The 40 Foods Challenge: What It Is and Why It Works

Most of us think we eat a varied diet. We have our favourite fruits, a few go-to vegetables, and we rotate through the same handful of meals week after week. But research suggests the average person eats fewer than 20 different foods in any given week — and that lack of variety has real consequences for the trillions of microbes that live in our gut.

The 40 Foods Challenge is a simple weekly habit: try to eat 40 different whole foods before Sunday. That's it. No calorie counting, no meal plans, no restriction. Just variety.

Where the idea comes from

Research into traditional hunter-gatherer communities — including the Hadza of Tanzania, one of the most-studied populations in gut microbiome science — suggests our ancestors regularly ate several hundred different whole foods over the course of a year. Most people in Western countries today rotate through roughly the same 20–30 foods week after week. The 40 Foods Challenge is about closing that gap.

The scientific case for food diversity has been building for over a decade. Professor Tim Spector, gut microbiome researcher at King's College London and co-founder of the nutrition company ZOE, was one of the first scientists to connect the dots between what we eat and the richness of our gut microbial community.

His large-scale research — including data from the American Gut Project involving thousands of participants — found something that changed the way many nutritionists think about diet:

It wasn't the specific diet that mattered most. It was the diversity. Other researchers have arrived at similar conclusions from different angles — NHS doctor Dr Rupy Aujla recommends focusing on plant diversity above almost any other dietary change, while Dr Michael Greger's Daily Dozen framework identifies 12 key food categories to eat every day. The common thread: eat from as many different whole food types as possible, as often as possible.

Why 40, not just 30?

To be clear: Tim Spector's research specifically supports 30 plant foods as the threshold where gut microbiome benefits become significant. There's no peer-reviewed study that compared 30 with 40 and declared 40 the winner. So why does the challenge use 40?

Two reasons. First, aiming higher means you're more likely to consistently clear the 30-plant threshold — even on a busy week. If you aim for 30 and fall a bit short, you might land on 22. Aim for 40 and a difficult week still leaves you in a good place.

Second, the 40 Foods Challenge counts all whole foods — not just plants. Fermented foods like kimchi, kefir, and miso bring their own microbial benefits. Minimally processed animal products contribute nutrients that plants alone can't always provide. Herbs and spices — often overlooked — are among the most polyphenol-rich foods on the planet. The 40 target reflects this broader scope.

Think of 30 plants as the science-backed baseline and 40 whole foods as the challenge that gets you well above it.

What actually counts

The challenge is designed to be generous, not restrictive. Here's what counts toward your 40:

• Fruits and vegetables — fresh, frozen, dried, or canned (no added sugar or preservatives)
• Whole grains and legumes — oats, brown rice, lentils, chickpeas, black beans
• Nuts and seeds — almonds, walnuts, chia seeds, sunflower seeds
• Herbs and spices — basil, turmeric, cinnamon, oregano — each one counts
• Fermented foods — kimchi, kefir, kombucha, miso, natural yogurt, sauerkraut
• Minimally processed animal products — eggs, fish, meat, cheese (up to about 10 per week)

What doesn't count: ultra-processed foods, store-bought smoothies, flavoured yogurts with added sugar, or anything with a long list of artificial additives.

The colour dimension

The app also tracks your food across 7 colour groups — red, orange, yellow, green, purple/black, white, and brown. This isn't just visual. Different colours in food indicate different families of plant chemicals called polyphenols, and different polyphenols feed different species of gut bacteria.

Professor Spector's research noted that polyphenol content can vary tenfold between differently coloured vegetables of the same type — which is why eating red, yellow, and orange capsicum is nutritionally different from eating three red ones. Variety of colour means variety of benefit.

How to actually do it

The 40 Foods Challenge resets every Monday. Your slate is wiped clean and you start fresh. This weekly rhythm keeps the habit from feeling overwhelming — you don't need to eat 40 different foods every day, just 40 different ones across the week.

A few practical tips that make hitting 40 much easier:

• Count herbs and spices individually — adding turmeric, cumin, and coriander to one dish gives you three foods at once
• Rotate your grains — have oats for breakfast one day, rye bread another, brown rice at dinner
• Go seasonal — seasonal produce naturally encourages variety week to week
• Think mixed bags — a handful of mixed nuts or a bag of mixed salad leaves can add 5–6 foods in one go
• Try one new food each week — just one unfamiliar ingredient keeps the habit fresh

The free 40 Foods Challenge app makes tracking effortless — browse 300+ foods by category, tap to log, and watch your weekly progress and colour diversity fill up in real time.