Much of the advice about gut health focuses on probiotics, fermented drinks, or individual “superfoods.” The evidence supports a simpler idea: a wider variety of plant foods is associated with a more diverse gut microbiome. What matters is not one particular food, but how many different plants appear across the week.
The 30-plants finding#
The American Gut Project examined the diets and gut bacteria of thousands of people. Those eating 30 or more different plants a week had noticeably more diverse gut microbiomes than those eating fewer than ten. The association held whether or not participants identified as vegetarian (McDonald 2018). The important factor was the number of different plant foods.
Why a number of plants rather than a list of “good” foods? Because your gut isn’t one organism — it’s hundreds of species, and different plants feed different ones. The fibres in oats aren’t the fibres in beans; the polyphenols in berries aren’t the ones in herbs. A narrow rotation feeds a narrow set of microbes and leaves the same gaps open week after week. Mainstream nutrition guidance keeps landing on the same additive shape — plenty of plants, a wide base, variety over perfection (World 2020) — and higher fruit-and-vegetable intake tracks with lower long-term risk across populations, not just a healthier gut (Aune 2017).
The useful reframe: the question is rarely what to cut, but what one new plant to add.
Are all foods equal? The nutrient-density angle#
Breadth comes first — but not every plant pulls the same weight. Some foods deliver far more nutrition per bite than others, and it helps to know which when you’re deciding what to add next.
Calk puts a small number on each food: a nutrient-density score. For each food it adds up how much of a day’s reference intake it brings across two dozen nutrients — vitamins, minerals, omega-3, fibre — but caps each nutrient at 100%, so a food that’s broadly rich beats one that merely spikes on a single thing. Higher number, more nutrition per bite. A few examples:
A low number isn’t a knock on the food. Cucumber is low-calorie, hydrating, and genuinely worth eating — the same goes for lettuce, celery, and watermelon. They just don’t carry the dense payload a “superfood” does. The score rates nutrients per bite, not whether a food belongs on your plate: you want both kinds — the rich ones for coverage, the light ones for volume, water, and freshness.
So the move is two-step: go wider first (more different plants — that’s what your gut responds to), then lean richer where you can (favour the higher numbers). A spinach at 62 returns far more than a cucumber at 10 for the same place on your plate — but both still count toward your 30.
How to actually hit 30#
It’s easier than it sounds, because the small things count:
- Herbs and spices each count. Seasoning generously is the cheapest way to climb the number — a mixed-herb dressing can add three or four plants on its own.
- Mix, don’t single. A salad with five vegetables, a stir-fry with several, a trail mix of assorted nuts and seeds — each meal can carry a handful of distinct plants.
- Rotate by colour. Green, orange, red, purple — different pigments mean different nutrients and different microbes fed.
- Frozen and dried count. Frozen vegetables and tinned beans are nutritionally on par and keep the variety up when fresh isn’t convenient.
See your own week#
You don’t have to count by hand. Your Calk Nutrition Report tallies the distinct plants and food groups across your month and shows the emptiest groups as the easiest, most pleasant things to add — laid out as a Variety Map, a periodic table of the foods you actually eat. The deeper indicator reference lives in Variety, Produce & Plants.