The short answer? We don’t know the answer to either of those questions.
Low-carb salad
The gut bacteria (aka microbiome) seem to be able to decrease or increase inflammation that could cause or exacerbate Alzheimer’s dementia. The microbiome’s effect on inflammation depends on the species of bacteria present, and the amount of those bacteria. At least one study found that Alzheimer’s patients have a greater abundance of the pro-inflammatory species and less of the anti-inflammatory species, compared to other folks.
Researchers with Wake Forest School of Medicine tried to find answers to the questions in the title of this post. (Click for full text.) They studied 17 experimental subjects, average age 64, who had mild cognitive impairment (11) or “cogni/subjective memory complaints” (6). God bless them for submitting to three spinal taps apiece. The experimental diets were 1) Mediterranean-Ketogenic (under 20 g carb/day), or 2) Low-fat American Heart Association diet (under 40 g fat/day). Participants were on each diet for six weeks.
The investigators didn’t find anything useful for those of us trying today to avoid Alzheimer’s or prevent the progression of mild cognitive impairment to dementia. Their bottom line is, “The data suggest that specific gut microbial signatures may depict [characterize] the mild cognitive impairment and that the modified Mediterranean-ketogenic diet can modulate the gut microbiome and metabolites in association with improved Alzheimer’s disease biomarkers in cerebrospinal fluid.”
So we won’t know for several more years, if ever, whether intentional modification of diet will “improve” our gut microbiomes, leading to lower risk of dementia.
What we have known for many year, however, is that the traditional Mediterranean diet is linked to lower risk of Alzhiemer’s dementia.
For more details, see Science Daily:
In a small pilot study, the researchers identified several distinct gut microbiome signatures — the chemicals produced by bacteria — in study participants with mild cognitive impairment (MCI) but not in their counterparts with normal cognition, and found that these bacterial signatures correlated with higher levels of markers of Alzheimer’s disease in the cerebrospinal fluid of the participants with MCI.
Through cross-group dietary intervention, the study also showed that a modified Mediterranean-ketogenic diet produced changes in the gut microbiome and its metabolites that correlated with reduced levels of Alzheimer’s markers in the members of both study groups.
Source: Diet’s effect on gut bacteria could play role in reducing Alzheimer’s risk — ScienceDaily
Steve Parker, M.D.
Two diets in one book, including the Ketogenic Mediterranean Diet
Click the pic to purchase the world’s first practical ketogenic Mediterranean diet at Amazon.com
Advanced Mediterranean Diet 
Eat more Greek yogurt?
Steve,Just want to say hi and thank you— I guess you know how many people are reading your posts, but I do not, so just wanted to let you know your efforts are appreciated. You may already listen to these fellows, but a “friend” on a political blog where I “hang out” in the comment section posted it the other day, and I am over the moon about this!
http://lowcarbmd.com/episode-49-dr-chris-palmer-treating-schizophrenia-with-lifestyle Well worth a listen on your next hike. Thanks for all you read and share. Lee Ann McGinnis, MD (Btw—many of the frequent commenters at the blog have “gone keto” (HFLC) as they call it, with just wonderful health benefits—and via them, many family members. I figure I have done more by posting “carbs kill” as a sign off (as well as PLENTY of links and info) as far as positive health influence than the forty years I’ve spent taking care of patients in the OR!)
Great to hear from you, Lee Ann. Thanks for the encouragement. I often feel like a prophet crying out in the wilderness.
-Steve