This 2020 rodent study investigated effects of different diets:

“The present study aimed to evaluate effects of β-glucan on the microbiota gut-brain axis and cognitive function in an obese mouse model induced by a high-fat and fiber-deficient diet (HFFD). After long-term supplementation for 15 weeks, β-glucan prevented HFFD-induced cognitive impairment, assessed behaviorally by object location, novel object recognition, and nesting building tests:

• Long-term β-glucan supplementation suppressed microglia activation and inflammation in hippocampus of HFFD-fed mice;
• β-glucan attenuated deleterious engulfment of synapses by activation of microglia seen in HFFD mice;
• β-glucan significantly prevented upregulation of TNF-α, IL-1β, and IL-6 mRNA expression in hippocampus; and
  
• A broad-spectrum antibiotic intervention abrogated β-glucan-induced improvement in cognitive function, highlighting the essential role of gut microbiota to mediate cognitive function and behavior.

We found that short-term β-glucan supplementation did not change cognitive behavior in HFFD fed mice. HFFD feeding for 7 days dramatically changed gut microbial profile, with β-glucan-fed mice clustered apart from HFFD-fed mice sample, suggesting:

• Quick changes in gut microbiota are induced by short-term β-glucan consumption and
• Possible causality of gut microbiota profile on cognition.

β-glucan supplementation increased place discrimination ratio in object location test compared with HFFD mice; however, there was no significant difference in total exploration time with objects during test phases between the two groups. Higher place discrimination index in β-glucan supplementation group was not due to better general performance, but increased recognition memory.

Results provide consistent evidence linking increased β-glucan intake to improved:

• Gut microbiota profile;
• Intestinal barrier function;
• Reduced endotoxemia; and
• Enhanced cognitive function via more optimized synaptic and signaling pathways in critical brain areas.

It is speculative that β-glucan improvement of gut microbiota composition, but not necessarily diversity per se, may be most critical for improved cognition. Enhanced consumption of β-glucan-rich foods is an easily implementable nutritional strategy to attenuate diet-induced cognitive decline.”

https://microbiomejournal.biomedcentral.com/articles/10.1186/s40168-020-00920-y “β-glucan attenuates cognitive impairment via the gut-brain axis in diet-induced obese mice”

This study did well by elaborating It’s the fiber, not the fat and Eat oats to prevent diabetes related findings. How many humans eat themselves into essentially the same situation as this HFFD group with no gut-microbiota-friendly dietary fiber?

Experiments were with β-glucan 1,3/1,4 found in oats. β-glucan 1,3/1,6 has separate effects, especially on innate immunity.

It’s a coin toss on whether observed cognitive improvement was due to 7% β-glucan soluble fiber, 7% indigestible fiber, or both since they were part of the same HFBG diet. I eat both fibers, beginning with Avena nuda oats for breakfast.