The Large Intestine: The Hidden Key to Health, Immunity and Longevity

Farzollah Gholizadeh and Fariba Bayrami

From resistant starch in bread and rice to Akkermansia, polyphenols and rebuilding the gut microbiome

Introduction: the large intestine is much more than an organ of elimination

The large intestine, or colon, is home to trillions of microorganisms, together called the “gut microbiome”. This living ecosystem plays an important role in digesting food, regulating immunity, metabolism, the health of the gut lining and controlling the chronic, age-related inflammation known as “inflammaging”.

One of the most important ways to support this ecosystem is to feed the beneficial bacteria properly, with fibre, resistant starches, polyphenols and suitable fats.

1. The main bacterial families of the large intestine

Most gut bacteria belong to a few main groups (phyla):

• Firmicutes (Bacillota): include butyrate-producing bacteria such as Faecalibacterium prausnitzii and some lactobacilli.
• Bacteroidetes (Bacteroidota): such as Bacteroides and Prevotella, which break down complex carbohydrates and produce acetate and propionate.
• Actinobacteria: include bifidobacteria, which ferment oligosaccharides and support immunity.
• Proteobacteria (Pseudomonadota): a disproportionate rise in some members of this group may go with inflammation and microbial imbalance.
• Verrucomicrobia: include the well-known Akkermansia muciniphila, linked to the gut’s mucus layer and to metabolic health.

The proportions of these groups differ from person to person, and gut health cannot be judged by one fixed ratio.

2. Butyrate, acetate and propionate: valuable products of gut bacteria

When gut bacteria ferment fibre and resistant starch, they produce short-chain fatty acids (SCFAs).

• Butyrate: one of the main energy sources for the cells lining the colon. It helps strengthen the mucosal barrier, regulate inflammation and keep gut cells healthy.
• Propionate: helps regulate liver metabolism, appetite and pathways linked to blood sugar and blood fats.
• Acetate: the most abundant short-chain fatty acid; it serves as an energy source and a raw material for some metabolic pathways.

Balanced production of these compounds is one reason feeding the gut bacteria matters.

3. What is resistant starch, and why does type 3 matter?

Resistant starch is the part of starch that is not fully digested in the small intestine and reaches the large intestine. There are several main types:

• RS1: starch locked inside the physical structure of seeds and fairly intact grains.
• RS2: naturally resistant starch found in green bananas and some raw starches.
• RS3: starch that regains an ordered, resistant structure after cooking and cooling.
• RS4: chemically modified starch.
• RS5: resistant structures formed when starch, especially amylose, combines with fats.

How RS3 forms

When rice, bread or potatoes are cooked, the starch gelatinises in the presence of water and heat. After cooling, some of the starch chains, especially amylose, line up again and form a more resistant structure. This process is called retrogradation.

As a result, part of the starch reaches the large intestine instead of being absorbed in the small intestine, and is fermented there by microorganisms. It becomes food for the beneficial bacteria that produce short-chain fatty acids, especially butyrate.

A practical way to increase type 3 resistant starch

Cook starchy foods thoroughly, cool them quickly and keep them in the fridge at 4 °C or below for about 12 to 24 hours. You can then eat them cold or reheated. How much RS3 forms depends on the food, its amylose content, the cooking method and the cooling conditions.

Do white bread and white rice produce resistant starch too?

Yes. Both white and brown rice, and products made from white and wholemeal flour, including some cakes, can form some RS3 after cooking and cooling.

The main difference is that whole grains contain more fibre, minerals and plant compounds alongside the resistant starch, so they are usually the better nutritional choice. The amount of RS3 depends more on the type of starch, the amylose-to-amylopectin ratio, and how it is cooked and cooled; a grain being whole does not on its own guarantee more RS3.

How to prepare common foods

• White or brown rice: after cooking, cool quickly and refrigerate at about 4 °C for 12 to 24 hours. Eat it cold or thoroughly reheated. Cooked rice should not sit at room temperature for long.
• Wholemeal or white bread: after baking and cooling, keeping it in the fridge or freezer and then toasting it can affect the amount of resistant starch. Wholemeal bread is preferable because of its higher fibre.
• Potatoes: boil or steam them in their skins, cool quickly, refrigerate for about 24 hours, then eat as a salad or reheated.
• Green bananas: naturally contain RS2. Cooking and then cooling can turn some of their starch into RS3.
• Cake: cakes made with wholemeal or white flour can also form RS3 after baking and cooling, although the amount depends on the recipe, the water, fat and sugar content, and the baking conditions.

Other food sources of resistant starch

Resistant starch, especially type 3, is not found only in bread, rice, pasta and potatoes. Many pulses, grains, starchy roots and some vegetables can also be sources. Cooking and then cooling them rearranges part of the starch and makes it more resistant to digestion in the small intestine.

• Pulses: chickpeas, green and brown lentils, white, red, black and pinto beans, broad beans and peas are good sources of resistant starch and fermentable fibre. Cooking them thoroughly and then refrigerating can increase RS3.
• Grains and pseudo-grains: pearl barley, oats, bulgur wheat, rye, millet, sorghum, corn, buckwheat and quinoa can also form some resistant starch after cooking and cooling. Whole grains have more nutritional value thanks to their fibre and plant compounds.
• Starchy roots and tubers: sweet potato, cassava, taro and green plantain can form RS3 after thorough cooking and cooling. Unripe bananas and plantains naturally contain type 2 resistant starch (RS2).
• Starchy vegetables: green peas, fresh broad beans and sweetcorn are the main options. Pumpkin and some winter squashes contain some starch but are not rich sources of resistant starch.
• Prebiotic vegetables: garlic, onion, leek, asparagus, globe artichoke and Jerusalem artichoke mainly contain inulin and fructans. Okra, broccoli and cauliflower also provide useful fibre. These are not resistant starch, but they help feed the microbiome and support gut health.

4. Akkermansia: the bacterium that guards the gut lining

Akkermansia muciniphila feeds on mucin, one of the main components of the gut’s mucus layer, and takes part in the natural cycle that renews this layer.

In many studies its presence has been linked to metabolic health, better insulin sensitivity and a healthier gut lining. However, having more of it does not on its own guarantee weight loss or a longer life.

Foods linked to supporting Akkermansia

• Pomegranate: contains punicalagins and ellagitannins, which the microbiome can turn into compounds such as urolithins.
• Green and black tea: contain catechins, EGCG and other polyphenols linked to helpful changes in the microbiome.
• Extra virgin olive oil: contains oleic acid and polyphenols such as hydroxytyrosol and oleuropein.
• Berries and dark grapes: sources of anthocyanins and phenolic compounds that gut bacteria can transform.

Citrus fruits can also help feed the microbiome thanks to their pectin, hesperidin and naringin. Whole fruit usually provides more fibre than juice.

The evidence that these foods specifically increase Akkermansia in people is not yet conclusive for all of them, and some of the findings come from laboratory and animal studies.

5. The five-step gut-rebuilding protocol (5R)

This model is a nutrition and lifestyle framework, not a proven treatment for every bowel disease.

1. Remove: cut down on added sugar, ultra-processed foods, trans fats and unnecessary use of medicines that harm the digestive tract, with your doctor’s advice.
2. Replace: improve eating habits, chew well and address real digestive problems. Bitter compounds and apple cider vinegar appear in some traditional approaches, but they are not suitable for everyone.
3. Reinoculate: eat healthy fermented foods such as kefir and sauerkraut and, if needed, probiotics with a named strain.
4. Repair: get enough protein, fibre, resistant starch and polyphenols. L-glutamine and bone broth are also suggested, but their effect on repairing the gut lining, especially bone broth’s, has not been proven for everyone.
5. Rebalance: regular sleep, physical activity, stress management and breathing exercises to support the gut–brain axis.

6. Omega-3: calming inflammation and supporting the gut barrier

Omega-3 fatty acids, especially EPA and DHA, help regulate inflammatory pathways and the production of mediators such as resolvins and protectins. Getting enough of them may help maintain the gut barrier and microbial diversity.

Key sources are sardines, mackerel, salmon and herring, as well as walnuts, chia seeds and ground flaxseed, which contain plant omega-3 (ALA).

7. Cooking oils and fats

• Extra virgin olive oil: thanks to its oleic acid and polyphenols, a good choice for daily use, salads and ordinary cooking.
• Cold-pressed sesame oil: contains sesamin and sesamol, which have been studied for antioxidant and anti-inflammatory effects.
• Virgin coconut oil: contains lauric acid and other saturated fats. Some of its components show antimicrobial activity in the laboratory.
• Black seed oil: contains thymoquinone and has a well-known place in humoral medicine. Early studies have looked at its anti-inflammatory effects.
• Flaxseed oil: a source of ALA. Use it raw, never heated, and store it in a dark bottle. Flax lignans are found mainly in whole or ground seeds, not necessarily in refined oil.
• Oils to limit: trans fats, oils reused for frying and oxidised fats. However, not all vegetable oils such as sunflower, soybean or canola are inherently harmful to the microbiome; quality, amount and how they are heated matter.

Conclusion

The health of the large intestine does not depend only on taking probiotics; it needs the beneficial microorganisms to be fed continuously.

A balanced intake of whole grains, type 3 resistant starch, vegetables, polyphenol-rich fruit, extra virgin olive oil, omega-3 sources and fermented foods, together with regular sleep and less stress, can support the health of the microbiome and the gut lining.

To feed the microbiome of the large intestine well, a varied mix of resistant starch, prebiotic fibres and plant polyphenols matters. Eating cooked and cooled pulses and grains together with vegetables such as garlic, onion, leek, artichoke and okra creates good conditions for beneficial bacteria and butyrate production.

Please note: if you have irritable bowel syndrome (IBS), increasing pulses and prebiotic vegetables suddenly can make bloating and pain worse, so increase them gradually. Blood in the stool, unexplained weight loss or a lasting change in bowel habits should always be checked by a doctor.

For a related read, see our article on intermittent fasting and temperament.

References

• 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.
• Costantini L, Molinari R, Farinon B, Merendino N. Impact of omega-3 fatty acids on the gut microbiota. Int J Mol Sci. 2017;18(12):2645.
• Cani PD, de Vos WM. Next-generation beneficial microbes: the case of Akkermansia muciniphila. Front Microbiol. 2017;8:1765.