What Effects Do Carbohydrate Foods Have on Beneficial Gut Bacteria?

Effects of Carbohydrates on the Microbiota

Simple carbohydrates are completely absorbed in the small intestine, whereas, because of their resistance to the enzymatic activity of human digestive enzymes, complex carbohydratesare not completely digested in the upper gastrointestinal tract.

Intake of complex carbohydrates, called fiber, is associated in both rodents and humans with various beneficial effects in conditions including obesity, inflammatory bowel disease, and asthma, through the microbiota and its various beneficial effects .

What Is Sugar / Carbohydrate?

Carbohydrates, sugar compounds, are among the most important organic nutrients for humans and animals, alongside fats and proteins. Their main purpose is to provide energy. Sugars are the most abundant organic compounds in our food and consist of carbon, oxygen, and hydrogen. Depending on the numbers and arrangement of these three elements and how humans can use them, sugars have different structures and names.

The figure above shows the building blocks of carbohydrates. Their smallest units are monosaccharides (glucose, fructose, and galactose). Their combinations form disaccharides (sucrose, maltose, lactose) and polysaccharides (starch, glycogen, cellulose, chitin).

Sugars:

1. Monosaccharides (glucose, fructose, galactose)
2. Disaccharides (sucrose, maltose, lactose)
3. Polysaccharides (starch, non-starch polysaccharides)
4. Sugar alcohols (sorbitol, maltitol, lactitol) and others form the various groups

Starchis found mainly in legumes, bread, potatoes, and pasta and is the main source of energy obtained from food.

Complex carbohydrates—for example, digestible starch, resistant starch and cellulose—pass through the small intestine and serve the colonic microbiota (beneficial gut bacteria) as substrates, biological raw materials. These compounds undergo a process carried out by several gut bacterial species, producing metabolites such as short-chain fatty acids (KZYA), CO2, and H2 that shape the intestinal environment: fermentation .

What Is Resistant Starch?

Starch is a digestible polysaccharide, but digestion-resistant starches also exist. Resistant starch can be defined as starch that is not absorbed in the small intestine of healthy people. It may occur naturally in foods, such as legumes, be added later as an ingredient, or be increased through appropriate food-processing methods. Some processed foods we consume daily, such as bread, pasta, and breakfast cereals, contain very little resistant starch. Food engineers and product developers are working to increase the resistant starch content of foods.

Starches that resist digestion reach the colon, where they are fermented by the gut microbiota and produce various products containing short-chain fatty acids (KZYA), which can provide physiological benefits. Several factors can affect a carbohydrate’s resistant starch content, including starch granule morphology, the amylose-to-amylopectin ratio, and its relationship with other food components. One current area of interest concerning resistant starch is its potential as a nondigestible food component benefiting the host by stimulating the growth or activity of one or a limited number of beneficial bacteria in the colon, that is, as a prebiotic . To be classified as a prebiotic, a resistant starch must meet three criteria: resistance to the upper gastrointestinal environment, fermentation by the gut microbiota, and selective stimulation of beneficial bacterial growth and/or activity.

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- Related Topic: Why and how are foods and animals modified?

Consumption of a fiber-rich plant-based diet is associated with increased bacterial genera Roseburia, Lachnospira, and Prevotella and increased KZYA production.

The abundance of gut microbiota and metabolites associated with fiber intake may independently affect disease outcomes. However, fiber also has effects independent of bowel movements, such as adding fecal bulk.

- Related Topic: How much dietary fiber should we consume?

Not All Fibers Affect the Microbiota Equally

Certain fiber types serve as substrates / raw materials for particular gut bacteria, and some biotransformed products can serve as substrates for other bacteria in the community.

Because most members of the gut microbiota tend to metabolize mono- and oligosaccharides, simple sugars, and because bacteria differ in their carbohydrate-processing abilities, cross-feeding between bacteria is essential for optimal digestion of complex polysaccharides.

Cross-feeding between bacteria leads to the development of distinct bacterial community structures depending on the types of fiber consumed. A change in the intake of fiber types can therefore produce complex changes in overall structure and function.

Gut bacteria differ greatly in their numbers of polysaccharide-degrading enzymes. Some bacteria are considered generalists and can therefore metabolize a wide variety of carbohydrates, whereas others are highly specialized. Bacteroides thetaiotaomicron and Bacteroides intestinalis are among the many members of the Bacteroidetes phylum that are putative generalists.

Other members of the gut microbiota, such as Prevotella copri and Rosaburia intestinalis are putative specialists.

A study examined fiber intake and bacterial diversity. Human feces were incubated with 15 nondigestible carbohydrates. It showed that not only substrate type but also bacterial interactions are important in producing different KZYA types. We know that the main KZYA produced—acetate, propionate, and butyrate—have important effects on gut microbiota and health (see: The important role of gut microbiota in nutrition and health).

Without Certain Keystone Bacteria, Digestion Becomes Deficient

Researchers suggested that the absence of some keystone bacteria could reduce nondigestible carbohydrate fermentation. They also reported that the environment's initial pH could control butyrate production by affecting the activity of transferases involved in butyrate formation.

These findings illustrate the complexity of understanding the role of fiber types in shaping gut microbiota composition and KZYA production.

In one study, 19 healthy obese male volunteers received a diet low in carbohydrates / fiber. Examination of their feces containing gut microbiota showed decreased numbers of Roseburia spp, Eubacterium rectale, and Bifidobacteria, with simultaneous reductions in KZYA and butyrate levels.

Changes in fiber intake may have intergenerational effects on gut microbiota composition. For example, a positive association was observed between fiber intake and microbiota diversity and richness in overweight pregnant women. A mouse study found that low intake of nondigestible carbohydrates reduced microbiota diversity. This condition persisted for 4 generations and was observed to be irreversible even after nondigestible carbohydrate intake increased.

Conclusion

Considering the effects of environmental factors on offspring helps us better understand why the typical Western diet, low in fiber, increases the long-term incidence of noncommunicable diseases.

The conclusion from this article is that fiber intake is essential for the development and maintenance of the gut microbiota. Fiber serves as a substrate for bacterial survival and proliferation. Beneficial bacteria thereby increase in number and diversity.

We have also understood the importance of consuming digestible substances such as starch and nondigestible substances such as fiber, both complex carbohydrates . Fiber intake should be diversified, meaning it should come from various sources rather than a single food. This allows different bacteria to persist and interactions between different bacteria to produce substances beneficial to health.