Two papers on whole grains, with the first a 2021 review:

“Whole grains are more complex than refined grains and are promoted as part of a healthy and sustainable diet, mainly because the contribution of indigestible carbohydrates, and their co-passenger nutrients, is significantly higher. Changing composition and availability of grain carbohydrates and phytochemicals during processing may positively affect gut microbiota and improve health.

Processing is required for virtually all cereals that humans consume. However, eliminating bran has resulted in grain-based products that contribute to a lower-quality diet.

Currently, there are no specific recommendations on relative proportions of different dietary fiber types (based on variability in fermentability or degree of solubility). Switching from refined grain to whole grain will deliver more dietary fiber and nutrients associated with bran and germ, and improve diet quality.

Carbohydrate-rich foods that are higher in slowly digested starches, resistant starch, oligosaccharides with prebiotic potential, and dietary fiber are considered to have a higher quality. Foods can be awarded an overall carbohydrate quality index (CQI). The optimum ratio of total carbohydrate (CHO) to dietary fiber should be ≤10:1.

Mostly only oligosaccharides and polysaccharides reach the colon. Even though larger molecules were fermented slower, they were still fermented within the proximal colon.

It is not surprising that there are conflicting reports with respect to effects of whole grains on gut microbiota. Part of this is due to whole grains comprising a diverse group of staple cereal foods, including wheat, corn, rice, oats, barley and rye, and hence different effects on gut microbiota are expected. Differences in study design, with respect to dose, duration, and study populations make it difficult to compare between studies and distill overarching similarities.

Enzymes can modify less fermentable dietary fiber to improve its fermentability by microbiota. Using different enzymes, dietary fibers can contribute to fermentation throughout the colon.”

https://onlinelibrary.wiley.com/doi/10.1111/1541-4337.12728 “Health benefits of whole grain: effects on dietary carbohydrate quality, the gut microbiome, and consequences of processing”

This review cited a 2019 paper as “an elegant study where oat bran (including co-passengers) was shown to be effective in increasing Bifidobacterium populations in the gut, whereas purified bioactive β-glucans did not show a bifidogenic effect”:

“Whole grain oats are known to modulate human gut microbiota and have prebiotic properties. Research todate mainly attributes these effects to fibre content. However, oats are also a rich dietary source of polyphenols, which may contribute to positive modulation of gut microbiota.

We found that oats increased bifidobacteria, acetic acid and propionic acid. This was mediated by synergy of all oat compounds within the complex food matrix, rather than its main bioactive β-glucan or polyphenols.

While human digestive enzymes cannot degrade plant cell wall polysaccharides, gut xylanolytic bacteria can, producing SCFA with health-beneficial effects. Certain strains down-regulate gene and protein expression of pro-inflammatory cytokines, notably isoform of nitric oxide synthase and PPAR-γ and interferon-γ, resulting in reduced inflammatory status, suggesting that oat β-glucan have beneficial effects on human health.

Oats as a whole food led to the greatest impact on microbiota.”

https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/oat-bran-but-not-its-isolated-bioactive-glucans-or-polyphenols-have-a-bifidogenic-effect-in-an-in-vitro-fermentation-model-of-the-gut-microbiota/B23FAE2C7EED702132FC72F1C9CE990E “Oat bran, but not its isolated bioactive β-glucans or polyphenols, have a bifidogenic effect in an in vitro fermentation model of the gut microbiota”

The Avena nuda oats I eat for breakfast start out as 81.0 grams (1/2 cup). The only processing I do from an Illinois farmer is soaking them for 16 hours, draining then changing out to 1 1/2 cups water, then cooking for 20 minutes in a 1000W microwave at 80% power. They end up weighing 154.7 g.

I eat 51.9 g of 3-day-old sprouted Avena sativa oats from a Montana farmer at the same time, and concurrently take 2.5 g inulin. Pretty sure this 154.7 + 51.9 + 2.5 = 209.1 g combination meets an “optimum ratio of total carbohydrate to dietary fiber ≤10:1.”

Also pretty sure sprouted Avena sativa oats supply enzymes that facilitate breaking down Avena nuda complex molecules. Haven’t experienced any complaints over the past 3+ months. 🙂