At the end of the small intestine, the bean has almost disappeared from view.
Its recognizable shape is gone. Much of its accessible starch has become glucose. Many of its proteins have been reduced to peptides and amino acids. Absorbed nutrients and other compounds have crossed into blood or lymph.
But digestion is not complete in the sense of leaving nothing behind.
The material approaching the colon contains plant cell walls, resistant starch, oligosaccharides, partly intact cells, and compounds that remain attached to or enclosed within those structures. It also contains water and substances contributed by the body itself.
This is not a purified ingredient called fiber.
It is a varied biological inheritance produced by the bean, the kitchen, the meal, and the digestive tract.
The governing principle of this chapter is:
What human enzymes cannot use becomes the starting material for another biological system.
The Bean’s Journey Continues: at the entrance to the colon
Imagine examining the remains of the bean just before they pass through the ileocecal valve into the large intestine.
There would be no miniature beans waiting there.
Instead, we would find a hydrated mixture of molecules and particles. Some fragments would contain recognizable plant cell walls. Some cells would still enclose starch or protein. Other structures would be torn open. Soluble carbohydrates would move in the surrounding fluid. Phenolic compounds might be free, chemically transformed, or still associated with the food matrix.
The mixture would also contain secretions and materials from the person who ate the meal.
This is the substrate presented to the microbiome.
To understand what microorganisms can make from it, we must first understand what actually arrived.
Fiber is not one substance
The word fiber is useful, but it can conceal important differences.
Dietary fiber includes carbohydrate polymers and associated substances that resist digestion in the human small intestine. In beans, much of this material comes from plant cell walls.
Those walls are not simple shells. They contain networks of cellulose, hemicelluloses, pectic polysaccharides, proteins, and other components. Their proportions, chemical linkages, physical arrangement, and accessibility vary among tissues and beans.
This diversity matters because microbial enzymes do not act on every fiber in the same way.
A polymer that dissolves or swells readily in water presents a different substrate from a tightly organized cellulose-rich structure. A carbohydrate exposed at the surface of a broken cell differs from the same carbohydrate embedded within an intact wall. Particle size, porosity, hydration, and association with protein or phenolic compounds can all influence access.
The familiar categories soluble and insoluble describe some physical behavior, but they do not fully predict microbial use or physiological effect.
Nor does the number of grams on a food label reveal the entire structure.
Two foods can contain similar measured amounts of fiber while delivering different polymers, particle structures, and associated compounds to the colon.
The microbiome does not receive a fiber number.
It receives a material.
Resistant starch is starch with a different destination
Starch is usually discussed as a source of glucose, but some starch escapes digestion in the small intestine.
This is called resistant starch.
Resistance can arise for different reasons. Starch may remain physically enclosed within intact or partly intact plant cells. Native starch granules may have structures that enzymes reach poorly. Cooked starch chains may reassociate during cooling into more enzyme-resistant arrangements. Some foods may contain chemically modified starches produced for specific uses.
These forms share a destination, not a single structure.
They reach the colon without having been fully converted to glucose.
Beans provide a particularly clear example of physical protection. Microscopy and digestion studies show that cotyledon cell walls and the protein-rich material around starch can restrict enzyme access.[1,2]
Human measurements demonstrate that this is more than a laboratory idea. In one study, healthy volunteers ate cooked white beans and researchers collected material at the end of the small intestine. An average of about one-sixth of the ingested bean starch was recovered as resistant starch, although the investigators noted limitations in the collection method.[3]
An earlier study in people with ileostomies also recovered dietary fiber and resistant starch in ileal output after bean-containing diets.[4]
The precise amount should not be treated as a universal bean value. It depends on variety, cooking, cooling, particle size, meal composition, transit, and measurement method.
The important conclusion is simpler:
Some bean starch reaches the colon because its structure or surroundings protected it from human enzymes.
Its chemical identity is still starch. Its biological opportunity has changed.
Oligosaccharides pass a human limitation
Beans also contain small carbohydrates in the raffinose family, including raffinose and stachyose.
Humans lack sufficient alpha-galactosidase in the small intestine to completely hydrolyze the bonds that characterize these compounds. They are therefore poorly digested before reaching the colon.
There, microorganisms may possess the missing enzymatic capacity.
This is one reason beans can increase gas. Microbial fermentation can generate hydrogen, carbon dioxide, and—in some people—methane, along with other products. The amount and experience vary according to the serving, preparation, habitual diet, microbial community, intestinal transit, and sensitivity to distention.
Preparation changes the dose delivered.
Raffinose-family oligosaccharides are water soluble. Soaking, discarding water, cooking, germination, and other processes can reduce their concentration, but the effect varies with method and bean.[5,6]
This creates a direct connection between kitchen and colon.
The cook helps determine how much of this microbial substrate survives to reach the community.
Intact cells carry packages into the colon
Some of the most consequential material is not a free molecule.
It is a cell.
Cooking softens the bean and allows many cotyledon cells to separate from one another without necessarily rupturing every wall. Chewing breaks more tissue, but some individual cells and cell clusters survive.
Within those structures, starch, protein, and other compounds remain partly encapsulated.
Human enzymes may penetrate and digest some of the contents while leaving other material behind. The cell arriving at the colon is therefore not identical to the plant cell that entered the mouth. It may be depleted, porous, fractured, or chemically altered.
Even so, the surviving structure organizes microbial access.
Microorganisms and their enzymes first encounter exposed surfaces. As walls are degraded, previously enclosed substrates may become available. Different resources can therefore be released at different times and places during colonic transit.
The bean does not deliver all of its remaining carbohydrate at once.
Its architecture can create a sequence.
Polyphenols travel with the matrix
Common beans contain a range of phenolic compounds, particularly in colored seed coats but also elsewhere in the seed.
Some become accessible during cooking and small-intestinal digestion. Some are absorbed and transformed by intestinal cells and the liver. Others remain associated with starch, protein, fiber, or cell-wall structures and continue toward the colon.
Laboratory digestion of rice-and-bean mixtures found that only a small fraction of the measured bean phenolics became available under the simulated small-intestinal conditions, with results depending on bean type and food mixture.[7]
Human evidence also supports more than one route. After volunteers ate cooked common beans, some phenolic metabolites appeared early, consistent with upper-intestinal absorption and transformation. Other metabolites appeared later, consistent with microbial metabolism in the colon. Responses varied considerably among individuals.[8]
This is an important correction to the common picture of a “bean antioxidant” entering the bloodstream unchanged.
The original compound may remain trapped, be absorbed and conjugated, or reach microorganisms and become a different molecule.
The colon may receive not just fiber accompanied by polyphenols, but fiber physically and chemically associated with them.
The matrix delivers several physically and chemically associated substrates together.
The body contributes to the mixture
The material entering the colon is not made entirely from the meal.
The digestive tract continuously adds to it.
Mucus lubricates and protects the intestinal surface. Epithelial cells are shed as the lining renews itself. Digestive enzymes and other proteins are secreted into the lumen. Bile acids enter the small intestine to support fat digestion; most are reabsorbed, but a fraction continues onward. Water, electrolytes, microbial cells from higher regions of the tract, and other endogenous materials also arrive.
The colon therefore receives two inheritances:
- what human digestion left from the bean and the rest of the meal; and
- what the host added during digestion and tissue renewal.
This distinction matters because colonic microorganisms can use host-derived substrates as well as dietary ones.
When fermentable dietary material is scarce, some organisms may rely more heavily on mucus-associated carbohydrates or proteins reaching the colon. When bean-derived substrates are abundant, they change the competitive environment.
The microbiome does not interact with diet in isolation from the host.
It lives at the meeting point between them.
Water determines the environment
Water surrounds the remaining bean material and gives the colon its working medium.
Some fiber holds water within or around its structure. Soluble polymers may increase viscosity. Insoluble particles can contribute bulk. As the contents move through the colon, water and electrolytes are absorbed, and the material becomes progressively more concentrated.
Transit time affects the opportunity for microbial transformation.
Rapid movement may carry substrates onward before extensive use. Slow movement allows more time for microbial metabolism and water removal, but it can also change which substrates remain available in later parts of the colon.
The same gram of carbohydrate does not encounter one unchanging fermentation chamber.
It moves through a changing physical and chemical landscape.
Location and time are part of the exposure.
Preparation determines the inheritance
The colon receives the consequences of earlier choices.
Consider several paths:
- A bean cooked until tender but kept largely intact may preserve more cellular encapsulation.
- Mashing disrupts more cells and can increase small-intestinal enzyme access.
- Fine milling may expose starch rapidly, leaving less physically protected material for the colon.
- Cooling can allow some cooked starch to retrograde and become more resistant.
- Soaking and draining may reduce raffinose-family oligosaccharides and some other soluble compounds.
- Retaining the cooking liquid preserves material that moved out of the seed.
- Germination or fermentation changes the bean through plant or microbial metabolism before it is eaten.
These are not instructions for finding one perfect preparation.
They demonstrate causation.
The physical and chemical form created in the kitchen helps determine the substrates delivered to the microbiome.
Processing is therefore part of the Food → Microbiome pathway, even though the microorganisms have not yet entered the story directly.
“Undigested” does not mean “unused”
Human nutrition was once described mainly in terms of what crossed the small-intestinal wall.
From that perspective, material remaining in the lumen could appear to be nutritional residue—what the body failed to obtain.
That description is incomplete.
The human genome does not encode enzymes capable of dismantling every carbohydrate structure in a bean. Microbial genomes add many biochemical capabilities. Material unavailable to human enzymes may therefore become available to organisms in the colon.
Some fiber is fermented extensively. Some is used slowly. Some contributes mainly to stool structure and transit. Resistant starch and oligosaccharides can become microbial substrates. Associated polyphenols can be released and transformed. Nitrogen-containing material can also enter microbial metabolism.
None of this means that everything reaching the colon produces a beneficial effect.
It means that not absorbed in the small intestine is not the end of the Evidence Pathway.
The biological recipient has changed.
The Bean’s Journey: digestion passes the inheritance onward
The bean began as a seed assembled by a plant.
The kitchen hydrated and heated it. The mouth fractured it. The stomach mixed it. Pancreatic and intestinal enzymes dismantled accessible starch and protein. Transporters moved selected products across the intestinal wall.
What remains reflects every one of those events.
The colon now receives:
- diverse plant-cell-wall polysaccharides;
- resistant starch;
- raffinose-family oligosaccharides;
- intact and partly intact cells;
- residual protein and nitrogen-containing material;
- polyphenols associated with the food matrix;
- water and minerals;
- bile-acid derivatives;
- mucus, shed cells, and other host material; and
- compounds contributed by the rest of the meal.
This is not the original bean.
It is the bean after human digestion has selected, transformed, absorbed, and passed onward.
What the evidence can establish at this stage
Researchers can estimate or measure what reaches the end of the small intestine using laboratory digestion models, microscopy, chemical analysis, intubation studies, and samples from people with ileostomies.
These methods can establish:
- that particular structures resist upper-gut digestion;
- how much starch, fiber, or another component survives under defined conditions;
- whether compounds remain free, enclosed, or associated with the matrix;
- how preparation changes the material delivered; and
- how people differ in the amount and timing of that delivery.
But arrival in the colon is not a health outcome.
Calling a carbohydrate fermentable or prebiotic requires evidence beyond its presence in the bean. Researchers must show that microorganisms use it under relevant conditions and determine what changes result. Even then, a microbial or metabolite change does not automatically establish a clinical benefit.
This chapter identifies the input.
The next chapters must follow the transformation.
The Bean at This Stage
The remaining bean is crossing into the colon.
It is a mixture of structures rather than a single nutrient. Some components are soluble and immediately accessible. Others remain protected within plant cells. Some will be used rapidly by microorganisms. Some will become available only after other structures are dismantled. Some may pass through with limited fermentation.
The host has added mucus, bile-acid derivatives, shed cells, and other materials to the same environment.
Human digestion has finished its leading role.
A community with different genes and enzymes is about to receive the inheritance.
Next — The Microbiome: A Community Receives the Bean
The colon is not an empty vessel waiting to ferment fiber.
It is an ecosystem populated by organisms that compete, cooperate, exchange metabolites, and respond to the resources that arrive.
The same bean material can be transformed differently by different communities.
To understand what the bean becomes next, we must meet the organisms that receive it.
Notes and selected references
- Rovalino-Córdova AM, Fogliano V, Capuano E. A closer look to cell structural barriers affecting starch digestibility in beans. Carbohydrate Polymers. 2018;181:994–1002. doi:10.1016/j.carbpol.2017.11.050.
- Staes E, Duijsens D, Daems R, Mikhalski M, Van Loey A, Grauwet T. Dose-dependent impact of intact cell fraction on in vitro starch digestion of common bean-based flour blends. Food Chemistry. 2025;472:142901. doi:10.1016/j.foodchem.2025.142901.
- Noah L, Guillon F, Bouchet B, et al. Digestion of carbohydrate from white beans (Phaseolus vulgaris L.) in healthy humans. Journal of Nutrition. 1998;128(6):977–985. doi:10.1093/jn/128.6.977.
- Schweizer TF, Andersson H, Langkilde AM, Reimann S, Torsdottir I. Nutrients excreted in ileostomy effluents after consumption of mixed diets with beans or potatoes. II. Starch, dietary fibre and sugars. European Journal of Clinical Nutrition. 1990;44(8):567–575. PMID:2170104.
- Siva N, Thavarajah P, Thavarajah D. Prebiotic carbohydrate concentrations of common bean and chickpea change during cooking, cooling, and reheating. Journal of Food Science. 2020;85(4):980–988. doi:10.1111/1750-3841.15066.
- Helbig E, Oliveira AC, Queiroz KS, Reis SMPM. Effect of soaking prior to cooking on the levels of phytate and tannin of the common bean (Phaseolus vulgaris L.) and the protein value. Journal of Nutritional Science and Vitaminology. 2003;49(2):81–86. doi:10.3177/jnsv.49.81.
- Lindemann IS, Dittgen CL, Batista CS, et al. Rice and common bean blends: effect of cooking on in vitro starch digestibility and phenolics profile. Food Chemistry. 2021;340:127908. doi:10.1016/j.foodchem.2020.127908.
- Mecha E, Feliciano RP, Rodriguez-Mateos A, et al. Human bioavailability of phenolic compounds found in common beans: the use of high-resolution MS to evaluate inter-individual variability. British Journal of Nutrition. 2020;123(3):273–292. doi:10.1017/S0007114519002836.