Why is the intake of only bran pointless?

Bran — is mainly insoluble fiber, that is, polysaccharides that the human body does not digest, but which can be partially fermented by intestinal bacteria (unlike soluble fiber, such as inulin).

They are a prebiotic, but their prebiotic effect is limited compared to more "easily digestible" food for microbes, such as resistant starch, pectin, beta-glucans, etc.

If the diet is low in other nutrients — especially proteins and fats — bran can quickly pass through the intestines without lingering and creating conditions for the growth of beneficial bacteria.

Proteins: building material and source of metabolites

When you consume protein, part of it reaches the large intestine, where it is used by the microbiota.

Some amino acids are key substrates for the growth of certain types of bacteria, for example:

• Glutamine and arginine — promote the growth of Lactobacillus and Bifidobacterium;
• Phenylalanine, tyrosine, tryptophan — are used for the synthesis of neurotransmitters (e.g., serotonin), which also regulate the microbiota.

However, with an excess of protein and a deficiency of fats and fiber, the opposite effect is possible — the growth of opportunistic pathogenic bacteria and the formation of toxic metabolites (ammonia, phenols, indoles). Therefore, fats are also needed along with protein.

Fats: regulators of motility and environment in the intestine

Fats, especially saturated and medium-chain, perform several functions:

• Reduce the speed of food transit — this gives bacteria more time to ferment fiber;
• Stimulate bile secretion — and bile acids are modified by bacteria into active metabolites that inhibit pathogens and stimulate the growth of Akkermansia and other beneficial strains;
• Create a lipid matrix — which supports the integrity of the mucous membrane and intestinal epithelium.

Why is the combination of bran + fats + protein effective?

This combination creates ideal conditions:

• Bran — provides bulk and part of the substrate for fermentation;
• Proteins — supply amino acids, stimulate the growth of symbiotic bacteria;
• Fats — slow down transit, promote bile production, and create a metabolically active environment.

Moreover, this combination mechanically and chemically activates the intestinal mucosa and immune system, stimulating the production of IgA and antimicrobial peptides, which enhances the selection of the "right" microflora.

Conclusion

Isolated consumption of bran — is only a half measure. To increase the quantity and activity of beneficial microflora, it is necessary to simultaneously:

• provide substrate (bran, soluble fiber);
• provide building materials and signals (amino acids from protein);
• ensure a favorable environment (fats, especially saturated and MCT).