants feed different groups of microbes, leading to broader microbial diversity and more stable metabolism.
  • Provides the mechanistic explanation behind the American Gut finding — showing how diversity in fibre types supports diversity in microbes.

Together, these studies show both sides of the story:

Eating a wider range of plants supplies a wider range of fibres and polyphenols — feeding more types of microbes and creating a more resilient, balanced gut ecosystem.

🧠 Gut Microbiome Diversity and Insulin

A growing body of research connects microbial diversity to metabolic health — including how well the body handles glucose and insulin.
While the American Gut Project did not measure insulin directly, more recent work links gut composition and diet-driven diversity with insulin resistance and glycaemic control.

Key studies:

  • Chen et al. (2021) — Association of Insulin Resistance and Type 2 Diabetes With Gut Microbial Diversity.
    JAMA Network Open, 4(8):e2115641.
    Large cohort study showing that people with lower microbial diversity had higher insulin resistance (HOMA-IR) and more type 2 diabetes.
    Read on JAMA Network

  • Shoer et al. (2023) — Impact of Dietary Interventions on Pre-Diabetic Oral and Gut Microbiome, Cardiometabolic Profile and Immune Response of the Host.
    Nature Communications, 14, 5083.
    Demonstrated that dietary changes improving microbiome composition were linked to better glycaemic and metabolic markers.
    Read on Nature

Together, these studies suggest that supporting microbiome diversity through diet may also support healthier insulin sensitivity — helping to keep metabolism and energy steadier over time.

References

  • McDonald et al. (2018). American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems, 3(3):e00031-18. Full text
  • Bowyer et al. (2019). Socioeconomic and lifestyle factors shape the gut microbiome of urban populations in the UK. Microbiome, 7: 39. Full text
  • Armet et al. (2022). Rethinking Healthy Eating in Light of the Gut Microbiome. Cell Host & Microbe, 30(6):820-837. Full text
  • Chen et al. (2021). Association of Insulin Resistance and Type 2 Diabetes With Gut Microbial Diversity. JAMA Network Open, 4(8):e2115641. Full text
  • Shoer et al. (2023). Impact of Dietary Interventions on Pre-Diabetic Oral and Gut Microbiome, Cardiometabolic Profile and Immune Response of the Host. Nature Communications, 14, 5083. Full text

In short:

The science shows a clear link between plant variety and gut diversity — and growing evidence ties gut diversity to better metabolic balance.
Aim for 30 different plants a week, and your body will thank you.