Spend any time browsing health and wellness content, and you are likely to come across the Rainbow Diet. “Eat the rainbow” has become one of nutrition’s catchiest pieces of advice. The idea is simple and appealing: fill your plate with different colours by eating a wider range of fruit, vegetables and other plant foods.
Colourful meals naturally look more inviting, can make eating more enjoyable and are certainly easier to remember than a long list of nutrition rules. Online charts often take the idea further, suggesting that red foods support the heart, orange and yellow foods benefit the eyes and immune system, green foods aid digestion, and blue or purple foods support the brain and healthy ageing. These neat colour codes are easy to follow, but this is where a helpful idea can sometimes be stretched too far. The science behind the Rainbow Diet is not quite so neatly divided by colour.
What Does “Eating the Rainbow” Really Mean?
The Rainbow Diet is not a strict meal plan. It is a visual guide that encourages us to eat plant foods of different colours. These colours come from natural plant substances, including carotenoids, anthocyanins and chlorophylls.
One large review brought together 86 studies covering 449 health outcomes and more than 37 million participants. It found that colour-related plant pigments were linked with improvements in 42% of the outcomes studied. However, 91% of the evidence ratings were classed as low or very low certainty.¹ The science therefore does not support turning the rainbow into a medical colour chart.
Although individual colours cannot be neatly matched with specific health benefits, the Rainbow Diet still encourages one important habit: eating a wider variety of plant foods. Colour is the visual cue, but dietary VARIETY is the real nutritional principle.
Beyond Colour: Why Dietary Variety Matters?
It is hardly news that different foods bring different nutritional strengths. Leafy greens can provide folate and vitamin K, dairy foods and fortified alternatives can contribute calcium, and nuts and seeds supply minerals such as magnesium. Fruit and vegetables, meanwhile, offer their own varied combinations of vitamins and natural plant compounds.
Most of us already understand that vitamins and minerals have different jobs in the body. What receives less attention is that plant foods also contain different prebiotic fibres and other fermentable dietary fibres, together with polyphenols. Their chemical shapes differ, so they interact with our gut bacteria in different ways. To understand why that matters, we first need to meet the gut microbiome.
Meet the Gut Microbiome
Our digestive tract is home to trillions of microorganisms, including bacteria, viruses and fungi. The gut microbiome is the name used for this living community and all the work it carries out inside us.
Many of these tiny residents help finish the digestion of food that our own enzymes cannot fully break down. In doing so, they make useful substances, interact with the immune system and help support the lining of the gut. Emerging evidence shows that the gut microbiome is closely linked with many areas of health, including digestion, immunity and metabolism.² Scientists often use diversity as one of the signs of a healthy gut microbiome. Not every bacterium in the gut plays the same role. Some are beneficial, some are largely harmless, and others may cause problems when they grow out of balance. In this article, supporting gut bacteria means creating conditions in which beneficial bacteria can thrive as part of a diverse, balanced gut community. In simple terms, helpful bacteria need to be plentiful, but they should also be varied. One type of bacterium cannot do every job.
For example, some friendly bacteria help break down particular fibres. Some release acetate or lactate, which other bacteria can use. Butyrate-producing bacteria make a short-chain fatty acid that provides energy for the cells lining the colon. Other microbes live close to the gut’s protective mucus layer and interact with the gut lining.
It is a little like a garden: a mixture of useful residents can perform more jobs than a single species. A varied microbial community can carry out a wider range of tasks and is less dependent on any one member.
This helps explain an interesting finding from the American Gut Project. People who reported eating more than 30 types of plants a week had a more varied mix of gut bacteria than those eating ten or fewer.³ Greater microbial variety is often linked with a healthier, more balanced and more resilient gut ecosystem.
To support a varied community of beneficial gut bacteria, we also need to give them a variety of foods. This is where dietary fibre comes in.
How Fibres and Polyphenols Shape the Gut Ecosystem
Dietary fibre is not one single ingredient. Inulin and fructo-oligosaccharides occur naturally in onions, garlic, leeks, asparagus and chicory. Oats and barley contain beta-glucan. Pulses, green bananas and cooked then cooled potatoes or rice can provide resistant starch. Apples, citrus fruit and berries contain pectin.
Some fibres are classed as prebiotics because gut microorganisms use them in a way that provides a health benefit. Other fibres can also be fermented by gut bacteria without meeting the strict definition of a prebiotic.
Different bacteria have different sets of tools for breaking these fibres down. A bacterium that works well with one type of fibre can respond differently to another. In a controlled human study, researchers gave volunteers different forms of resistant starch. Each form changed the mix and activity of their gut bacteria in its own way, including which short-chain fatty acids were produced.⁴ In other words, fibres that sound similar on a food label do not necessarily do exactly the same job inside the gut.
Polyphenols add another part to the story. They are natural plant compounds found in foods such as berries, apples, tea, cocoa, pomegranate, walnuts, grapes and red cabbage. Some also help give plants their colour or slightly bitter taste.
Unlike fermentable fibre, polyphenols do not simply act as food for gut bacteria. Instead, the relationship works in two directions. Gut bacteria break some polyphenols into smaller compounds, making them easier for the body to absorb and use. Meanwhile, the polyphenols that reach the colon, together with some of the compounds produced from them, can encourage certain beneficial bacteria to grow or become more active, while helping to keep some potentially harmful bacteria in check. Because different plants contain different types of polyphenols, eating a wider variety creates more opportunities for these interactions within the gut microbiome.
In other words, polyphenols do more than provide the raw materials for useful compounds: they can also change the make-up and activity of the gut community. Together with the different fibres found in plant foods, this means that a more varied diet gives our gut bacteria a wider selection of fibres and plant compounds to work with.⁵
Cross-Feeding: When Microbial Diversity Creates More Diversity
Within a balanced gut community, beneficial bacteria do not work alone. They share food and by-products with their neighbours. Scientists call this cross-feeding, but the idea is straightforward: what one bacterium leaves behind can become another bacterium’s meal.
For example, certain Bifidobacterium strains break down carbohydrates and release acetate and lactate. Other bacteria can use these substances to grow or to make butyrate. In a laboratory study, Bifidobacterium adolescentis helped Faecalibacterium prausnitzii produce more butyrate when the two were grown together under specific conditions.⁶
Two randomised controlled trials from The Chinese University of Hong Kong show how a focused intervention can lead to wider changes across the gut community.
In the IMPACT study, 453 older adults or people with type 2 diabetes received three Bifidobacterium strains with selected prebiotic compounds, or a placebo, for three months. The researchers found increases in bifidobacteria and butyrate-producing bacteria, along with stronger connections within the microbial community.⁷
The six-month RECOVERY trial studied 463 adults with long COVID, also known as post-acute COVID-19 syndrome. The intervention contained the same three bacterial species and selected prebiotic compounds. Bacterial diversity increased, together with several species that had not been added directly, including Roseburia intestinalis, Roseburia hominis, F. prausnitzii and Akkermansia muciniphila.⁸
These bacteria are scientifically interesting for different reasons. F. prausnitzii is an important butyrate producer, and laboratory and animal research shows how its butyrate can affect signals involved in inflammation.⁹ A. muciniphila lives in the mucus layer of the gut. In laboratory research using cells and mice, a protein it produced encouraged intestinal L cells to release GLP-1, a hormone involved in appetite and blood sugar control.¹⁰
Neither F. prausnitzii nor A. muciniphila was included in the mixture given to participants, yet both increased during the RECOVERY trial. This shows that the effect extended beyond the three Bifidobacterium species that participants consumed. The mixture also changed conditions inside the gut in ways that helped other beneficial bacteria grow. Cross-feeding is one way this can happen, as bacteria can use substances made by their neighbours. Changes in acidity, available nutrients and competition for space can also influence which bacteria thrive.
This idea is particularly relevant for bacteria that are difficult to put into everyday supplements. F. prausnitzii, for example, is extremely sensitive to oxygen, making it challenging to manufacture and deliver alive.¹¹ Helping the bacteria already living in the gut to support one another can therefore be just as important as trying to add every helpful species directly.
Bringing More Variety to the Plate
Variety deserves particular attention for people following a vegetarian or vegan diet. Once animal foods are removed, it can be easy to fall back on the same few meals or ingredients. Making fuller use of vegetables, fruit, pulses, whole grains, nuts and seeds can help create a plant-based diet that is more varied, balanced and enjoyable.
Plant foods are a treasure trove of flavours, colours and textures, but how we prepare and combine them can make all the difference. Disliking plain boiled leafy greens does not mean that those vegetables must disappear from the menu. They can be stir-fried with garlic, blended into soups, tossed through pasta, added to wraps or paired with ingredients that bring sweetness, crunch or acidity.
Think of the difference between eating the same plain boiled vegetables every day and enjoying a colourful salad made with peppery leaves, roasted vegetables, fresh fruit, chickpeas, toasted nuts, seeds, herbs and a flavoursome dressing. Both contain plant foods, but the second offers far more variety in taste, texture, fibres and natural plant compounds.
Naturally, food choices should always take account of individual medical conditions, allergies and intolerances. Beyond these needs, experimenting with different ingredients, combinations and cooking methods can turn an unnecessarily narrow diet into something healthier and much more enjoyable.
Eating the rainbow is not about giving every colour a medical superpower. It is about making room for more foods, more flavours and more ways to nourish both ourselves and the microbial community living inside us.
References
-
Blumfield M, Mayr H, de Vlieger N, Abbott K, Starck C, Fayet-Moore F, et al. Should we “eat a rainbow”? An umbrella review of the health effects of colorful bioactive pigments in fruits and vegetables. Molecules. 2022;27(13):4061. doi:10.3390/molecules27134061.
-
Valdes AM, Walter J, Segal E, Spector TD. Role of the gut microbiota in nutrition and health. BMJ. 2018;361:k2179. doi:10.1136/bmj.k2179.
-
McDonald D, Hyde E, Debelius JW, Morton JT, Gonzalez A, Ackermann G, et al. American Gut: an open platform for citizen science microbiome research. mSystems. 2018;3(3):e00031-18. doi:10.1128/mSystems.00031-18.
-
Deehan EC, Yang C, Perez-Muñoz ME, Nguyen NK, Cheng CC, Triador L, et al. Precision microbiome modulation with discrete dietary fiber structures directs short-chain fatty acid production. Cell Host Microbe. 2020;27(3):389-404.e6. doi:10.1016/j.chom.2020.01.006.
-
Cardona F, Andrés-Lacueva C, Tulipani S, Tinahones FJ, Queipo-Ortuño MI. Benefits of polyphenols on gut microbiota and implications in human health. J Nutr Biochem. 2013;24(8):1415-1422. doi:10.1016/j.jnutbio.2013.05.001.
-
Rios-Covian D, Gueimonde M, Duncan SH, Flint HJ, de los Reyes-Gavilan CG. Enhanced butyrate formation by cross-feeding between Faecalibacterium prausnitzii and Bifidobacterium adolescentis. FEMS Microbiol Lett. 2015;362(21):fnv176. doi:10.1093/femsle/fnv176.
-
Wong MCS, Zhang L, Ching JYL, Mak JWY, Huang J, Wang S, et al. Effects of gut microbiome modulation on reducing adverse health outcomes among elderly and diabetes patients during the COVID-19 pandemic: a randomised, double-blind, placebo-controlled trial (IMPACT study). Nutrients. 2023;15(8):1982. doi:10.3390/nu15081982.
-
Lau RI, Su Q, Lau ISF, Ching JYL, Wong MCS, Lau LHS, et al. A synbiotic preparation (SIM01) for post-acute COVID-19 syndrome in Hong Kong (RECOVERY): a randomised, double-blind, placebo-controlled trial. Lancet Infect Dis. 2024;24(3):256-265. doi:10.1016/S1473-3099(23)00685-0.
-
Lenoir M, Martín R, Torres-Maravilla E, Chadi S, González-Dávila P, Sokol H, et al. Butyrate mediates anti-inflammatory effects of Faecalibacterium prausnitzii in intestinal epithelial cells through Dact3. Gut Microbes. 2020;12(1):1-16. doi:10.1080/19490976.2020.1826748.
-
Yoon HS, Cho CH, Yun MS, Jang SJ, You HJ, Kim JH, et al. Akkermansia muciniphila secretes a glucagon-like peptide-1-inducing protein that improves glucose homeostasis and ameliorates metabolic disease in mice. Nat Microbiol. 2021;6(5):563-573. doi:10.1038/s41564-021-00880-5.
-
Khan MT, Dwibedi C, Sundh D, Pradhan M, Kraft JD, Caesar R, et al. Synergy and oxygen adaptation for development of next-generation probiotics. Nature. 2023;620(7973):381-385. doi:10.1038/s41586-023-06378-w.
